2012-01-30 Robert Dewar <dewar@adacore.com>

[pf3gnuchains/gcc-fork.git] / gcc / ada / projects.texi

@set gprconfig GPRconfig

@c ------ projects.texi
@c Copyright (C) 2002-2012, Free Software Foundation, Inc.
@c This file is shared between the GNAT user's guide and gprbuild. It is not
@c compilable on its own, you should instead compile the other two manuals.
@c For that reason, there is no toplevel @menu

@c ---------------------------------------------
@node GNAT Project Manager
@chapter GNAT Project Manager
@c ---------------------------------------------

@noindent
@menu
* Introduction::
* Building With Projects::
* Organizing Projects into Subsystems::
* Scenarios in Projects::
* Library Projects::
* Project Extension::
* Aggregate Projects::
* Aggregate Library Projects::
* Project File Reference::
@end menu

@c ---------------------------------------------
@node Introduction
@section Introduction
@c ---------------------------------------------

@noindent
This chapter describes GNAT's @emph{Project Manager}, a facility that allows
you to manage complex builds involving a number of source files, directories,
and options for different system configurations. In particular,
project files allow you to specify:

@itemize @bullet
@item The directory or set of directories containing the source files, and/or the
  names of the specific source files themselves
@item The directory in which the compiler's output
  (@file{ALI} files, object files, tree files, etc.) is to be placed
@item The directory in which the executable programs are to be placed
@item Switch settings for any of the project-enabled tools;
  you can apply these settings either globally or to individual compilation units.
@item The source files containing the main subprogram(s) to be built
@item The source programming language(s)
@item Source file naming conventions; you can specify these either globally or for
  individual compilation units (@pxref{Naming Schemes}).
@item Change any of the above settings depending on external values, thus enabling
  the reuse of the projects in various @b{scenarios} (@pxref{Scenarios
  in Projects}).
@item Automatically build libraries as part of the build process
  (@pxref{Library Projects}).

@end itemize

@noindent
Project files are written in a syntax close to that of Ada, using familiar
notions such as packages, context clauses, declarations, default values,
assignments, and inheritance (@pxref{Project File Reference}).

Project files can be built hierarchically from other project files, simplifying
complex system integration and project reuse (@pxref{Organizing Projects into
Subsystems}).

@itemize @bullet
@item One project can import other projects containing needed source files.
  More generally, the Project Manager lets you structure large development
  efforts into hierarchical subsystems, where build decisions are delegated
  to the subsystem level, and thus different compilation environments
  (switch settings) used for different subsystems.
@item You can organize GNAT projects in a hierarchy: a child project
  can extend a parent project, inheriting the parent's source files and
  optionally overriding any of them with alternative versions
  (@pxref{Project Extension}).

@end itemize

@noindent
Several tools support project files, generally in addition to specifying
the information on the command line itself). They share common switches
to control the loading of the project (in particular
@option{^-P^/PROJECT_FILE=^@emph{projectfile}} and
@option{^-X^/EXTERNAL_REFERENCE=^@emph{vbl}=@emph{value}}).
@xref{Switches Related to Project Files}.

The Project Manager supports a wide range of development strategies,
for systems of all sizes.  Here are some typical practices that are
easily handled:

@itemize @bullet
@item Using a common set of source files and generating object files in different
  directories via different switch settings. It can be used for instance, for
  generating separate sets of object files for debugging and for production.
@item Using a mostly-shared set of source files with different versions of
  some units or subunits. It can be used for instance, for grouping and hiding
@end itemize

@noindent
all OS dependencies in a small number of implementation units.

Project files can be used to achieve some of the effects of a source
versioning system (for example, defining separate projects for
the different sets of sources that comprise different releases) but the
Project Manager is independent of any source configuration management tool
that might be used by the developers.

The various sections below introduce the different concepts related to
projects. Each section starts with examples and use cases, and then goes into
the details of related project file capabilities.

@c ---------------------------------------------
@node Building With Projects
@section Building With Projects
@c ---------------------------------------------

@noindent
In its simplest form, a unique project is used to build a single executable.
This section concentrates on such a simple setup. Later sections will extend
this basic model to more complex setups.

The following concepts are the foundation of project files, and will be further
detailed later in this documentation. They are summarized here as a reference.

@table @asis
@item @b{Project file}:
  A text file using an Ada-like syntax, generally using the @file{.gpr}
  extension. It defines build-related characteristics of an application.
  The characteristics include the list of sources, the location of those
  sources, the location for the generated object files, the name of
  the main program, and the options for the various tools involved in the
  build process.

@item @b{Project attribute}:
  A specific project characteristic is defined by an attribute clause. Its
  value is a string or a sequence of strings. All settings in a project
  are defined through a list of predefined attributes with precise
  semantics. @xref{Attributes}.

@item @b{Package in a project}:
  Global attributes are defined at the top level of a project.
  Attributes affecting specific tools are grouped in a
  package whose name is related to tool's function. The most common
  packages are @code{Builder}, @code{Compiler}, @code{Binder},
  and @code{Linker}. @xref{Packages}.

@item @b{Project variables}:
  In addition to attributes, a project can use variables to store intermediate
  values and avoid duplication in complex expressions. It can be initialized
  with a value coming from the environment.
  A frequent use of variables is to define scenarios.
  @xref{External Values}, @xref{Scenarios in Projects}, and @xref{Variables}.

@item @b{Source files} and @b{source directories}:
  A source file is associated with a language through a naming convention. For
  instance, @code{foo.c} is typically the name of a C source file;
  @code{bar.ads} or @code{bar.1.ada} are two common naming conventions for a
  file containing an Ada spec. A compilation unit is often composed of a main
  source file and potentially several auxiliary ones, such as header files in C.
  The naming conventions can be user defined @xref{Naming Schemes}, and will
  drive the builder to call the appropriate compiler for the given source file.
  Source files are searched for in the source directories associated with the
  project through the @b{Source_Dirs} attribute. By default, all the files (in
  these source directories) following the naming conventions associated with the
  declared languages are considered to be part of the project. It is also
  possible to limit the list of source files using the @b{Source_Files} or
  @b{Source_List_File} attributes. Note that those last two attributes only
  accept basenames with no directory information.

@item @b{Object files} and @b{object directory}:
  An object file is an intermediate file produced by the compiler from a
  compilation unit. It is used by post-compilation tools to produce
  final executables or libraries. Object files produced in the context of
  a given project are stored in a single directory that can be specified by the
  @b{Object_Dir} attribute. In order to store objects in
  two or more object directories, the system must be split into
  distinct subsystems with their own project file.

@end table

The following subsections introduce gradually all the attributes of interest
for simple build needs. Here is the simple setup that will be used in the
following examples.

The Ada source files @file{pack.ads}, @file{pack.adb}, and @file{proc.adb} are in
the @file{common/} directory. The file @file{proc.adb} contains an Ada main
subprogram @code{Proc} that @code{with}s package @code{Pack}. We want to compile
these source files with the switch @option{-O2}, and put the resulting files in
the directory @file{obj/}.

@smallexample
@group
^common/^[COMMON]^
  pack.ads
  pack.adb
  proc.adb
@end group
@group
^common/release/^[COMMON.RELEASE]^
  proc.ali, proc.o pack.ali, pack.o
@end group
@end smallexample

@noindent
Our project is to be called @emph{Build}. The name of the
file is the name of the project (case-insensitive) with the
@file{.gpr} extension, therefore the project file name is @file{build.gpr}. This
is not mandatory, but a warning is issued when this convention is not followed.

This is a very simple example, and as stated above, a single project
file is enough for it. We will thus create a new file, that for now
should contain the following code:

@smallexample
@b{project} Build @b{is}
@b{end} Build;
@end smallexample

@menu
* Source Files and Directories::
* Object and Exec Directory::
* Main Subprograms::
* Tools Options in Project Files::
* Compiling with Project Files::
* Executable File Names::
* Avoid Duplication With Variables::
* Naming Schemes::
@end menu

@c ---------------------------------------------
@node Source Files and Directories
@subsection Source Files and Directories
@c ---------------------------------------------

@noindent
When you create a new project, the first thing to describe is how to find the
corresponding source files. This is the only settings that are needed by all
the tools that will use this project (builder, compiler, binder and linker for
the compilation, IDEs to edit the source files,@dots{}).

@cindex Source directories
First step is to declare the source directories, which are the directories
to be searched to find source files. In the case of the example,
the @file{common} directory is the only source directory.

@cindex @code{Source_Dirs}
There are several ways of defining source directories:

@itemize @bullet
@item When the attribute @b{Source_Dirs} is not used, a project contains a
  single source directory which is the one where the project file itself
  resides. In our example, if @file{build.gpr} is placed in the @file{common}
  directory, the project has the needed implicit source directory.

@item The attribute @b{Source_Dirs} can be set to a list of path names, one
  for each of the source directories. Such paths can either be absolute
  names (for instance @file{"/usr/local/common/"} on UNIX), or relative to the
  directory in which the project file resides (for instance "." if
  @file{build.gpr} is inside @file{common/}, or "common" if it is one level up).
  Each of the source directories must exist and be readable.

@cindex portability
  The syntax for directories is platform specific. For portability, however,
  the project manager will always properly translate UNIX-like path names to
  the native format of specific platform. For instance, when the same project
  file is to be used both on Unix and Windows, "/" should be used as the
  directory separator rather than "\".

@item The attribute @b{Source_Dirs} can automatically include subdirectories
  using a special syntax inspired by some UNIX shells. If any of the path in
  the list ends with @emph{"**"}, then that path and all its subdirectories
  (recursively) are included in the list of source directories. For instance,
  @file{**} and @file{./**} represent the complete directory tree rooted at ".".
@cindex Source directories, recursive

@cindex @code{Excluded_Source_Dirs}
  When using that construct, it can sometimes be convenient to also use the
  attribute @b{Excluded_Source_Dirs}, which is also a list of paths. Each entry
  specifies a directory whose immediate content, not including subdirs, is to
  be excluded. It is also possible to exclude a complete directory subtree
  using the "**" notation.

@cindex @code{Ignore_Source_Sub_Dirs}
  It is often desirable to remove, from the source directories, directory
  subtrees rooted at some subdirectories. An example is the subdirectories
  created by a Version Control System such as Subversion that creates directory
  subtrees .svn/**. To do that, attribute  @b{Ignore_Source_Sub_Dirs} can be
  used. It specifies the list of simple file names for the root of these
  undesirable directory subtrees.

@end itemize

@noindent
When applied to the simple example, and because we generally prefer to have
the project file at the toplevel directory rather than mixed with the sources,
we will create the following file

@smallexample
   build.gpr
   @b{project} Build @b{is}
      @b{for} Source_Dirs @b{use} ("common");  --  <<<<
   @b{end} Build;
@end smallexample

@noindent
Once source directories have been specified, one may need to indicate
source files of interest. By default, all source files present in the source
directories are considered by the project manager. When this is not desired,
it is possible to specify the list of sources to consider explicitly.
In such a case, only source file base names are indicated and not
their absolute or relative path names. The project manager is in charge of
locating the specified source files in the specified source directories.

@itemize @bullet
@item By default, the project manager  search for all source files of all
  specified languages in all the source directories.

  Since the project manager was initially developed for Ada environments, the
  default language is usually Ada and the above project file is complete: it
  defines without ambiguity the sources composing the project: that is to say,
  all the sources in subdirectory "common" for the default language (Ada) using
  the default naming convention.

@cindex @code{Languages}
  However, when compiling a multi-language application, or a pure C
  application, the project manager must be told which languages are of
  interest, which is done by setting the @b{Languages} attribute to a list of
  strings, each of which is the name of a language. Tools like
  @command{gnatmake} only know about Ada, while other tools like
  @command{gprbuild} know about many more languages such as C, C++, Fortran,
  assembly and others can be added dynamically.

@cindex Naming scheme
  Even when using only Ada, the default naming might not be suitable. Indeed,
  how does the project manager recognizes an "Ada file" from any other
  file? Project files can describe the naming scheme used for source files,
  and override the default (@pxref{Naming Schemes}). The default is the
  standard GNAT extension (@file{.adb} for bodies and @file{.ads} for
  specs), which is what is used in our example, explaining why no naming scheme
  is explicitly specified.
  @xref{Naming Schemes}.

@item @code{Source Files}
  @cindex @code{Source_Files}
  In some cases, source directories might contain files that should not be
  included in a project. One can specify the explicit list of file names to
  be considered through the @b{Source_Files} attribute.
  When this attribute is defined, instead of looking at every file in the
  source directories, the project manager takes only those names into
  consideration  reports  errors if they cannot be found in the source
  directories or does not correspond to the naming scheme.

@item For various reasons, it is sometimes useful to have a project with no
  sources (most of the time because the attributes defined in the project
  file will be reused in other projects, as explained in @pxref{Organizing
  Projects into Subsystems}. To do this, the attribute
  @emph{Source_Files} is set to the empty list, i.e. @code{()}. Alternatively,
  @emph{Source_Dirs} can be set to the empty list, with the same
  result.

@item @code{Source_List_File}
@cindex @code{Source_List_File}
  If there is a great number of files, it might be more convenient to use
  the attribute @b{Source_List_File}, which specifies the full path of a file.
  This file must contain a list of source file names (one per line, no
  directory information) that are searched as if they had been defined
  through @emph{Source_Files}. Such a file can easily be created through
  external tools.

  A warning is issued if both attributes @code{Source_Files} and
  @code{Source_List_File} are given explicit values. In this case, the
  attribute @code{Source_Files} prevails.

@item @code{Excluded_Source_Files}
@cindex @code{Excluded_Source_Files}
@cindex @code{Locally_Removed_Files}
@cindex @code{Excluded_Source_List_File}
  Specifying an explicit list of files is not always convenient.It might be
  more convenient to use the default search rules with specific exceptions.
  This can be done thanks to the attribute @b{Excluded_Source_Files}
  (or its synonym @b{Locally_Removed_Files}).
  Its value is the list of file names that should not be taken into account.
  This attribute is often used when extending a project, @xref{Project
  Extension}. A similar attribute @b{Excluded_Source_List_File} plays the same
  role but takes the name of file containing file names similarly to
  @code{Source_List_File}.

@end itemize

@noindent
In most simple cases, such as the above example, the default source file search
behavior provides the expected result, and we do not need to add anything after
setting @code{Source_Dirs}. The project manager automatically finds
@file{pack.ads}, @file{pack.adb} and @file{proc.adb} as source files of the
project.

Note that it is considered an error for a project file to have no sources
attached to it unless explicitly declared as mentioned above.

If the order of the source directories is known statically, that is if
@code{"**"} is not used in the string list @code{Source_Dirs}, then there may
be several files with the same source file name sitting in different
directories of the project. In this case, only the file in the first directory
is considered as a source of the project and the others are hidden. If
@code{"**"} is not used in the string list @code{Source_Dirs}, it is an error
to have several files with the same source file name in the same directory
@code{"**"} subtree, since there would be an ambiguity as to which one should
be used. However, two files with the same source file name may in two single
directories or directory subtrees. In this case, the one in the first directory
or directory subtree is a source of the project.

@c ---------------------------------------------
@node Object and Exec Directory
@subsection Object and Exec Directory
@c ---------------------------------------------

@noindent
The next step when writing a project is to indicate where the compiler should
put the object files. In fact, the compiler and other tools might create
several different kind of files (for GNAT, there is the object file and the ALI
file for instance). One of the important concepts in projects is that most
tools may consider source directories as read-only and do not attempt to create
new or temporary files there. Instead, all files are created in the object
directory. It is of course not true for project-aware IDEs, whose purpose it is
to create the source files.

@cindex @code{Object_Dir}
The object directory is specified through the @b{Object_Dir} attribute.
Its value is the path to the object directory, either absolute or
relative to the directory containing the project file. This
directory must already exist and be readable and writable, although
some tools have a switch to create the directory if needed (See
the switch @code{-p} for @command{gnatmake} and @command{gprbuild}).

If the attribute @code{Object_Dir} is not specified, it defaults to
the project directory, that is the directory containing the project file.

For our example, we can specify the object dir in this way:

@smallexample
   @b{project} Build @b{is}
      @b{for} Source_Dirs @b{use} ("common");
      @b{for} Object_Dir @b{use} "obj";   --  <<<<
   @b{end} Build;
@end smallexample

@noindent
As mentioned earlier, there is a single object directory per project. As a
result, if you have an existing system where the object files are spread in
several directories, you can either move all of them into the same directory if
you want to build it with a single project file, or study the section on
subsystems (@pxref{Organizing Projects into Subsystems}) to see how each
separate object directory can be associated with one of the subsystem
constituting the application.

When the @command{linker} is called, it usually creates an executable. By
default, this executable is placed in the object directory of the project. It
might be convenient to store it in its own directory.

@cindex @code{Exec_Dir}
This can be done through the @code{Exec_Dir} attribute, which, like
@emph{Object_Dir} contains a single absolute or relative path and must point to
an existing and writable directory, unless you ask the tool to create it on
your behalf. When not specified, It defaults to the object directory and
therefore to the project file's directory if neither @emph{Object_Dir} nor
@emph{Exec_Dir} was specified.

In the case of the example, let's place the executable in the root
of the hierarchy, ie the same directory as @file{build.gpr}. Hence
the project file is now

@smallexample
   @b{project} Build @b{is}
      @b{for} Source_Dirs @b{use} ("common");
      @b{for} Object_Dir @b{use} "obj";
      @b{for} Exec_Dir @b{use} ".";  --   <<<<
   @b{end} Build;
@end smallexample

@c ---------------------------------------------
@node Main Subprograms
@subsection Main Subprograms
@c ---------------------------------------------

@noindent
In the previous section, executables were mentioned. The project manager needs
to be taught what they are. In a project file, an executable is indicated by
pointing to source file of the main subprogram. In C this is the file that
contains the @code{main} function, and in Ada the file that contains the main
unit.

There can be any number of such main files within a given project, and thus
several executables can be built in the context of a single project file. Of
course, one given executable might not (and in fact will not) need all the
source files referenced by the project. As opposed to other build environments
such as @command{makefile}, one does not need to specify the list of
dependencies of each executable, the project-aware builders knows enough of the
semantics of the languages to build ands link only the necessary elements.

@cindex @code{Main}
The list of main files is specified via the @b{Main} attribute. It contains
a list of file names (no directories). If a project defines this
attribute, it is not necessary to identify  main files on the
command line when invoking a builder, and editors like
@command{GPS} will be able to create extra menus to spawn or debug the
corresponding executables.

@smallexample
   @b{project} Build @b{is}
      @b{for} Source_Dirs @b{use} ("common");
      @b{for} Object_Dir @b{use} "obj";
      @b{for} Exec_Dir @b{use} ".";
      @b{for} Main @b{use} ("proc.adb");  --   <<<<
   @b{end} Build;
@end smallexample

@noindent
If this attribute is defined in the project, then spawning the builder
with a command such as

@smallexample
   gnatmake ^-Pbuild^/PROJECT_FILE=build^
@end smallexample

@noindent
automatically builds all the executables corresponding to the files
listed in the @emph{Main} attribute. It is possible to specify one
or more executables on the command line to build a subset of them.

@c ---------------------------------------------
@node Tools Options in Project Files
@subsection Tools Options in Project Files
@c ---------------------------------------------

@noindent
We now have a project file that fully describes our environment, and can be
used to build the application with a simple @command{gnatmake} command as seen
in the previous section. In fact, the empty project we showed immediately at
the beginning (with no attribute at all) could already fulfill that need if it
was put in the @file{common} directory.

Of course, we always want more control. This section will show you how to
specify the compilation switches that the various tools involved in the
building of the executable should use.

@cindex command line length
Since source names and locations are described into the project file, it is not
necessary to use switches on the command line for this purpose (switches such
as -I for gcc). This removes a major source of command line length overflow.
Clearly, the builders will have to communicate this information one way or
another to the underlying compilers and tools they call but they usually use
response files for this and thus should not be subject to command line
overflows.

Several tools are participating to the creation of an executable: the compiler
produces object files from the source files; the binder (in the Ada case)
creates an source file that takes care, among other things, of elaboration
issues and global variables initialization; and the linker gathers everything
into a single executable that users can execute. All these tools are known by
the project manager and will be called with user defined switches from the
project files. However, we need to introduce a new project file concept to
express which switches to be used for any of the tools involved in the build.

@cindex project file packages
A project file is subdivided into zero or more @b{packages}, each of which
contains the attributes specific to one tool (or one set of tools). Project
files use an Ada-like syntax for packages. Package names permitted in project
files are restricted to a predefined set (@pxref{Packages}), and the contents
of packages are limited to a small set of constructs and attributes
(@pxref{Attributes}).

Our example project file can be extended with the following empty packages. At
this stage, they could all be omitted since they are empty, but they show which
packages would be involved in the build process.

@smallexample
   @b{project} Build @b{is}
      @b{for} Source_Dirs @b{use} ("common");
      @b{for} Object_Dir @b{use} "obj";
      @b{for} Exec_Dir @b{use} ".";
      @b{for} Main @b{use} ("proc.adb");

      @b{package} Builder @b{is}  --<<<  for gnatmake and gprbuild
      @b{end} Builder;

      @b{package} Compiler @b{is} --<<<  for the compiler
      @b{end} Compiler;

      @b{package} Binder @b{is}   --<<<  for the binder
      @b{end} Binder;

      @b{package} Linker @b{is}   --<<<  for the linker
      @b{end} Linker;
   @b{end} Build;
@end smallexample

@noindent
Let's first examine the compiler switches. As stated in the initial description
of the example, we want to compile all files with @option{-O2}. This is a
compiler switch, although it is usual, on the command line, to pass it to the
builder which then passes it to the compiler. It is recommended to use directly
the right package, which will make the setup easier to understand for other
people.

Several attributes can be used to specify the switches:

@table @asis
@item @b{Default_Switches}:
@cindex @code{Default_Switches}
  This is the first mention in this manual of an @b{indexed attribute}. When
  this attribute is defined, one must supply an @emph{index} in the form of a
  literal string.
  In the case of @emph{Default_Switches}, the index is the name of the
  language to which the switches apply (since a different compiler will
  likely be used for each language, and each compiler has its own set of
  switches). The value of the attribute is a list of switches.

  In this example, we want to compile all Ada source files with the
  @option{-O2} switch, and the resulting project file is as follows
  (only the @code{Compiler} package is shown):

  @smallexample
  @b{package} Compiler @b{is}
    @b{for} Default_Switches ("Ada") @b{use} ("-O2");
  @b{end} Compiler;
  @end smallexample

@item @b{Switches}:
@cindex @code{Switches}
  in some cases, we might want to use specific switches
  for one or more files. For instance, compiling @file{proc.adb} might not be
  possible at high level of optimization because of a compiler issue.
  In such a case, the @emph{Switches}
  attribute (indexed on the file name) can be used and will override the
  switches defined by @emph{Default_Switches}. Our project file would
  become:

  @smallexample
  @b{package} Compiler @b{is}
    @b{for} Default_Switches ("Ada") @b{use} ("-O2");
    @b{for} Switches ("proc.adb") @b{use} ("-O0");
  @b{end} Compiler;
  @end smallexample

  @noindent
  @code{Switches} may take a pattern as an index, such as in:

  @smallexample
  @b{package} Compiler @b{is}
    @b{for} Default_Switches ("Ada") @b{use} ("-O2");
    @b{for} Switches ("pkg*") @b{use} ("-O0");
  @b{end} Compiler;
  @end smallexample

  @noindent
  Sources @file{pkg.adb} and @file{pkg-child.adb} would be compiled with -O0,
  not -O2.

  @noindent
  @code{Switches} can also be given a language name as index instead of a file
  name in which case it has the same semantics as @emph{Default_Switches}.
  However, indexes with wild cards are never valid for language name.

@item @b{Local_Configuration_Pragmas}:
@cindex @code{Local_Configuration_Pragmas}
  this attribute may specify the path
  of a file containing configuration pragmas for use by the Ada compiler,
  such as @code{pragma Restrictions (No_Tasking)}. These pragmas will be
  used for all the sources of the project.

@end table

The switches for the other tools are defined in a similar manner through the
@b{Default_Switches} and @b{Switches} attributes, respectively in the
@emph{Builder} package (for @command{gnatmake} and @command{gprbuild}),
the @emph{Binder} package (binding Ada executables) and the @emph{Linker}
package (for linking executables).

@c ---------------------------------------------
@node Compiling with Project Files
@subsection Compiling with Project Files
@c ---------------------------------------------

@noindent
Now that our project files are written, let's build our executable.
Here is the command we would use from the command line:

@smallexample
   gnatmake ^-Pbuild^/PROJECT_FILE=build^
@end smallexample

@noindent
This will automatically build the executables specified through the
@emph{Main} attribute: for each, it will compile or recompile the
sources for which the object file does not exist or is not up-to-date; it
will then run the binder; and finally run the linker to create the
executable itself.

@command{gnatmake} only knows how to handle Ada files. By using
@command{gprbuild} as a builder, you could automatically manage C files the
same way: create the file @file{utils.c} in the @file{common} directory,
set the attribute @emph{Languages} to @code{"(Ada, C)"}, and run

@smallexample
   gprbuild ^-Pbuild^/PROJECT_FILE=build^
@end smallexample

@noindent
Gprbuild knows how to recompile the C files and will
recompile them only if one of their dependencies has changed. No direct
indication on how to build the various elements is given in the
project file, which describes the project properties rather than a
set of actions to be executed. Here is the invocation of
@command{gprbuild} when building a multi-language program:

@smallexample
$ gprbuild -Pbuild
gcc -c proc.adb
gcc -c pack.adb
gcc -c utils.c
gprbind proc
...
gcc proc.o -o proc
@end smallexample

@noindent
Notice the three steps described earlier:

@itemize @bullet
@item The first three gcc commands correspond to the compilation phase.
@item The gprbind command corresponds to the post-compilation phase.
@item The last gcc command corresponds to the final link.

@end itemize

@noindent
@cindex @option{-v} option (for GPRbuild)
The default output of GPRbuild's execution is kept reasonably simple and easy
to understand. In particular, some of the less frequently used commands are not
shown, and some parameters are abbreviated. So it is not possible to rerun the
effect of the @command{gprbuild} command by cut-and-pasting its output.
GPRbuild's option @code{-v} provides a much more verbose output which includes,
among other information, more complete compilation, post-compilation and link
commands.

@c ---------------------------------------------
@node Executable File Names
@subsection Executable File Names
@c ---------------------------------------------

@noindent
@cindex @code{Executable}
By default, the executable name corresponding to a main file is
computed from the main source file name. Through the attribute
@b{Builder.Executable}, it is possible to change this default.

For instance, instead of building @command{proc} (or @command{proc.exe}
on Windows), we could configure our project file to build "proc1"
(resp proc1.exe) with the following addition:

@smallexample @c projectfile
   project Build is
      ...  --  same as before
      package Builder is
         for Executable ("proc.adb") use "proc1";
      end Builder
   end Build;
@end smallexample

@noindent
@cindex @code{Executable_Suffix}
Attribute @b{Executable_Suffix}, when specified, may change the suffix
of the executable files, when no attribute @code{Executable} applies:
its value replace the platform-specific executable suffix.
The default executable suffix is empty on UNIX and ".exe" on Windows.

It is also possible to change the name of the produced executable by using the
command line switch @option{-o}. When several mains are defined in the project,
it is not possible to use the @option{-o} switch and the only way to change the
names of the executable is provided by Attributes @code{Executable} and
@code{Executable_Suffix}.

@c ---------------------------------------------
@node Avoid Duplication With Variables
@subsection Avoid Duplication With Variables
@c ---------------------------------------------

@noindent
To illustrate some other project capabilities, here is a slightly more complex
project using similar sources and a main program in C:

@smallexample @c projectfile
project C_Main is
   for Languages    use ("Ada", "C");
   for Source_Dirs  use ("common");
   for Object_Dir   use  "obj";
   for Main         use ("main.c");
   package Compiler is
      C_Switches := ("-pedantic");
      for Default_Switches ("C")   use C_Switches;
      for Default_Switches ("Ada") use ("-gnaty");
      for Switches ("main.c") use C_Switches & ("-g");
   end Compiler;
end C_Main;
@end smallexample

@noindent
This project has many similarities with the previous one.
As expected, its @code{Main} attribute now refers to a C source.
The attribute @emph{Exec_Dir} is now omitted, thus the resulting
executable will be put in the directory @file{obj}.

The most noticeable difference is the use of a variable in the
@emph{Compiler} package to store settings used in several attributes.
This avoids text duplication, and eases maintenance (a single place to
modify if we want to add new switches for C files). We will revisit
the use of variables in the context of scenarios (@pxref{Scenarios in
Projects}).

In this example, we see how the file @file{main.c} can be compiled with
the switches used for all the other C files, plus @option{-g}.
In this specific situation the use of a variable could have been
replaced by a reference to the @code{Default_Switches} attribute:

@smallexample @c projectfile
   for Switches ("c_main.c") use Compiler'Default_Switches ("C") & ("-g");
@end smallexample

@noindent
Note the tick (@emph{'}) used to refer to attributes defined in a package.

Here is the output of the GPRbuild command using this project:

@smallexample
$gprbuild -Pc_main
gcc -c -pedantic -g main.c
gcc -c -gnaty proc.adb
gcc -c -gnaty pack.adb
gcc -c -pedantic utils.c
gprbind main.bexch
...
gcc main.o -o main
@end smallexample

@noindent
The default switches for Ada sources,
the default switches for C sources (in the compilation of @file{lib.c}),
and the specific switches for @file{main.c} have all been taken into
account.

@c ---------------------------------------------
@node Naming Schemes
@subsection Naming Schemes
@c ---------------------------------------------

@noindent
Sometimes an Ada software system is ported from one compilation environment to
another (say GNAT), and the file are not named using the default GNAT
conventions. Instead of changing all the file names, which for a variety of
reasons might not be possible, you can define the relevant file naming scheme
in the @b{Naming} package of your project file.

The naming scheme has two distinct goals for the project manager: it
allows finding of source files when searching in the source
directories, and given a source file name it makes it possible to guess
the associated language, and thus the compiler to use.

Note that the use by the Ada compiler of pragmas Source_File_Name is not
supported when using project files. You must use the features described in this
paragraph. You can however specify other configuration pragmas
(@pxref{Specifying Configuration Pragmas}).

The following attributes can be defined in package @code{Naming}:

@table @asis
@item @b{Casing}:
@cindex @code{Casing}
  Its value must be one of @code{"lowercase"} (the default if
  unspecified), @code{"uppercase"} or @code{"mixedcase"}. It describes the
  casing of file names with regards to the Ada unit name. Given an Ada unit
  My_Unit, the file name will respectively be @file{my_unit.adb} (lowercase),
  @file{MY_UNIT.ADB} (uppercase) or @file{My_Unit.adb} (mixedcase).
  On Windows, file names are case insensitive, so this attribute is
  irrelevant.

@item @b{Dot_Replacement}:
@cindex @code{Dot_Replacement}
  This attribute specifies the string that should replace the "." in unit
  names. Its default value is @code{"-"} so that a unit
  @code{Parent.Child} is expected to be found in the file
  @file{parent-child.adb}. The replacement string must satisfy the following
  requirements to avoid ambiguities in the naming scheme:

  @itemize -
  @item It must not be empty
  @item It cannot start or end with an alphanumeric character
  @item It cannot be a single underscore
  @item It cannot start with an underscore followed by an alphanumeric
  @item It cannot contain a dot @code{'.'} except if the entire string
     is @code{"."}

  @end itemize

@item @b{Spec_Suffix} and @b{Specification_Suffix}:
@cindex @code{Spec_Suffix}
@cindex @code{Specification_Suffix}
  For Ada, these attributes give the suffix used in file names that contain
  specifications. For other languages, they give the extension for files
  that contain declaration (header files in C for instance). The attribute
  is indexed on the language.
  The two attributes are equivalent, but the latter is obsolescent.
  If @code{Spec_Suffix ("Ada")} is not specified, then the default is
  @code{"^.ads^.ADS^"}.
  The value must satisfy the following requirements:

  @itemize -
  @item It must not be empty
  @item It cannot start with an alphanumeric character
  @item It cannot start with an underscore followed by an alphanumeric character
  @item It must include at least one dot

  @end itemize

@item @b{Body_Suffix} and @b{Implementation_Suffix}:
@cindex @code{Body_Suffix}
@cindex @code{Implementation_Suffix}
  These attributes give the extension used for file names that contain
  code (bodies in Ada). They are indexed on the language. The second
  version is obsolescent and fully replaced by the first attribute.

  These attributes must satisfy the same requirements as @code{Spec_Suffix}.
  In addition, they must be different from any of the values in
  @code{Spec_Suffix}.
  If @code{Body_Suffix ("Ada")} is not specified, then the default is
  @code{"^.adb^.ADB^"}.

  If @code{Body_Suffix ("Ada")} and @code{Spec_Suffix ("Ada")} end with the
  same string, then a file name that ends with the longest of these two
  suffixes will be a body if the longest suffix is @code{Body_Suffix ("Ada")}
  or a spec if the longest suffix is @code{Spec_Suffix ("Ada")}.

  If the suffix does not start with a '.', a file with a name exactly equal
  to the suffix will also be part of the project (for instance if you define
  the suffix as @code{Makefile}, a file called @file{Makefile} will be part
  of the project. This capability is usually not interesting  when building.
  However, it might become useful when a project is also used to
  find the list of source files in an editor, like the GNAT Programming System
  (GPS).

@item @b{Separate_Suffix}:
@cindex @code{Separate_Suffix}
  This attribute is specific to Ada. It denotes the suffix used in file names
  that contain separate bodies. If it is not specified, then it defaults to
  same value as @code{Body_Suffix ("Ada")}. The same rules apply as for the
  @code{Body_Suffix} attribute. The only accepted index is "Ada".

@item @b{Spec} or @b{Specification}:
@cindex @code{Spec}
@cindex @code{Specification}
  This attribute @code{Spec} can be used to define the source file name for a
  given Ada compilation unit's spec. The index is the literal name of the Ada
  unit (case insensitive). The value is the literal base name of the file that
  contains this unit's spec (case sensitive or insensitive depending on the
  operating system). This attribute allows the definition of exceptions to the
  general naming scheme, in case some files do not follow the usual
  convention.

  When a source file contains several units, the relative position of the unit
  can be indicated. The first unit in the file is at position 1

  @smallexample @c projectfile
   for Spec ("MyPack.MyChild") use "mypack.mychild.spec";
   for Spec ("top") use "foo.a" at 1;
   for Spec ("foo") use "foo.a" at 2;
  @end smallexample

@item @b{Body} or @b{Implementation}:
@cindex @code{Body}
@cindex @code{Implementation}
  These attribute play the same role as @emph{Spec} for Ada bodies.

@item @b{Specification_Exceptions} and @b{Implementation_Exceptions}:
@cindex @code{Specification_Exceptions}
@cindex @code{Implementation_Exceptions}
  These attributes define exceptions to the naming scheme for languages
  other than Ada. They are indexed on the language name, and contain
  a list of file names respectively for headers and source code.

@end table

@ifclear vms
For example, the following package models the Apex file naming rules:

@smallexample @c projectfile
@group
  package Naming is
    for Casing               use "lowercase";
    for Dot_Replacement      use ".";
    for Spec_Suffix ("Ada")  use ".1.ada";
    for Body_Suffix ("Ada")  use ".2.ada";
  end Naming;
@end group
@end smallexample
@end ifclear

@ifset vms
For example, the following package models the DEC Ada file naming rules:

@smallexample @c projectfile
@group
  package Naming is
    for Casing               use "lowercase";
    for Dot_Replacement      use "__";
    for Spec_Suffix ("Ada")  use "_.ada";
    for Body_Suffix ("Ada")  use ".ada";
  end Naming;
@end group
@end smallexample

@noindent
(Note that @code{Casing} is @code{"lowercase"} because GNAT gets the file
names in lower case)
@end ifset

@c ---------------------------------------------
@node Organizing Projects into Subsystems
@section Organizing Projects into Subsystems
@c ---------------------------------------------

@noindent
A @b{subsystem} is a coherent part of the complete system to be built. It is
represented by a set of sources and one single object directory. A system can
be composed of a single subsystem when it is simple as we have seen in the
first section. Complex systems are usually composed of several interdependent
subsystems. A subsystem is dependent on another subsystem if knowledge of the
other one is required to build it, and in particular if visibility on some of
the sources of this other subsystem is required. Each subsystem is usually
represented by its own project file.

In this section, the previous example is being extended. Let's assume some
sources of our @code{Build} project depend on other sources.
For instance, when building a graphical interface, it is usual to depend upon
a graphical library toolkit such as GtkAda. Furthermore, we also need
sources from a logging module we had previously written.

@menu
* Project Dependencies::
* Cyclic Project Dependencies::
* Sharing Between Projects::
* Global Attributes::
@end menu

@c ---------------------------------------------
@node Project Dependencies
@subsection Project Dependencies
@c ---------------------------------------------

@noindent
GtkAda comes with its own project file (appropriately called
@file{gtkada.gpr}), and we will assume we have already built a project
called @file{logging.gpr} for the logging module. With the information provided
so far in @file{build.gpr}, building the application would fail with an error
indicating that the gtkada and logging units that are relied upon by the sources
of this project cannot be found.

This is easily solved by adding the following @b{with} clauses at the beginning
of our project:

@smallexample @c projectfile
  with "gtkada.gpr";
  with "a/b/logging.gpr";
  project Build is
     ...  --  as before
  end Build;
@end smallexample

@noindent
@cindex @code{Externally_Built}
When such a project is compiled, @command{gnatmake} will automatically
check the other projects and recompile their sources when needed. It will also
recompile the sources from @code{Build} when needed, and finally create the
executable. In some cases, the implementation units needed to recompile a
project are not available, or come from some third-party and you do not want to
recompile it yourself. In this case, the attribute @b{Externally_Built} to
"true" can be set, indicating to the builder that this project can be assumed
to be up-to-date, and should not be considered for recompilation. In Ada, if
the sources of this externally built project were compiled with another version
of the compiler or with incompatible options, the binder will issue an error.

The project's @code{with} clause has several effects. It provides source
visibility between projects during the compilation process. It also guarantees
that the necessary object files from @code{Logging} and @code{GtkAda} are
available when linking @code{Build}.

As can be seen in this example, the syntax for importing projects is similar
to the syntax for importing compilation units in Ada. However, project files
use literal strings instead of names, and the @code{with} clause identifies
project files rather than packages.

Each literal string after @code{with} is the path
(absolute or relative) to a project file. The @code{.gpr} extension is
optional, although we recommend adding it. If no extension is specified,
and no project file with the @file{^.gpr^.GPR^} extension is found, then
the file is searched for exactly as written in the @code{with} clause,
that is with no extension.

As mentioned above, the path after a @code{with} has to be a literal
string, and you cannot use concatenation, or lookup the value of external
variables to change the directories from which a project is loaded.
A solution if you need something like this is to use aggregate projects
(@pxref{Aggregate Projects}).

@cindex project path
When a relative path or a base name is used, the
project files are searched relative to each of the directories in the
@b{project path}. This path includes all the directories found with the
following algorithm, in that order, as soon as a matching file is found,
the search stops:

@itemize @bullet
@item First, the file is searched relative to the directory that contains the
  current project file.

@item
@cindex @code{ADA_PROJECT_PATH}
@cindex @code{GPR_PROJECT_PATH}
  Then it is searched relative to all the directories specified in the
  ^environment variables^logical names^ @b{GPR_PROJECT_PATH} and
  @b{ADA_PROJECT_PATH} (in that order) if they exist. The former is
  recommended, the latter is kept for backward compatibility.

@item Finally, it is searched relative to the default project directories.
  Such directories depends on the tool used. The different locations searched
  in the specified order are:

  @itemize @bullet
  @item @file{<prefix>/<target>/lib/gnat}
  (for @command{gnatmake} in all cases, and for @command{gprbuild} if option
  @option{--target} is specified)
  @item @file{<prefix>/share/gpr/}
  (for @command{gnatmake} and @command{gprbuild})
  @item @file{<prefix>/lib/gnat/}
  (for @command{gnatmake} and @command{gprbuild})
  @end itemize

  In our example, @file{gtkada.gpr} is found in the predefined directory if
  it was installed at the same root as GNAT.
@end itemize

@noindent
Some tools also support extending the project path from the command line,
generally through the @option{-aP}. You can see the value of the project
path by using the @command{gnatls -v} command.

Any symbolic link will be fully resolved in the directory of the
importing project file before the imported project file is examined.

Any source file in the imported project can be used by the sources of the
importing project, transitively.
Thus if @code{A} imports @code{B}, which imports @code{C}, the sources of
@code{A} may depend on the sources of @code{C}, even if @code{A} does not
import @code{C} explicitly. However, this is not recommended, because if
and when @code{B} ceases to import @code{C}, some sources in @code{A} will
no longer compile. @command{gprbuild} has a switch @option{--no-indirect-imports}
that will report such indirect dependencies.

One very important aspect of a project hierarchy is that
@b{a given source can only belong to one project} (otherwise the project manager
would not know which settings apply to it and when to recompile it). It means
that different project files do not usually share source directories or
when they do, they need to specify precisely which project owns which sources
using attribute @code{Source_Files} or equivalent. By contrast, 2 projects
can each own a source with the same base file name as long as they live in
different directories. The latter is not true for Ada Sources because of the
correlation between source files and Ada units.

@c ---------------------------------------------
@node Cyclic Project Dependencies
@subsection Cyclic Project Dependencies
@c ---------------------------------------------

@noindent
Cyclic dependencies are mostly forbidden:
if @code{A} imports @code{B} (directly or indirectly) then @code{B}
is not allowed to import @code{A}. However, there are cases when cyclic
dependencies would be beneficial. For these cases, another form of import
between projects exists: the @b{limited with}.  A project @code{A} that
imports a project @code{B} with a straight @code{with} may also be imported,
directly or indirectly, by @code{B} through a @code{limited with}.

The difference between straight @code{with} and @code{limited with} is that
the name of a project imported with a @code{limited with} cannot be used in the
project importing it. In particular, its packages cannot be renamed and
its variables cannot be referred to.

@smallexample @c 0projectfile
with "b.gpr";
with "c.gpr";
project A is
    For Exec_Dir use B'Exec_Dir; -- ok
end A;

limited with "a.gpr";   --  Cyclic dependency: A -> B -> A
project B is
   For Exec_Dir use A'Exec_Dir; -- not ok
end B;

with "d.gpr";
project C is
end C;

limited with "a.gpr";  --  Cyclic dependency: A -> C -> D -> A
project D is
   For Exec_Dir use A'Exec_Dir; -- not ok
end D;
@end smallexample

@c ---------------------------------------------
@node Sharing Between Projects
@subsection Sharing Between Projects
@c ---------------------------------------------

@noindent
When building an application, it is common to have similar needs in several of
the projects corresponding to the subsystems under construction. For instance,
they will all have the same compilation switches.

As seen before (@pxref{Tools Options in Project Files}), setting compilation
switches for all sources of a subsystem is simple: it is just a matter of
adding a @code{Compiler.Default_Switches} attribute to each project files with
the same value. Of course, that means duplication of data, and both places need
to be changed in order to recompile the whole application with different
switches. It can become a real problem if there are many subsystems and thus
many project files to edit.

There are two main approaches to avoiding this duplication:

@itemize @bullet
@item Since @file{build.gpr} imports @file{logging.gpr}, we could change it
  to reference the attribute in Logging, either through a package renaming,
  or by referencing the attribute. The following example shows both cases:

  @smallexample @c projectfile
  project Logging is
     package Compiler is
        for Switches ("Ada") use ("-O2");
     end Compiler;
     package Binder is
        for Switches ("Ada") use ("-E");
     end Binder;
  end Logging;

  with "logging.gpr";
  project Build is
     package Compiler renames Logging.Compiler;
     package Binder is
        for Switches ("Ada") use Logging.Binder'Switches ("Ada");
     end Binder;
  end Build;
  @end smallexample

  @noindent
  The solution used for @code{Compiler} gets the same value for all
  attributes of the package, but you cannot modify anything from the
  package (adding extra switches or some exceptions). The second
  version is more flexible, but more verbose.

  If you need to refer to the value of a variable in an imported
  project, rather than an attribute, the syntax is similar but uses
  a "." rather than an apostrophe. For instance:

  @smallexample @c projectfile
  with "imported";
  project Main is
     Var1 := Imported.Var;
  end Main;
  @end smallexample

@item The second approach is to define the switches in a third project.
  That project is setup without any sources (so that, as opposed to
  the first example, none of the project plays a special role), and
  will only be used to define the attributes. Such a project is
  typically called @file{shared.gpr}.

  @smallexample @c projectfile
  abstract project Shared is
     for Source_Files use ();   --  no project
     package Compiler is
        for Switches ("Ada") use ("-O2");
     end Compiler;
  end Shared;

  with "shared.gpr";
  project Logging is
     package Compiler renames Shared.Compiler;
  end Logging;

  with "shared.gpr";
  project Build is
     package Compiler renames Shared.Compiler;
  end Build;
  @end smallexample

  @noindent
  As for the first example, we could have chosen to set the attributes
  one by one rather than to rename a package. The reason we explicitly
  indicate that @code{Shared} has no sources is so that it can be created
  in any directory and we are sure it shares no sources with @code{Build}
  or @code{Logging}, which of course would be invalid.

@cindex project qualifier
  Note the additional use of the @b{abstract} qualifier in @file{shared.gpr}.
  This qualifier is optional, but helps convey the message that we do not
  intend this project to have sources (@pxref{Qualified Projects} for
  more qualifiers).
@end itemize

@c ---------------------------------------------
@node Global Attributes
@subsection Global Attributes
@c ---------------------------------------------

@noindent
We have already seen many examples of attributes used to specify a special
option of one of the tools involved in the build process. Most of those
attributes are project specific. That it to say, they only affect the invocation
of tools on the sources of the project where they are defined.

There are a few additional attributes that apply to all projects in a
hierarchy as long as they are defined on the "main" project.
The main project is the project explicitly mentioned on the command-line.
The project hierarchy is the "with"-closure of the main project.

Here is a list of commonly used global attributes:

@table @asis
@item @b{Builder.Global_Configuration_Pragmas}:
@cindex @code{Global_Configuration_Pragmas}
  This attribute points to a file that contains configuration pragmas
  to use when building executables. These pragmas apply for all
  executables build from this project hierarchy. As we have seen before,
  additional pragmas can be specified on a per-project basis by setting the
  @code{Compiler.Local_Configuration_Pragmas} attribute.

@item @b{Builder.Global_Compilation_Switches}:
@cindex @code{Global_Compilation_Switches}
  This attribute is a list of compiler switches to use when compiling any
  source file in the project hierarchy. These switches are used in addition
  to the ones defined in the @code{Compiler} package, which only apply to
  the sources of the corresponding project. This attribute is indexed on
  the name of the language.

@end table

Using such global capabilities is convenient. It can also lead to unexpected
behavior. Especially when several subsystems are shared among different main
projects and the different global attributes are not
compatible. Note that using aggregate projects can be a safer and more powerful
replacement to global attributes.

@c ---------------------------------------------
@node Scenarios in Projects
@section Scenarios in Projects
@c ---------------------------------------------

@noindent
Various aspects of the projects can be modified based on @b{scenarios}. These
are user-defined modes that change the behavior of a project. Typical
examples are the setup of platform-specific compiler options, or the use of
a debug and a release mode (the former would activate the generation of debug
information, when the second will focus on improving code optimization).

Let's enhance our example to support a debug and a release modes.The issue is to
let the user choose what kind of system he is building:
use @option{-g} as compiler switches in debug mode and @option{-O2}
in release mode. We will also setup the projects so that we do not share the
same object directory in both modes, otherwise switching from one to the other
might trigger more recompilations than needed or mix objects from the 2 modes.

One naive approach is to create two different project files, say
@file{build_debug.gpr} and @file{build_release.gpr}, that set the appropriate
attributes as explained in previous sections. This solution does not scale well,
because in presence of multiple projects depending on each other,
you will also have to duplicate the complete hierarchy and adapt the project
files to point to the right copies.

@cindex scenarios
Instead, project files support the notion of scenarios controlled
by external values. Such values can come from several sources (in decreasing
order of priority):

@table @asis
@item @b{Command line}:
@cindex @option{-X}
  When launching @command{gnatmake} or @command{gprbuild}, the user can pass
  extra @option{-X} switches to define the external value. In
  our case, the command line might look like

  @smallexample
       gnatmake -Pbuild.gpr -Xmode=debug
   or  gnatmake -Pbuild.gpr -Xmode=release
  @end smallexample

@item @b{^Environment variables^Logical names^}:
  When the external value does not come from the command line, it can come from
  the value of ^environment variables^logical names^ of the appropriate name.
  In our case, if ^an environment variable^a logical name^ called "mode"
  exist, its value will be taken into account.

@item @b{External function second parameter}

@end table

@cindex @code{external}
We now need to get that value in the project. The general form is to use
the predefined function @b{external} which returns the current value of
the external. For instance, we could setup the object directory to point to
either @file{obj/debug} or @file{obj/release} by changing our project to

@smallexample @c projectfile
   project Build is
       for Object_Dir use "obj/" & external ("mode", "debug");
       ... --  as before
   end Build;
@end smallexample

@noindent
The second parameter to @code{external} is optional, and is the default
value to use if "mode" is not set from the command line or the environment.

In order to set the switches according to the different scenarios, other
constructs have to be introduced such as typed variables and case statements.

@cindex typed variable
@cindex case statement
A @b{typed variable} is a variable that
can take only a limited number of values, similar to an enumeration in Ada.
Such a variable can then be used in a @b{case statement} and create conditional
sections in the project. The following example shows how this can be done:

@smallexample @c projectfile
   project Build is
      type Mode_Type is ("debug", "release");  --  all possible values
      Mode : Mode_Type := external ("mode", "debug"); -- a typed variable

      package Compiler is
         case Mode is
            when "debug" =>
               for Switches ("Ada") use ("-g");
            when "release" =>
               for Switches ("Ada") use ("-O2");
         end case;
      end Compiler;
   end Build;
@end smallexample

@noindent
The project has suddenly grown in size, but has become much more flexible.
@code{Mode_Type} defines the only valid values for the @code{mode} variable. If
any other value is read from the environment, an error is reported and the
project is considered as invalid.

The @code{Mode} variable is initialized with an external value
defaulting to @code{"debug"}. This default could be omitted and that would
force the user to define the value. Finally, we can use a case statement to set the
switches depending on the scenario the user has chosen.

Most aspects of the projects can depend on scenarios. The notable exception
are project dependencies (@code{with} clauses), which may not depend on a scenario.

Scenarios work the same way with @b{project hierarchies}: you can either
duplicate a variable similar to @code{Mode} in each of the project (as long
as the first argument to @code{external} is always the same and the type is
the same), or simply set the variable in the @file{shared.gpr} project
(@pxref{Sharing Between Projects}).

@c ---------------------------------------------
@node Library Projects
@section Library Projects
@c ---------------------------------------------

@noindent
So far, we have seen examples of projects that create executables. However,
it is also possible to create libraries instead. A @b{library} is a specific
type of subsystem where, for convenience, objects are grouped together
using system-specific means such as archives or windows DLLs.

Library projects provide a system- and language-independent way of building both @b{static}
and @b{dynamic} libraries. They also support the concept of @b{standalone
libraries} (SAL) which offers two significant properties: the elaboration
(e.g. initialization) of the library is either automatic or very simple;
a change in the
implementation part of the library implies minimal post-compilation actions on
the complete system and potentially no action at all for the rest of the
system in the case of dynamic SALs.

The GNAT Project Manager takes complete care of the library build, rebuild and
installation tasks, including recompilation of the source files for which
objects do not exist or are not up to date, assembly of the library archive, and
installation of the library (i.e., copying associated source, object and
@file{ALI} files to the specified location).

@menu
* Building Libraries::
* Using Library Projects::
* Stand-alone Library Projects::
* Installing a library with project files::
@end menu

@c ---------------------------------------------
@node Building Libraries
@subsection Building Libraries
@c ---------------------------------------------

@noindent
Let's enhance our example and transform the @code{logging} subsystem into a
library.  In order to do so, a few changes need to be made to @file{logging.gpr}.
A number of specific attributes needs to be defined: at least @code{Library_Name}
and @code{Library_Dir}; in addition, a number of other attributes can be used
to specify specific aspects of the library. For readability, it is also
recommended (although not mandatory), to use the qualifier @code{library} in
front of the @code{project} keyword.

@table @asis
@item @b{Library_Name}:
@cindex @code{Library_Name}
  This attribute is the name of the library to be built. There is no
  restriction on the name of a library imposed by the project manager, except
  for stand-alone libraries whose names must follow the syntax of Ada
  identifiers; however, there may be system specific restrictions on the name.
  In general, it is recommended to stick to alphanumeric characters (and
  possibly single underscores) to help portability.

@item @b{Library_Dir}:
@cindex @code{Library_Dir}
  This attribute  is the path (absolute or relative) of the directory where
  the library is to be installed. In the process of building a library,
  the sources are compiled, the object files end up  in the explicit or
  implicit @code{Object_Dir} directory. When all sources of a library
  are compiled, some of the compilation artifacts, including the library itself,
  are copied to the library_dir directory. This directory must exists and be
  writable. It must also be different from the object directory so that cleanup
  activities in the Library_Dir do not affect recompilation needs.

@end table

Here is the new version of @file{logging.gpr} that makes it a library:

@smallexample @c projectfile
library project Logging is          --  "library" is optional
   for Library_Name use "logging";  --  will create "liblogging.a" on Unix
   for Object_Dir   use "obj";
   for Library_Dir  use "lib";      --  different from object_dir
end Logging;
@end smallexample

@noindent
Once the above two attributes are defined, the library project is valid and
is enough for building a library with default characteristics.
Other library-related attributes can be used to change the defaults:

@table @asis
@item @b{Library_Kind}:
@cindex @code{Library_Kind}
  The value of this attribute must be either @code{"static"}, @code{"dynamic"} or
  @code{"relocatable"} (the latter is a synonym for dynamic). It indicates
  which kind of library should be build (the default is to build a
  static library, that is an archive of object files that can potentially
  be linked into a static executable). When the library is set to be dynamic,
  a separate image is created that will be loaded independently, usually
  at the start of the main program execution. Support for dynamic libraries is
  very platform specific, for instance on Windows it takes the form of a DLL
  while on GNU/Linux, it is a dynamic elf image whose suffix is usually
  @file{.so}. Library project files, on the other hand, can be written in
  a platform independent way so that the same project file can be used to build
  a library on different operating systems.

  If you need to build both a static and a dynamic library, it is recommended
  use two different object directories, since in some cases some extra code
  needs to be generated for the latter. For such cases, one can
  either define two different project files, or a single one which uses scenarios
  to indicate at the various kinds of library to be build and their
  corresponding object_dir.

@cindex @code{Library_ALI_Dir}
@item @b{Library_ALI_Dir}:
  This attribute may be specified to indicate the directory where the ALI
  files of the library are installed. By default, they are copied into the
  @code{Library_Dir} directory, but as for the executables where we have a
  separate @code{Exec_Dir} attribute, you might want to put them in a separate
  directory since there can be hundreds of them. The same restrictions as for
  the @code{Library_Dir} attribute apply.

@cindex @code{Library_Version}
@item @b{Library_Version}:
  This attribute is platform dependent, and has no effect on VMS and Windows.
  On Unix, it is used only for dynamic libraries as the internal
  name of the library (the @code{"soname"}). If the library file name (built
  from the @code{Library_Name}) is different from the @code{Library_Version},
  then the library file will be a symbolic link to the actual file whose name
  will be @code{Library_Version}. This follows the usual installation schemes
  for dynamic libraries on many Unix systems.

@smallexample @c projectfile
@group
  project Logging is
     Version := "1";
     for Library_Dir use "lib";
     for Library_Name use "logging";
     for Library_Kind use "dynamic";
     for Library_Version use "liblogging.so." & Version;
  end Logging;
@end group
@end smallexample

  @noindent
  After the compilation, the directory @file{lib} will contain both a
  @file{libdummy.so.1} library and a symbolic link to it called
  @file{libdummy.so}.

@cindex @code{Library_GCC}
@item @b{Library_GCC}:
  This attribute is the name of the tool to use instead of "gcc" to link shared
  libraries. A common use of this attribute is to define a wrapper script that
  accomplishes specific actions before calling gcc (which itself is calling the
  linker to build the library image).

@item @b{Library_Options}:
@cindex @code{Library_Options}
  This attribute may be used to specify additional switches (last switches)
  when linking a shared library.

@item @b{Leading_Library_Options}:
@cindex @code{Leading_Library_Options}
  This attribute, that is taken into account only by @command{gprbuild}, may be
  used to specified leading options (first switches) when linking a shared
  library.

@cindex @code{Linker_Options}
@item @b{Linker.Linker_Options}:
  This attribute specifies additional switches to be given to the linker when
  linking an executable. It is ignored when defined in the main project and
  taken into account in all other projects that are imported directly or
  indirectly. These switches complement the @code{Linker.Switches}
  defined in the main project. This is useful when a particular subsystem
  depends on an external library: adding this dependency as a
  @code{Linker_Options} in the project of the subsystem is more convenient than
  adding it to all the @code{Linker.Switches} of the main projects that depend
  upon this subsystem.
@end table

@c ---------------------------------------------
@node Using Library Projects
@subsection Using Library Projects
@c ---------------------------------------------

@noindent
When the builder detects that a project file is a library project file, it
recompiles all sources of the project that need recompilation and rebuild the
library if any of the sources have been recompiled. It then groups all object
files into a single file, which is a shared or a static library. This library
can later on be linked with multiple executables. Note that the use
of shard libraries reduces the size of the final executable and can also reduce
the memory footprint at execution time when the library is shared among several
executables.

It is also possible to build @b{multi-language libraries}. When using
@command{gprbuild} as a builder, multi-language library projects allow naturally
the creation of multi-language libraries . @command{gnatmake}, does not try to
compile non Ada sources. However, when the project is multi-language, it will
automatically link all object files found in the object directory, whether or
not they were compiled from an Ada source file. This specific behavior does not
apply to Ada-only projects which only take into account the objects
corresponding to the sources of the project.

A non-library project can import a library project. When the builder is invoked
on the former, the library of the latter is only rebuilt when absolutely
necessary. For instance, if a unit of the
library is not up-to-date but non of the executables need this unit, then the
unit is not recompiled and the library is not reassembled.
For instance, let's assume in our example that logging has the following
sources: @file{log1.ads}, @file{log1.adb}, @file{log2.ads} and
@file{log2.adb}. If @file{log1.adb} has been modified, then the library
@file{liblogging} will be rebuilt when compiling all the sources of
@code{Build} only if @file{proc.ads}, @file{pack.ads} or @file{pack.adb}
include a @code{"with Log1"}.

To ensure that all the sources in the @code{Logging} library are
up to date, and that all the sources of @code{Build} are also up to date,
the following two commands needs to be used:

@smallexample
gnatmake -Plogging.gpr
gnatmake -Pbuild.gpr
@end smallexample

@noindent
All @file{ALI} files will also be copied from the object directory to the
library directory. To build executables, @command{gnatmake} will use the
library rather than the individual object files.

@ifclear vms
Library projects can also be useful to describe a library that need to be used
but, for some reason, cannot be rebuilt. For instance, it is the case when some
of the library sources are not available. Such library projects need simply to
use the @code{Externally_Built} attribute as in the example below:

@smallexample @c projectfile
library project Extern_Lib is
   for Languages    use ("Ada", "C");
   for Source_Dirs  use ("lib_src");
   for Library_Dir  use "lib2";
   for Library_Kind use "dynamic";
   for Library_Name use "l2";
   for Externally_Built use "true";  --  <<<<
end Extern_Lib;
@end smallexample

@noindent
In the case of externally built libraries, the @code{Object_Dir}
attribute does not need to be specified because it will never be
used.

The main effect of using such an externally built library project is mostly to
affect the linker command in order to reference the desired library. It can
also be achieved by using @code{Linker.Linker_Options} or @code{Linker.Switches}
in the project corresponding to the subsystem needing this external library.
This latter method is more straightforward in simple cases but when several
subsystems depend upon the same external library, finding the proper place
for the @code{Linker.Linker_Options} might not be easy and if it is
not placed properly, the final link command is likely to present ordering issues.
In such a situation, it is better to use the externally built library project
so that all other subsystems depending on it can declare this dependency thanks
to a project @code{with} clause, which in turn will trigger the builder to find
the proper order of libraries in the final link command.
@end ifclear

@c ---------------------------------------------
@node Stand-alone Library Projects
@subsection Stand-alone Library Projects
@c ---------------------------------------------

@noindent
@cindex standalone libraries
A @b{stand-alone library} is a library that contains the necessary code to
elaborate the Ada units that are included in the library. A stand-alone
library is a convenient way to add an Ada subsystem to a more global system
whose main is not in Ada since it makes the elaboration of the Ada part mostly
transparent. However, stand-alone libraries are also useful when the main is in
Ada: they provide a means for minimizing relinking & redeployment of complex
systems when localized changes are made.

The name of a stand-alone library, specified with attribute
@code{Library_Name}, must have the syntax of an Ada identifier.

The most prominent characteristic of a stand-alone library is that it offers a
distinction between interface units and implementation units. Only the former
are visible to units outside the library. A stand-alone library project is thus
characterised by a third attribute, usually @b{Library_Interface}, in addition
to the two attributes that make a project a Library Project
(@code{Library_Name} and @code{Library_Dir}). This third attribute may also be
@b{Interfaces}. @b{Library_Interface} only works when the interface is in Ada
and takes a list of units as parameter. @b{Interfaces} works for any supported
language and takes a list of sources as parameter.

@table @asis
@item @b{Library_Interface}:
@cindex @code{Library_Interface}
  This attribute defines an explicit subset of the units of the project.
  Projects importing this library project may only "with" units whose sources
  are listed in the @code{Library_Interface}. Other sources are considered
  implementation units.

@smallexample @c projectfile
@group
     for Library_Dir use "lib";
     for Library_Name use "loggin";
     for Library_Interface use ("lib1", "lib2");  --  unit names
@end group
@end smallexample

@item @b{Interfaces}
  This attribute defnes an explicit subset of the source files of a project.
  It may be used as a replacement for attribute @code{Library_Interface}. For
  multi-language library projects, it is the only way to make the project a
  Stand-Alone Library project and at the same time to reduce the non Ada
  interfacing sources.

@item @b{Library_Standalone}:
@cindex @code{Library_Standalone}
  This attribute defines the kind of standalone library to
  build. Values are either @code{standard} (the default), @code{no} or
  @code{encapsulated}. When @code{standard} is used the code to elaborate and
  finalize the library is embedded, when @code{encapsulated} is used the
  library can furthermore only depends on static libraries (including
  the GNAT runtime). This attribute can be set to @code{no} to make it clear
  that the library should not be standalone in which case the
  @code{Library_Interface} should not defined.

@smallexample @c projectfile
@group
     for Library_Dir use "lib";
     for Library_Name use "loggin";
     for Library_Interface use ("lib1", "lib2");  --  unit names
     for Library_Standalone use "encapsulated";
@end group
@end smallexample

@end table

In order to include the elaboration code in the stand-alone library, the binder
is invoked on the closure of the library units creating a package whose name
depends on the library name (^b~logging.ads/b^B$LOGGING.ADS/B^ in the example).
This binder-generated package includes @b{initialization} and @b{finalization}
procedures whose names depend on the library name (@code{logginginit} and
@code{loggingfinal} in the example). The object corresponding to this package is
included in the library.

@table @asis
@item @b{Library_Auto_Init}:
@cindex @code{Library_Auto_Init}
  A dynamic stand-alone Library is automatically initialized
  if automatic initialization of Stand-alone Libraries is supported on the
  platform and if attribute @b{Library_Auto_Init} is not specified or
  is specified with the value "true". A static Stand-alone Library is never
  automatically initialized. Specifying "false" for this attribute
  prevent automatic initialization.

  When a non-automatically initialized stand-alone library is used in an
  executable, its initialization procedure must be called before any service of
  the library is used. When the main subprogram is in Ada, it may mean that the
  initialization procedure has to be called during elaboration of another
  package.

@item @b{Library_Dir}:
@cindex @code{Library_Dir}
  For a stand-alone library, only the @file{ALI} files of the interface units
  (those that are listed in attribute @code{Library_Interface}) are copied to
  the library directory. As a consequence, only the interface units may be
  imported from Ada units outside of the library. If other units are imported,
  the binding phase will fail.

@item @b{Binder.Default_Switches}:
  When a stand-alone library is bound, the switches that are specified in
  the attribute @b{Binder.Default_Switches ("Ada")} are
  used in the call to @command{gnatbind}.

@item @b{Library_Src_Dir}:
@cindex @code{Library_Src_Dir}
  This attribute defines the location (absolute or relative to the project
  directory) where the sources of the interface units are copied at
  installation time.
  These sources includes the specs of the interface units along with the closure
  of sources necessary to compile them successfully. That may include bodies and
  subunits, when pragmas @code{Inline} are used, or when there is a generic
  units in the spec. This directory cannot point to the object directory or
  one of the source directories, but it can point to the library directory,
  which is the default value for this attribute.

@item @b{Library_Symbol_Policy}:
@cindex @code{Library_Symbol_Policy}
  This attribute controls the export of symbols and, on some platforms (like
  VMS) that have the notions of major and minor IDs built in the library
  files, it controls the setting of these IDs. It is not supported on all
  platforms (where it will just have no effect). It may have one of the
  following values:

  @itemize -
  @item @code{"autonomous"} or @code{"default"}: exported symbols are not controlled
  @item @code{"compliant"}: if attribute @b{Library_Reference_Symbol_File}
     is not defined, then it is equivalent to policy "autonomous". If there
     are exported symbols in the reference symbol file that are not in the
     object files of the interfaces, the major ID of the library is increased.
     If there are symbols in the object files of the interfaces that are not
     in the reference symbol file, these symbols are put at the end of the list
     in the newly created symbol file and the minor ID is increased.
  @item @code{"controlled"}: the attribute @b{Library_Reference_Symbol_File} must be
     defined. The library will fail to build if the exported symbols in the
     object files of the interfaces do not match exactly the symbol in the
     symbol file.
  @item @code{"restricted"}: The attribute @b{Library_Symbol_File} must be defined.
     The library will fail to build if there are symbols in the symbol file that
     are not in the exported symbols of the object files of the interfaces.
     Additional symbols in the object files are not added to the symbol file.
  @item @code{"direct"}: The attribute @b{Library_Symbol_File} must be defined and
     must designate an existing file in the object directory. This symbol file
     is passed directly to the underlying linker without any symbol processing.

  @end itemize

@item @b{Library_Reference_Symbol_File}
@cindex @code{Library_Reference_Symbol_File}
  This attribute may define the path name of a reference symbol file that is
  read when the symbol policy is either "compliant" or "controlled", on
  platforms that support symbol control, such as VMS, when building a
  stand-alone library. The path may be an absolute path or a path relative
  to the project directory.

@item @b{Library_Symbol_File}
@cindex @code{Library_Symbol_File}
  This attribute may define the name of the symbol file to be created when
  building a stand-alone library when the symbol policy is either "compliant",
  "controlled" or "restricted", on platforms that support symbol control,
  such as VMS. When symbol policy is "direct", then a file with this name
  must exist in the object directory.
@end table

@c ---------------------------------------------
@node Installing a library with project files
@subsection Installing a library with project files
@c ---------------------------------------------

@noindent
When using project files, library installation is part of the library build
process. Thus no further action is needed in order to make use of the
libraries that are built as part of the general application build. A usable
version of the library is installed in the directory specified by the
@code{Library_Dir} attribute of the library project file.

You may want to install a library in a context different from where the library
is built. This situation arises with third party suppliers, who may want
to distribute a library in binary form where the user is not expected to be
able to recompile the library. The simplest option in this case is to provide
a project file slightly different from the one used to build the library, by
using the @code{externally_built} attribute. @ref{Using Library Projects}

@c ---------------------------------------------
@node Project Extension
@section Project Extension
@c ---------------------------------------------

@noindent
During development of a large system, it is sometimes necessary to use
modified versions of some of the source files, without changing the original
sources. This can be achieved through the @b{project extension} facility.

Suppose for instance that our example @code{Build} project is build every night
for the whole team, in some shared directory. A developer usually need to work
on a small part of the system, and might not want to have a copy of all the
sources and all the object files (mostly because that would require too much
disk space, time to recompile everything). He prefers to be able to override
some of the source files in his directory, while taking advantage of all the
object files generated at night.

Another example can be taken from large software systems, where it is common to have
multiple implementations of a common interface; in Ada terms, multiple
versions of a package body for the same spec.  For example, one implementation
might be safe for use in tasking programs, while another might only be used
in sequential applications.  This can be modeled in GNAT using the concept
of @emph{project extension}.  If one project (the ``child'') @emph{extends}
another project (the ``parent'') then by default all source files of the
parent project are inherited by the child, but the child project can
override any of the parent's source files with new versions, and can also
add new files or remove unnecessary ones.
This facility is the project analog of a type extension in
object-oriented programming.  Project hierarchies are permitted (an extending
project may itself be extended), and a project that
extends a project can also import other projects.

A third example is that of using project extensions to provide different
versions of the same system. For instance, assume that a @code{Common}
project is used by two development branches. One of the branches has now
been frozen, and no further change can be done to it or to @code{Common}.
However, the other development branch still needs evolution of @code{Common}.
Project extensions provide a flexible solution to create a new version
of a subsystem while sharing and reusing as much as possible from the original
one.

A project extension inherits implicitly all the sources and objects from the
project it extends. It is possible to create a new version of some of the
sources in one of the additional source dirs of the extending project. Those new
versions hide the original versions. Adding new sources or removing existing
ones is also possible. Here is an example on how to extend the project
@code{Build} from previous examples:

@smallexample @c projectfile
   project Work extends "../bld/build.gpr" is
   end Work;
@end smallexample

@noindent
The project after @b{extends} is the one being extended. As usual, it can be
specified using an absolute path, or a path relative to any of the directories
in the project path (@pxref{Project Dependencies}). This project does not
specify source or object directories, so the default value for these attribute
will be used that is to say the current directory (where project @code{Work} is
placed). We can already compile that project with

@smallexample
   gnatmake -Pwork
@end smallexample

@noindent
If no sources have been placed in the current directory, this command
won't do anything, since this project does not change the
sources it inherited from @code{Build}, therefore all the object files
in @code{Build} and its dependencies are still valid and are reused
automatically.

Suppose we now want to supply an alternate version of @file{pack.adb}
but use the existing versions of @file{pack.ads} and @file{proc.adb}.
We can create the new file Work's current directory  (likely
by copying the one from the @code{Build} project and making changes to
it. If new packages are needed at the same time, we simply create
new files in the source directory of the extending project.

When we recompile, @command{gnatmake} will now automatically recompile
this file (thus creating @file{pack.o} in the current directory) and
any file that depends on it (thus creating @file{proc.o}). Finally, the
executable is also linked locally.

Note that we could have obtained the desired behavior using project import
rather than project inheritance. A @code{base} project would contain the
sources for @file{pack.ads} and @file{proc.adb}, and @code{Work} would
import @code{base} and add @file{pack.adb}. In this scenario,  @code{base}
cannot contain the original version of @file{pack.adb} otherwise there would be
2 versions of the same unit in the closure of the project and this is not
allowed. Generally speaking, it is not recommended to put the spec and the
body of a unit in different projects since this affects their autonomy and
reusability.

In a project file that extends another project, it is possible to
indicate that an inherited source is @b{not part} of the sources of the
extending project. This is necessary sometimes when a package spec has
been overridden and no longer requires a body: in this case, it is
necessary to indicate that the inherited body is not part of the sources
of the project, otherwise there will be a compilation error
when compiling the spec.

@cindex @code{Excluded_Source_Files}
@cindex @code{Excluded_Source_List_File}
For that purpose, the attribute @b{Excluded_Source_Files} is used.
Its value is a list of file names.
It is also possible to use attribute @code{Excluded_Source_List_File}.
Its value is the path of a text file containing one file name per
line.

@smallexample @c @projectfile
project Work extends "../bld/build.gpr" is
   for Source_Files use ("pack.ads");
   --  New spec of Pkg does not need a completion
   for Excluded_Source_Files use ("pack.adb");
end Work;
@end smallexample

@noindent
All packages that are not declared in the extending project are inherited from
the project being extended, with their attributes, with the exception of
@code{Linker'Linker_Options} which is never inherited. In particular, an
extending project retains all the switches specified in the project being
extended.

At the project level, if they are not declared in the extending project, some
attributes are inherited from the project being extended. They are:
@code{Languages}, @code{Main} (for a root non library project) and
@code{Library_Name} (for a project extending a library project)

@menu
* Project Hierarchy Extension::
@end menu

@c ---------------------------------------------
@node Project Hierarchy Extension
@subsection Project Hierarchy Extension
@c ---------------------------------------------

@noindent
One of the fundamental restrictions in project extension is the following:
@b{A project is not allowed to import directly or indirectly at the same time an
extending project and one of its ancestors}.

By means of example, consider the following hierarchy of projects.

@smallexample
   a.gpr  contains package A1
   b.gpr, imports a.gpr and contains B1, which depends on A1
   c.gpr, imports b.gpr and contains C1, which depends on B1
@end smallexample

@noindent
If we want to locally extend the packages @code{A1} and @code{C1}, we need to
create several extending projects:

@smallexample
   a_ext.gpr which extends a.gpr, and overrides A1
   b_ext.gpr which extends b.gpr and imports a_ext.gpr
   c_ext.gpr which extends c.gpr, imports b_ext.gpr and overrides C1
@end smallexample

@noindent
@smallexample @c projectfile
   project A_Ext extends "a.gpr" is
      for Source_Files use ("a1.adb", "a1.ads");
   end A_Ext;

   with "a_ext.gpr";
   project B_Ext extends "b.gpr" is
   end B_Ext;

   with "b_ext.gpr";
   project C_Ext extends "c.gpr" is
      for Source_Files use ("c1.adb");
   end C_Ext;
@end smallexample

@noindent
The extension @file{b_ext.gpr} is required, even though we are not overriding
any of the sources of @file{b.gpr} because otherwise @file{c_expr.gpr} would
import @file{b.gpr} which itself knows nothing about @file{a_ext.gpr}.

@cindex extends all
When extending a large system spanning multiple projects, it is often
inconvenient to extend every project in the hierarchy that is impacted by a
small change introduced in a low layer. In such cases, it is possible to create
an @b{implicit extension} of entire hierarchy using @b{extends all}
relationship.

When the project is extended using @code{extends all} inheritance, all projects
that are imported by it, both directly and indirectly, are considered virtually
extended. That is, the project manager creates implicit projects
that extend every project in the hierarchy; all these implicit projects do not
control sources on their own and use the object directory of
the "extending all" project.

It is possible to explicitly extend one or more projects in the hierarchy
in order to modify the sources. These extending projects must be imported by
the "extending all" project, which will replace the corresponding virtual
projects with the explicit ones.

When building such a project hierarchy extension, the project manager will
ensure that both modified sources and sources in implicit extending projects
that depend on them, are recompiled.

Thus, in our example we could create the following projects instead:

@smallexample
   a_ext.gpr, extends a.gpr and overrides A1
   c_ext.gpr, "extends all" c.gpr, imports a_ext.gpr and overrides C1

@end smallexample

@noindent
@smallexample @c projectfile
   project A_Ext extends "a.gpr" is
      for Source_Files use ("a1.adb", "a1.ads");
   end A_Ext;

   with "a_ext.gpr";
   project C_Ext extends all "c.gpr" is
     for Source_Files use ("c1.adb");
   end C_Ext;
@end smallexample

@noindent
When building project @file{c_ext.gpr}, the entire modified project space is
considered for recompilation, including the sources of @file{b.gpr} that are
impacted by the changes in @code{A1} and @code{C1}.

@c ---------------------------------------------
@node Aggregate Projects
@section Aggregate Projects
@c ---------------------------------------------

@noindent

Aggregate projects are an extension of the project paradigm, and are
meant to solve a few specific use cases that cannot be solved directly
using standard projects. This section will go over a few of these use
cases to try and explain what you can use aggregate projects for.

@menu
* Building all main programs from a single project tree::
* Building a set of projects with a single command::
* Define a build environment::
* Performance improvements in builder::
* Syntax of aggregate projects::
* package Builder in aggregate projects::
@end menu

@c -----------------------------------------------------------
@node Building all main programs from a single project tree
@subsection Building all main programs from a single project tree
@c -----------------------------------------------------------

Most often, an application is organized into modules and submodules,
which are very conveniently represented as a project tree or graph
(the root project A @code{with}s the projects for each modules (say B and C),
which in turn @code{with} projects for submodules.

Very often, modules will build their own executables (for testing
purposes for instance), or libraries (for easier reuse in various
contexts).

However, if you build your project through gnatmake or gprbuild, using
a syntax similar to

@smallexample
   gprbuild -PA.gpr
@end smallexample

this will only rebuild the main programs of project A, not those of the
imported projects B and C. Therefore you have to spawn several
gnatmake commands, one per project, to build all executables.
This is a little inconvenient, but more importantly is inefficient
(since gnatmake needs to do duplicate work to ensure that sources are
up-to-date, and cannot easily compile things in parallel when using
the -j switch).

Also libraries are always rebuild when building a project.

You could therefore define an aggregate project Agg that groups A, B
and C. Then, when you build with

@smallexample
    gprbuild -PAgg.gpr
@end smallexample

this will build all mains from A, B and C.

@smallexample @c projectfile
   aggregate project Agg is
      for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
   end Agg;
@end smallexample

If B or C do not define any main program (through their Main
attribute), all their sources are build. When you do not group them
in the aggregate project, only those sources that are needed by A
will be build.

If you add a main to a project P not already explicitly referenced in the
aggregate project, you will need to add "p.gpr" in the list of project
files for the aggregate project, or the main will not be built when
building the aggregate project.

@c ---------------------------------------------------------
@node Building a set of projects with a single command
@subsection Building a set of projects with a single command
@c ---------------------------------------------------------

One other case is when you have multiple applications and libraries
that are build independently from each other (but they can be build in
parallel). For instance, you have a project tree rooted at A, and
another one (which might share some subprojects) rooted at B.

Using only gprbuild, you could do

@smallexample
  gprbuild -PA.gpr
  gprbuild -PB.gpr
@end smallexample

to build both. But again, gprbuild has to do some duplicate work for
those files that are shared between the two, and cannot truly build
things in parallel efficiently.

If the two projects are really independent, share no sources other
than through a common subproject, and have no source files with a
common basename, you could create a project C that imports A and
B. But these restrictions are often too strong, and one has to build
them independently. An aggregate project does not have these
limitations, and can aggregate two project trees that have common
sources.

@smallexample
Aggregate projects can group projects with duplicate file names
@end smallexample

This scenario is particularly useful in environment like VxWork 653
where the applications running in the multiple partitions can be build
in parallel through a single gprbuild command. This also works nicely
with Annex E.

@smallexample
   Aggregate projects can be used to build multiple partitions
@end smallexample

@c ---------------------------------------------
@node Define a build environment
@subsection Define a build environment
@c ---------------------------------------------

The environment variables at the time you launch gprbuild or gprbuild
will influence the view these tools have of the project (PATH to find
the compiler, ADA_PROJECT_PATH or GPR_PROJECT_PATH to find the
projects, environment variables that are referenced in project files
through the "external" statement,...). Several command line switches
can be used to override those (-X or -aP), but on some systems and
with some projects, this might make the command line too long, and on
all systems often make it hard to read.

An aggregate project can be used to set the environment for all
projects build through that aggregate. One of the nice aspects is that
you can put the aggregate project under configuration management, and
make sure all your user have a consistent environment when
building. The syntax looks like

@smallexample @c projectfile
   aggregate project Agg is
      for Project_Files use ("A.gpr", "B.gpr");
      for Project_Path use ("../dir1", "../dir1/dir2");
      for External ("BUILD") use "PRODUCTION";

      package Builder is
         for Switches ("Ada") use ("-q");
      end Builder;
   end Agg;
@end smallexample

One of the often requested features in projects is to be able to
reference external variables in @code{with} statements, as in

@smallexample @c projectfile
  with external("SETUP") & "path/prj.gpr";   --  ILLEGAL
  project MyProject is
     ...
  end MyProject;
@end smallexample

For various reasons, this isn't authorized. But using aggregate
projects provide an elegant solution. For instance, you could
use a project file like:

@smallexample @c projectfile
aggregate project Agg is
    for Project_Path use (external("SETUP") % "path");
    for Project_Files use ("myproject.gpr");
end Agg;

with "prj.gpr";  --  searched on Agg'Project_Path
project MyProject is
   ...
end MyProject;
@end smallexample

@c --------------------------------------------
@node Performance improvements in builder
@subsection Performance improvements in builder
@c --------------------------------------------

The loading of aggregate projects is optimized in gprbuild and
gnatmake, so that all files are searched for only once on the disk
(thus reducing the number of system calls and contributing to faster
compilation times especially on systems with sources on remote
servers). As part of the loading, gprbuild and gnatmake compute how
and where a source file should be compiled, and even if it is found
several times in the aggregated projects it will be compiled only
once.

Since there is no ambiguity as to which switches should be used, files
can be compiled in parallel (through the usual -j switch) and this can
be done while maximizing the use of CPUs (compared to launching
multiple gprbuild and gnatmake commands in parallel).

@c -------------------------------------
@node Syntax of aggregate projects
@subsection Syntax of aggregate projects
@c -------------------------------------

An aggregate project follows the general syntax of project files. The
recommended extension is still @file{.gpr}. However, a special
@code{aggregate} qualifier must be put before the keyword
@code{project}.

An aggregate project cannot @code{with} any other project (standard or
aggregate), except an abstract project which can be used to share
attribute values. Building other aggregate projects from an aggregate
project is done through the Project_Files attribute (see below).

An aggregate project does not have any source files directly (only
through other standard projects). Therefore a number of the standard
attributes and packages are forbidden in an aggregate project. Here is the
(non exhaustive) list:

@itemize @bullet
@item Languages
@item Source_Files, Source_List_File and other attributes dealing with
  list of sources.
@item Source_Dirs, Exec_Dir and Object_Dir
@item Library_Dir, Library_Name and other library-related attributes
@item Main
@item Roots
@item Externally_Built
@item Inherit_Source_Path
@item Excluded_Source_Dirs
@item Locally_Removed_Files
@item Excluded_Source_Files
@item Excluded_Source_List_File
@item Interfaces
@end itemize

The only package that is authorized (albeit optional) is
Builder. Other packages (in particular Compiler, Binder and Linker)
are forbidden. It is an error to have any of these
(and such an error prevents the proper loading of the aggregate
project).

Three new attributes have been created, which can only be used in the
context of aggregate projects:

@table @asis
@item @b{Project_Files}:
@cindex @code{Project_Files}

This attribute is compulsory (or else we are not aggregating any project,
and thus not doing anything). It specifies a list of @file{.gpr} files
that are grouped in the aggregate. The list may be empty. The project
files can be either other aggregate projects, or standard projects. When
grouping standard projects, you can have both the root of a project tree
(and you do not need to specify all its imported projects), and any project
within the tree.

Basically, the idea is to specify all those projects that have
main programs you want to build and link, or libraries you want to
build. You can even specify projects that do not use the Main
attribute nor the @code{Library_*} attributes, and the result will be to
build all their source files (not just the ones needed by other
projects).

The file can include paths (absolute or relative). Paths are
relative to the location of the aggregate project file itself (if
you use a base name, we expect to find the .gpr file in the same
directory as the aggregate project file). The extension @file{.gpr} is
mandatory, since this attribute contains file names, not project names.

Paths can also include the @code{"*"} and @code{"**"} globbing patterns. The
latter indicates that any subdirectory (recursively) will be
searched for matching files. The latter (@code{"**"}) can only occur at the
last position in the directory part (ie @code{"a/**/*.gpr"} is supported, but
not @code{"**/a/*.gpr"}). Starting the pattern with @code{"**"} is equivalent
to starting with @code{"./**"}.

For now, the pattern @code{"*"} is only allowed in the filename part, not
in the directory part. This is mostly for efficiency reasons to limit the
number of system calls that are needed.

Here are a few valid examples:

@smallexample @c projectfile
    for Project_Files use ("a.gpr", "subdir/b.gpr");
    --  two specific projects relative to the directory of agg.gpr

    for Project_Files use ("**/*.gpr");
    --  all projects recursively
@end smallexample

@item @b{Project_Path}:
@cindex @code{Project_Path}

This attribute can be used to specify a list of directories in
which to look for project files in @code{with} statements.

When you specify a project in Project_Files
say @code{"x/y/a.gpr"}), and this projects imports a project "b.gpr", only
b.gpr is searched in the project path. a.gpr must be exactly at
<dir of the aggregate>/x/y/a.gpr.

This attribute, however, does not affect the search for the aggregated
project files specified with @code{Project_Files}.

Each aggregate project has its own (that is if agg1.gpr includes
agg2.gpr, they can potentially both have a different project path).
This project path is defined as the concatenation, in that order, of
the current directory, followed by the command line -aP switches,
then the directories from the Project_Path attribute, then the
directories from the GPR_PROJECT_PATH and ADA_PROJECT_PATH env.
variables, and finally the predefined directories.

In the example above, agg2.gpr's project path is not influenced by
the attribute agg1'Project_Path, nor is agg1 influenced by
agg2'Project_Path.

This can potentially lead to errors. In the following example:

@smallexample
     +---------------+                  +----------------+
     | Agg1.gpr      |-=--includes--=-->| Agg2.gpr       |
     |  'project_path|                  |  'project_path |
     |               |                  |                |
     +---------------+                  +----------------+
           :                                   :
           includes                        includes
           :                                   :
           v                                   v
       +-------+                          +---------+
       | P.gpr |<---------- withs --------|  Q.gpr  |
       +-------+---------\                +---------+
           |             |
           withs         |
           |             |
           v             v
       +-------+      +---------+
       | R.gpr |      | R'.gpr  |
       +-------+      +---------+
@end smallexample

When looking for p.gpr, both aggregates find the same physical file on
the disk. However, it might happen that with their different project
paths, both aggregate projects would in fact find a different r.gpr.
Since we have a common project (p.gpr) "with"ing two different r.gpr,
this will be reported as an error by the builder.

Directories are relative to the location of the aggregate project file.

Here are a few valid examples:

@smallexample @c projectfile
   for Project_Path use ("/usr/local/gpr", "gpr/");
@end smallexample

@item @b{External}:
@cindex @code{External}

This attribute can be used to set the value of environment
variables as retrieved through the @code{external} statement
in projects. It does not affect the environment variables
themselves (so for instance you cannot use it to change the value
of your PATH as seen from the spawned compiler).

This attribute affects the external values as seen in the rest of
the aggreate projects, and in the aggregated projects.

The exact value of external a variable comes from one of three
sources (each level overrides the previous levels):

@itemize @bullet
@item An External attribute in aggregate project, for instance
    @code{for External ("BUILD_MODE") use "DEBUG"};

@item Environment variables

These override the value given by the attribute, so that
users can override the value set in the (presumably shared
with others in his team) aggregate project.

@item The -X command line switch to gprbuild and gnatmake

This always takes precedence.

@end itemize

This attribute is only taken into account in the main aggregate
project (i.e. the one specified on the command line to gprbuild or
natmake), and ignored in other aggregate projects. It is invalid
in standard projects.
The goal is to have a consistent value in all
projects that are build through the aggregate, which would not
be the case in the diamond case: A groups the aggregate
projects B and C, which both (either directly or indirectly)
build the project P. If B and C could set different values for
the environment variables, we would have two different views of
P, which in particular might impact the list of source files in P.

@end table

@c ----------------------------------------------
@node package Builder in aggregate projects
@subsection package Builder in aggregate projects
@c ----------------------------------------------

As we mentioned before, only the package Builder can be specified in
an aggregate project. In this package, only the following attributes
are valid:

@table @asis
@item @b{Switches}:
@cindex @code{Switches}
This attribute gives the list of switches to use for the builder
(gprbuild or gnatmake), depending on the language of the main file.
For instance,

@smallexample @c projectfile
for Switches ("Ada") use ("-d", "-p");
for Switches ("C")   use ("-p");
@end smallexample

These switches are only read from the main aggregate project (the
one passed on the command line), and ignored in all other aggregate
projects or projects.

It can only contain builder switches, not compiler switches.

@item @b{Global_Compilation_Switches}
@cindex @code{Global_Compilation_Switches}

This attribute gives the list of compiler switches for the various
languages. For instance,

@smallexample @c projectfile
for Global_Compilation_Switches ("Ada") use ("-O1", "-g");
for Global_Compilation_Switches ("C")   use ("-O2");
@end smallexample

This attribute is only taken into account in the aggregate project
specified on the command line, not in other aggregate projects.

In the projects grouped by that aggregate, the attribute
Builder.Global_Compilation_Switches is also ignored. However, the
attribute Compiler.Default_Switches will be taken into account (but
that of the aggregate have higher priority). The attribute
Compiler.Switches is also taken into account and can be used to
override the switches for a specific file. As a result, it always
has priority.

The rules are meant to avoid ambiguities when compiling. For
instance, aggregate project Agg groups the projects A and B, that
both depend on C. Here is an extra for all of these projects:

@smallexample @c projectfile
      aggregate project Agg is
          for Project_Files use ("a.gpr", "b.gpr");
          package Builder is
             for Global_Compilation_Switches ("Ada") use ("-O2");
          end Builder;
      end Agg;

      with "c.gpr";
      project A is
          package Builder is
             for Global_Compilation_Switches ("Ada") use ("-O1");
             --  ignored
          end Builder;

          package Compiler is
             for Default_Switches ("Ada") use ("-O1", "-g");
             for Switches ("a_file1.adb") use ("-O0");
          end Compiler;
      end A;

      with "c.gpr";
      project B is
          package Compiler is
             for Default_Switches ("Ada") use ("-O0");
          end Compiler;
      end B;

      project C is
          package Compiler is
             for Default_Switches ("Ada") use ("-O3", "-gnatn");
             for Switches ("c_file1.adb") use ("-O0", "-g");
          end Compiler;
      end C;
@end smallexample

then the following switches are used:

@itemize @bullet
@item all files from project A except a_file1.adb are compiled
      with "-O2 -g", since the aggregate project has priority.
@item the file a_file1.adb is compiled with
      "-O0", since the Compiler.Switches has priority
@item all files from project B are compiled with
      "-O2", since the aggregate project has priority
@item all files from C are compiled with "-O2 -gnatn", except for
      c_file1.adb which is compiled with "-O0 -g"
@end itemize

Even though C is seen through two paths (through A and through
B), the switches used by the compiler are unambiguous.

@item @b{Global_Configuration_Pragmas}
@cindex @code{Global_Configuration_Pragmas}

This attribute can be used to specify a file containing
configuration pragmas, to be passed to the compiler.  Since we
ignore the package Builder in other aggregate projects and projects,
only those pragmas defined in the main aggregate project will be
taken into account.

Projects can locally add to those by using the
@code{Compiler.Local_Configuration_Pragmas} attribute if they need.

@end table

For projects that are build through the aggregate, the package Builder
is ignored, except for the Executable attribute which specifies the
name of the executables resulting from the link of the main programs, and
for the Executable_Suffix.

@c ---------------------------------------------
@node Aggregate Library Projects
@section Aggregate Library Projects
@c ---------------------------------------------

@noindent

Aggregate library projects make it possible to build a single library
using object files built using other standard or library
projects. This gives the flexibility to describe an application as
having multiple modules (a GUI, database access, ...) using different
project files (so possibly built with different compiler options) and
yet create a single library (static or relocatable) out of the
corresponding object files.

@menu
* Building aggregate library projects::
* Syntax of aggregate library projects::
@end menu

@c ---------------------------------------------
@node Building aggregate library projects
@subsection Building aggregate library projects
@c ---------------------------------------------

For example, we can define an aggregate project Agg that groups A, B
and C:

@smallexample @c projectfile
   aggregate library project Agg is
      for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
      for Library_Name use ("agg");
      for Library_Dir use ("lagg");
   end Agg;
@end smallexample

Then, when you build with:

@smallexample
    gprbuild agg.gpr
@end smallexample

This will build all units from projects A, B and C and will create a
static library named @file{libagg.a} into the @file{lagg}
directory. An aggregate library project has the same set of
restriction as a standard library project.

Note that a shared aggregate library project cannot aggregates a
static library project. In platforms where a compiler option is
required to create relocatable object files, a Builder package in the
aggregate library project may be used:

@smallexample @c projectfile
   aggregate library project Agg is
      for Project_Files use ("a.gpr", "b.gpr", "c.gpr");
      for Library_Name use ("agg");
      for Library_Dir use ("lagg");
      for Library_Kind use "relocatable";

      package Builder is
         for Global_Compilation_Switches ("Ada") use ("-fPIC");
      end Builder;
   end Agg;
@end smallexample

With the above aggregate library Builder package, the @code{-fPIC}
option will be passed to the compiler when building any source code
from projects @file{a.gpr}, @file{b.gpr} and @file{c.gpr}.

@c ---------------------------------------------
@node Syntax of aggregate library projects
@subsection Syntax of aggregate library projects
@c ---------------------------------------------

An aggregate library project follows the general syntax of project
files. The recommended extension is still @file{.gpr}. However, a special
@code{aggregate library} qualifier must be put before the keyword
@code{project}.

An aggregate library project cannot @code{with} any other project
(standard or aggregate), except an abstract project which can be used
to share attribute values.

An aggregate library project does not have any source files directly (only
through other standard projects). Therefore a number of the standard
attributes and packages are forbidden in an aggregate library
project. Here is the (non exhaustive) list:

@itemize @bullet
@item Languages
@item Source_Files, Source_List_File and other attributes dealing with
  list of sources.
@item Source_Dirs, Exec_Dir and Object_Dir
@item Main
@item Roots
@item Externally_Built
@item Inherit_Source_Path
@item Excluded_Source_Dirs
@item Locally_Removed_Files
@item Excluded_Source_Files
@item Excluded_Source_List_File
@item Interfaces
@end itemize

The only package that is authorized (albeit optional) is Builder.

The Project_Files attribute (See @pxref{Aggregate Projects}) is used to
described the aggregated projects whose object files have to be
included into the aggregate library.

@c ---------------------------------------------
@node Project File Reference
@section Project File Reference
@c ---------------------------------------------

@noindent
This section describes the syntactic structure of project files, the various
constructs that can be used. Finally, it ends with a summary of all available
attributes.

@menu
* Project Declaration::
* Qualified Projects::
* Declarations::
* Packages::
* Expressions::
* External Values::
* Typed String Declaration::
* Variables::
* Attributes::
* Case Statements::
@end menu

@c ---------------------------------------------
@node Project Declaration
@subsection Project Declaration
@c ---------------------------------------------

@noindent
Project files have an Ada-like syntax. The minimal project file is:

@smallexample @c projectfile
@group
project Empty is
end Empty;
@end group
@end smallexample

@noindent
The identifier @code{Empty} is the name of the project.
This project name must be present after the reserved
word @code{end} at the end of the project file, followed by a semi-colon.

@b{Identifiers} (i.e.@: the user-defined names such as project or variable names)
have the same syntax as Ada identifiers: they must start with a letter,
and be followed by zero or more letters, digits or underscore characters;
it is also illegal to have two underscores next to each other. Identifiers
are always case-insensitive ("Name" is the same as "name").

@smallexample
simple_name ::= identifier
name        ::= simple_name @{ . simple_name @}
@end smallexample

@noindent
@b{Strings} are used for values of attributes or as indexes for these
attributes. They are in general case sensitive, except when noted
otherwise (in particular, strings representing file names will be case
insensitive on some systems, so that "file.adb" and "File.adb" both
represent the same file).

@b{Reserved words} are the same as for standard Ada 95, and cannot
be used for identifiers. In particular, the following words are currently
used in project files, but others could be added later on. In bold are the
extra reserved words in project files: @code{all, at, case, end, for, is,
limited, null, others, package, renames, type, use, when, with, @b{extends},
@b{external}, @b{project}}.

@b{Comments} in project files have the same syntax as in Ada, two consecutive
hyphens through the end of the line.

A project may be an @b{independent project}, entirely defined by a single
project file. Any source file in an independent project depends only
on the predefined library and other source files in the same project.
But a project may also depend on other projects, either by importing them
through @b{with clauses}, or by @b{extending} at most one other project. Both
types of dependency can be used in the same project.

A path name denotes a project file. It can be absolute or relative.
An absolute path name includes a sequence of directories, in the syntax of
the host operating system, that identifies uniquely the project file in the
file system. A relative path name identifies the project file, relative
to the directory that contains the current project, or relative to a
directory listed in the environment variables ADA_PROJECT_PATH and
GPR_PROJECT_PATH. Path names are case sensitive if file names in the host
operating system are case sensitive. As a special case, the directory
separator can always be "/" even on Windows systems, so that project files
can be made portable across architectures.
The syntax of the environment variable ADA_PROJECT_PATH and
GPR_PROJECT_PATH is a list of directory names separated by colons on UNIX and
semicolons on Windows.

A given project name can appear only once in a context clause.

It is illegal for a project imported by a context clause to refer, directly
or indirectly, to the project in which this context clause appears (the
dependency graph cannot contain cycles), except when one of the with clause
in the cycle is a @b{limited with}.
@c ??? Need more details here

@smallexample @c projectfile
with "other_project.gpr";
project My_Project extends "extended.gpr" is
end My_Project;
@end smallexample

@noindent
These dependencies form a @b{directed graph}, potentially cyclic when using
@b{limited with}. The subprogram reflecting the @b{extends} relations is a
tree.

A project's @b{immediate sources} are the source files directly defined by
that project, either implicitly by residing in the project source directories,
or explicitly through any of the source-related attributes.
More generally, a project sources are the immediate sources of the project
together with the immediate sources (unless overridden) of any
project on which it depends directly or indirectly.

A @b{project hierarchy} can be created, where projects are children of
other projects. The name of such a child project must be @code{Parent.Child},
where @code{Parent} is the name of the parent project. In particular, this
makes all @code{with} clauses of the parent project automatically visible
in the child project.

@smallexample
project        ::= context_clause project_declaration

context_clause ::= @{with_clause@}
with_clause    ::= @i{with} path_name @{ , path_name @} ;
path_name      ::= string_literal

project_declaration ::= simple_project_declaration | project_extension
simple_project_declaration ::=
  @i{project} @i{<project_>}name @i{is}
    @{declarative_item@}
  @i{end} <project_>simple_name;
@end smallexample

@c ---------------------------------------------
@node Qualified Projects
@subsection Qualified Projects
@c ---------------------------------------------

@noindent
Before the reserved @code{project}, there may be one or two @b{qualifiers}, that
is identifiers or reserved words, to qualify the project.
The current list of qualifiers is:

@table @asis
@item @b{abstract}: qualifies a project with no sources. Such a
  project must either have no declaration of attributes @code{Source_Dirs},
  @code{Source_Files}, @code{Languages} or @code{Source_List_File}, or one of
  @code{Source_Dirs}, @code{Source_Files}, or @code{Languages} must be declared
  as empty. If it extends another project, the project it extends must also be a
  qualified abstract project.
@item @b{standard}: a standard project is a non library project with sources.
  This is the default (implicit) qualifier.
@item @b{aggregate}: a project whose sources are aggregated from other
project files.
@item @b{aggregate library}: a library whose sources are aggregated
from other project or library project files.
@item @b{library}: a library project must declare both attributes
  @code{Library_Name} and @code{Library_Dir}.
@item @b{configuration}: a configuration project cannot be in a project tree.
  It describes compilers and other tools to @code{gprbuild}.
@end table

@c ---------------------------------------------
@node Declarations
@subsection Declarations
@c ---------------------------------------------

@noindent
Declarations introduce new entities that denote types, variables, attributes,
and packages. Some declarations can only appear immediately within a project
declaration. Others can appear within a project or within a package.

@smallexample
declarative_item ::= simple_declarative_item
  | typed_string_declaration
  | package_declaration

simple_declarative_item ::= variable_declaration
  | typed_variable_declaration
  | attribute_declaration
  | case_construction
  | empty_declaration

empty_declaration ::= @i{null} ;
@end smallexample

@noindent
An empty declaration is allowed anywhere a declaration is allowed. It has
no effect.

@c ---------------------------------------------
@node Packages
@subsection Packages
@c ---------------------------------------------

@noindent
A project file may contain @b{packages}, that group attributes (typically
all the attributes that are used by one of the GNAT tools).

A package with a given name may only appear once in a project file.
The following packages are currently supported in project files
(See @pxref{Attributes} for the list of attributes that each can contain).

@table @code
@item Binder
  This package specifies characteristics useful when invoking the binder either
  directly via the @command{gnat} driver or when using a builder such as
  @command{gnatmake} or @command{gprbuild}. @xref{Main Subprograms}.
@item Builder
  This package specifies the compilation options used when building an
  executable or a library for a project. Most of the options should be
  set in one of @code{Compiler}, @code{Binder} or @code{Linker} packages,
  but there are some general options that should be defined in this
  package. @xref{Main Subprograms}, and @pxref{Executable File Names} in
  particular.
@item Check
  This package specifies the options used when calling the checking tool
  @command{gnatcheck} via the @command{gnat} driver. Its attribute
  @b{Default_Switches} has the same semantics as for the package
  @code{Builder}. The first string should always be @code{-rules} to specify
  that all the other options belong to the @code{-rules} section of the
  parameters to @command{gnatcheck}.
@item Compiler
  This package specifies the compilation options used by the compiler for
  each languages. @xref{Tools Options in Project Files}.
@item Cross_Reference
  This package specifies the options used when calling the library tool
  @command{gnatxref} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Eliminate
  This package specifies the options used when calling the tool
  @command{gnatelim} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Finder
  This package specifies the options used when calling the search tool
  @command{gnatfind} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Gnatls
  This package the options to use when invoking @command{gnatls} via the
  @command{gnat} driver.
@item Gnatstub
  This package specifies the options used when calling the tool
  @command{gnatstub} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item IDE
  This package specifies the options used when starting an integrated
  development environment, for instance @command{GPS} or @command{Gnatbench}.
  @xref{The Development Environments}.
@item Linker
  This package specifies the options used by the linker.
  @xref{Main Subprograms}.
@item Makefile
@cindex Makefile package in projects
  This package is used by the GPS plugin Makefile.py. See the documentation
  in that plugin (from GPS: /Tools/Plug-ins).
@item Metrics
  This package specifies the options used when calling the tool
  @command{gnatmetric} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Naming
  This package specifies the naming conventions that apply
  to the source files in a project. In particular, these conventions are
  used to automatically find all source files in the source directories,
  or given a file name to find out its language for proper processing.
  @xref{Naming Schemes}.
@item Pretty_Printer
  This package specifies the options used when calling the formatting tool
  @command{gnatpp} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Stack
  This package specifies the options used when calling the tool
  @command{gnatstack} via the @command{gnat} driver. Its attributes
  @b{Default_Switches} and @b{Switches} have the same semantics as for the
  package @code{Builder}.
@item Synchronize
  This package specifies the options used when calling the tool
  @command{gnatsync} via the @command{gnat} driver.

@end table

In its simplest form, a package may be empty:

@smallexample @c projectfile
@group
project Simple is
  package Builder is
  end Builder;
end Simple;
@end group
@end smallexample

@noindent
A package may contain @b{attribute declarations},
@b{variable declarations} and @b{case constructions}, as will be
described below.

When there is ambiguity between a project name and a package name,
the name always designates the project. To avoid possible confusion, it is
always a good idea to avoid naming a project with one of the
names allowed for packages or any name that starts with @code{gnat}.

A package can also be defined by a @b{renaming declaration}. The new package
renames a package declared in a different project file, and has the same
attributes as the package it renames. The name of the renamed package
must be the same as the name of the renaming package. The project must
contain a package declaration with this name, and the project
must appear in the context clause of the current project, or be its parent
project. It is not possible to add or override attributes to the renaming
project. If you need to do so, you should use an @b{extending declaration}
(see below).

Packages that are renamed in other project files often come from project files
that have no sources: they are just used as templates. Any modification in the
template will be reflected automatically in all the project files that rename
a package from the template. This is a very common way to share settings
between projects.

Finally, a package can also be defined by an @b{extending declaration}. This is
similar to a @b{renaming declaration}, except that it is possible to add or
override attributes.

@smallexample
package_declaration ::= package_spec | package_renaming | package_extension
package_spec ::=
  @i{package} @i{<package_>}simple_name @i{is}
    @{simple_declarative_item@}
  @i{end} package_identifier ;
package_renaming ::==
  @i{package} @i{<package_>}simple_name @i{renames} @i{<project_>}simple_name.package_identifier ;
package_extension ::==
  @i{package} @i{<package_>}simple_name @i{extends} @i{<project_>}simple_name.package_identifier @i{is}
    @{simple_declarative_item@}
  @i{end} package_identifier ;
@end smallexample

@c ---------------------------------------------
@node Expressions
@subsection Expressions
@c ---------------------------------------------

@noindent
An expression is any value that can be assigned to an attribute or a
variable. It is either a literal value, or a construct requiring runtime
computation by the project manager. In a project file, the computed value of
an expression is either a string or a list of strings.

A string value is one of:
@itemize @bullet
@item A literal string, for instance @code{"comm/my_proj.gpr"}
@item The name of a variable that evaluates to a string (@pxref{Variables})
@item The name of an attribute that evaluates to a string (@pxref{Attributes})
@item An external reference (@pxref{External Values})
@item A concatenation of the above, as in @code{"prefix_" & Var}.

@end itemize

@noindent
A list of strings is one of the following:

@itemize @bullet
@item A parenthesized comma-separated list of zero or more string expressions, for
  instance @code{(File_Name, "gnat.adc", File_Name & ".orig")} or @code{()}.
@item The name of a variable that evaluates to a list of strings
@item The name of an attribute that evaluates to a list of strings
@item A concatenation of a list of strings and a string (as defined above), for
  instance @code{("A", "B") & "C"}
@item A concatenation of two lists of strings

@end itemize

@noindent
The following is the grammar for expressions

@smallexample
string_literal ::= "@{string_element@}"  --  Same as Ada
string_expression ::= string_literal
    | @i{variable_}name
    | external_value
    | attribute_reference
    | ( string_expression @{ & string_expression @} )
string_list  ::= ( string_expression @{ , string_expression @} )
   | @i{string_variable}_name
   | @i{string_}attribute_reference
term ::= string_expression | string_list
expression ::= term @{ & term @}     --  Concatenation
@end smallexample

@noindent
Concatenation involves strings and list of strings. As soon as a list of
strings is involved, the result of the concatenation is a list of strings. The
following Ada declarations show the existing operators:

@smallexample @c ada
  function "&" (X : String;      Y : String)      return String;
  function "&" (X : String_List; Y : String)      return String_List;
  function "&" (X : String_List; Y : String_List) return String_List;
@end smallexample

@noindent
Here are some specific examples:

@smallexample @c projectfile
@group
   List := () & File_Name; --  One string in this list
   List2 := List & (File_Name & ".orig"); -- Two strings
   Big_List := List & Lists2;  --  Three strings
   Illegal := "gnat.adc" & List2;  --  Illegal, must start with list
@end group
@end smallexample

@c ---------------------------------------------
@node External Values
@subsection External Values
@c ---------------------------------------------

@noindent
An external value is an expression whose value is obtained from the command
that invoked the processing of the current project file (typically a
gnatmake or gprbuild command).

There are two kinds of external values, one that returns a single string, and
one that returns a string list.

The syntax of a single string external value is:

@smallexample
external_value ::= @i{external} ( string_literal [, string_literal] )
@end smallexample

@noindent
The first string_literal is the string to be used on the command line or
in the environment to specify the external value. The second string_literal,
if present, is the default to use if there is no specification for this
external value either on the command line or in the environment.

Typically, the external value will either exist in the
^environment variables^logical name^
or be specified on the command line through the
@option{^-X^/EXTERNAL_REFERENCE=^@emph{vbl}=@emph{value}} switch. If both
are specified, then the command line value is used, so that a user can more
easily override the value.

The function @code{external} always returns a string. It is an error if the
value was not found in the environment and no default was specified in the
call to @code{external}.

An external reference may be part of a string expression or of a string
list expression, and can therefore appear in a variable declaration or
an attribute declaration.

Most of the time, this construct is used to initialize typed variables, which
are then used in @b{case} statements to control the value assigned to
attributes in various scenarios. Thus such variables are often called
@b{scenario variables}.

The syntax for a string list external value is:

@smallexample
external_value ::= @i{external_as_list} ( string_literal , string_literal )
@end smallexample

@noindent
The first string_literal is the string to be used on the command line or
in the environment to specify the external value. The second string_literal is
the separator between each component of the string list.

If the external value does not exist in the environment or on the command line,
the result is an empty list. This is also the case, if the separator is an
empty string or if the external value is only one separator.

Any separator at the beginning or at the end of the external value is
discarded. Then, if there is no separator in the external value, the result is
a string list with only one string. Otherwise, any string between the beginning
and the first separator, between two consecutive separators and between the
last separator and the end are components of the string list.

@smallexample
   @i{external_as_list} ("SWITCHES", ",")
@end smallexample

@noindent
If the external value is "-O2,-g", the result is ("-O2", "-g").

If the external value is ",-O2,-g,", the result is also ("-O2", "-g").

if the external value is "-gnav", the result is ("-gnatv").

If the external value is ",,", the result is ("").

If the external value is ",", the result is (), the empty string list.

@c ---------------------------------------------
@node Typed String Declaration
@subsection Typed String Declaration
@c ---------------------------------------------

@noindent
A @b{type declaration} introduces a discrete set of string literals.
If a string variable is declared to have this type, its value
is restricted to the given set of literals. These are the only named
types in project files. A string type may only be declared at the project
level, not inside a package.

@smallexample
typed_string_declaration ::=
  @i{type} @i{<typed_string_>}_simple_name @i{is} ( string_literal @{, string_literal@} );
@end smallexample

@noindent
The string literals in the list are case sensitive and must all be different.
They may include any graphic characters allowed in Ada, including spaces.
Here is an example of a string type declaration:

@smallexample @c projectfile
   type OS is ("NT", "nt", "Unix", "GNU/Linux", "other OS");
@end smallexample

@noindent
Variables of a string type are called @b{typed variables}; all other
variables are called @b{untyped variables}. Typed variables are
particularly useful in @code{case} constructions, to support conditional
attribute declarations. (@pxref{Case Statements}).

A string type may be referenced by its name if it has been declared in the same
project file, or by an expanded name whose prefix is the name of the project
in which it is declared.

@c ---------------------------------------------
@node Variables
@subsection Variables
@c ---------------------------------------------

@noindent
@b{Variables} store values (strings or list of strings) and can appear
as part of an expression. The declaration of a variable creates the
variable and assigns the value of the expression to it. The name of the
variable is available immediately after the assignment symbol, if you
need to reuse its old value to compute the new value. Before the completion
of its first declaration, the value of a variable defaults to the empty
string ("").

A @b{typed} variable can be used as part of a @b{case} expression to
compute the value, but it can only be declared once in the project file,
so that all case statements see the same value for the variable. This
provides more consistency and makes the project easier to understand.
The syntax for its declaration is identical to the Ada syntax for an
object declaration. In effect, a typed variable acts as a constant.

An @b{untyped} variable can be declared and overridden multiple times
within the same project. It is declared implicitly through an Ada
assignment. The first declaration establishes the kind of the variable
(string or list of strings) and successive declarations must respect
the initial kind. Assignments are executed in the order in which they
appear, so the new value replaces the old one and any subsequent reference
to the variable uses the new value.

A variable may be declared at the project file level, or within a package.

@smallexample
typed_variable_declaration ::=
  @i{<typed_variable_>}simple_name : @i{<typed_string_>}name := string_expression;
variable_declaration ::= @i{<variable_>}simple_name := expression;
@end smallexample

@noindent
Here are some examples of variable declarations:

@smallexample @c projectfile
@group
   This_OS : OS := external ("OS"); --  a typed variable declaration
   That_OS := "GNU/Linux";          --  an untyped variable declaration

   Name      := "readme.txt";
   Save_Name := Name & ".saved";

   Empty_List := ();
   List_With_One_Element := ("-gnaty");
   List_With_Two_Elements := List_With_One_Element & "-gnatg";
   Long_List := ("main.ada", "pack1_.ada", "pack1.ada", "pack2_.ada");
@end group
@end smallexample

@noindent
A @b{variable reference} may take several forms:

@itemize @bullet
@item The simple variable name, for a variable in the current package (if any)
  or in the current project
@item An expanded name, whose prefix is a context name.

@end itemize

@noindent
A @b{context} may be one of the following:

@itemize @bullet
@item The name of an existing package in the current project
@item The name of an imported project of the current project
@item The name of an ancestor project (i.e., a project extended by the current
  project, either directly or indirectly)
@item An expanded name whose prefix is an imported/parent project name, and
  whose selector is a package name in that project.
@end itemize

@c ---------------------------------------------
@node Attributes
@subsection Attributes
@c ---------------------------------------------

@noindent
A project (and its packages) may have @b{attributes} that define
the project's properties.  Some attributes have values that are strings;
others have values that are string lists.

@smallexample
attribute_declaration ::=
   simple_attribute_declaration | indexed_attribute_declaration
simple_attribute_declaration ::= @i{for} attribute_designator @i{use} expression ;
indexed_attribute_declaration ::=
  @i{for} @i{<indexed_attribute_>}simple_name ( string_literal) @i{use} expression ;
attribute_designator ::=
  @i{<simple_attribute_>}simple_name
  | @i{<indexed_attribute_>}simple_name ( string_literal )
@end smallexample

@noindent
There are two categories of attributes: @b{simple attributes}
and @b{indexed attributes}.
Each simple attribute has a default value: the empty string (for string
attributes) and the empty list (for string list attributes).
An attribute declaration defines a new value for an attribute, and overrides
the previous value. The syntax of a simple attribute declaration is similar to
that of an attribute definition clause in Ada.

Some attributes are indexed. These attributes are mappings whose
domain is a set of strings. They are declared one association
at a time, by specifying a point in the domain and the corresponding image
of the attribute.
Like untyped variables and simple attributes, indexed attributes
may be declared several times. Each declaration supplies a new value for the
attribute, and replaces the previous setting.

Here are some examples of attribute declarations:

@smallexample @c projectfile
   --  simple attributes
   for Object_Dir use "objects";
   for Source_Dirs use ("units", "test/drivers");

   --  indexed attributes
   for Body ("main") use "Main.ada";
   for Switches ("main.ada") use ("-v", "-gnatv");
   for Switches ("main.ada") use Builder'Switches ("main.ada") & "-g";

   --  indexed attributes copy (from package Builder in project Default)
   --  The package name must always be specified, even if it is the current
   --  package.
   for Default_Switches use Default.Builder'Default_Switches;
@end smallexample

@noindent
Attributes references may be appear anywhere in expressions, and are used
to retrieve the value previously assigned to the attribute. If an attribute
has not been set in a given package or project, its value defaults to the
empty string or the empty list.

@smallexample
attribute_reference ::= attribute_prefix ' @i{<simple_attribute>_}simple_name [ (string_literal) ]
attribute_prefix ::= @i{project}
  | @i{<project_>}simple_name
  | package_identifier
  | @i{<project_>}simple_name . package_identifier
@end smallexample

@noindent
Examples are:

@smallexample @c projectfile
  project'Object_Dir
  Naming'Dot_Replacement
  Imported_Project'Source_Dirs
  Imported_Project.Naming'Casing
  Builder'Default_Switches ("Ada")
@end smallexample

@noindent
The prefix of an attribute may be:

@itemize @bullet
@item @code{project} for an attribute of the current project
@item The name of an existing package of the current project
@item The name of an imported project
@item The name of a parent project that is extended by the current project
@item An expanded name whose prefix is imported/parent project name,
  and whose selector is a package name

@end itemize

@noindent
Legal attribute names are listed below, including the package in
which they must be declared. These names are case-insensitive. The
semantics for the attributes is explained in great details in other sections.

The column @emph{index} indicates whether the attribute is an indexed attribute,
and when it is whether its index is case sensitive (sensitive) or not (insensitive), or if case sensitivity depends is the same as file names sensitivity on the
system (file). The text is between brackets ([]) if the index is optional.

@multitable @columnfractions .3 .1 .2 .4
@headitem Attribute Name @tab Value @tab Package @tab Index
@headitem General attributes @tab @tab @tab @pxref{Building With Projects}
@item Name @tab string @tab - @tab (Read-only, name of project)
@item Project_Dir @tab string @tab - @tab (Read-only, directory of project)
@item Source_Files @tab list @tab - @tab -
@item Source_Dirs  @tab list @tab - @tab -
@item Source_List_File @tab string @tab - @tab -
@item Locally_Removed_Files @tab list @tab - @tab -
@item Excluded_Source_Files @tab list @tab - @tab -
@item Object_Dir   @tab string @tab - @tab -
@item Exec_Dir     @tab string @tab - @tab -
@item Excluded_Source_Dirs @tab list @tab - @tab -
@item Excluded_Source_Files @tab list @tab - @tab -
@item Excluded_Source_List_File @tab list @tab - @tab -
@item Inherit_Source_Path  @tab list @tab - @tab insensitive
@item Languages @tab list @tab - @tab -
@item Main      @tab list @tab - @tab -
@item Main_Language @tab string @tab - @tab -
@item Externally_Built      @tab string @tab - @tab -
@item Roots      @tab list @tab - @tab file
@headitem
   Library-related attributes @tab @tab @tab @pxref{Library Projects}
@item Library_Dir @tab string @tab - @tab -
@item Library_Name @tab string @tab - @tab -
@item Library_Kind @tab string @tab - @tab -
@item Library_Version @tab string @tab - @tab -
@item Library_Interface @tab string @tab - @tab -
@item Library_Auto_Init @tab string @tab - @tab -
@item Library_Options @tab list @tab - @tab -
@item Leading_Library_Options @tab list @tab - @tab -
@item Library_Src_Dir @tab string @tab - @tab -
@item Library_ALI_Dir @tab string @tab - @tab -
@item Library_GCC @tab string @tab - @tab -
@item Library_Symbol_File @tab string @tab - @tab -
@item Library_Symbol_Policy @tab string @tab - @tab -
@item Library_Reference_Symbol_File @tab string @tab - @tab -
@item Interfaces @tab list @tab - @tab -
@headitem
   Naming @tab @tab @tab @pxref{Naming Schemes}
@item Spec_Suffix @tab string @tab Naming @tab insensitive (language)
@item Body_Suffix @tab string @tab Naming @tab insensitive (language)
@item Separate_Suffix @tab string @tab Naming @tab -
@item Casing @tab string @tab Naming @tab -
@item Dot_Replacement @tab string @tab Naming @tab -
@item Spec @tab string @tab Naming @tab insensitive (Ada unit)
@item Body @tab string @tab Naming @tab insensitive (Ada unit)
@item Specification_Exceptions @tab list @tab Naming @tab insensitive (language)
@item Implementation_Exceptions @tab list @tab Naming @tab insensitive (language)
@headitem
   Building @tab @tab @tab @pxref{Switches and Project Files}
@item Default_Switches @tab list @tab Builder, Compiler, Binder, Linker, Cross_Reference, Finder, Pretty_Printer, gnatstub, Check, Synchronize, Eliminate, Metrics, IDE @tab insensitive (language name)
@item Switches @tab list @tab Builder, Compiler, Binder, Linker, Cross_Reference, Finder, gnatls, Pretty_Printer, gnatstub, Check, Synchronize, Eliminate, Metrics, Stack @tab [file] (file name)
@item Local_Configuration_Pragmas @tab string @tab Compiler @tab -
@item Local_Config_File @tab string @tab insensitive @tab -
@item Global_Configuration_Pragmas @tab list @tab Builder @tab -
@item Global_Compilation_Switches @tab list @tab Builder @tab language
@item Executable @tab string @tab Builder @tab [file]
@item Executable_Suffix @tab string @tab Builder @tab -
@item Global_Config_File @tab string @tab Builder @tab insensitive (language)
@headitem
   IDE (used and created by GPS) @tab @tab @tab
@item Remote_Host @tab string @tab IDE @tab -
@item Program_Host @tab string @tab IDE @tab -
@item Communication_Protocol @tab string @tab IDE @tab -
@item Compiler_Command @tab string @tab IDE @tab insensitive (language)
@item Debugger_Command @tab string @tab IDE @tab -
@item Gnatlist @tab string @tab IDE @tab -
@item Gnat @tab string @tab IDE @tab -
@item VCS_Kind @tab string @tab IDE @tab -
@item VCS_File_Check @tab string @tab IDE @tab -
@item VCS_Log_Check @tab string @tab IDE @tab -
@item Documentation_Dir @tab string @tab IDE @tab -
@headitem
   Configuration files @tab @tab @tab See gprbuild manual
@item Default_Language @tab string @tab - @tab -
@item Run_Path_Option @tab list @tab - @tab -
@item Run_Path_Origin @tab string @tab - @tab -
@item Separate_Run_Path_Options @tab string @tab - @tab -
@item Toolchain_Version @tab string @tab - @tab insensitive
@item Toolchain_Description @tab string @tab - @tab insensitive
@item Object_Generated @tab string @tab - @tab insensitive
@item Objects_Linked @tab string @tab - @tab insensitive
@item Target @tab string @tab - @tab -
@item Library_Builder @tab string @tab - @tab -
@item Library_Support @tab string @tab - @tab -
@item Archive_Builder @tab list @tab - @tab -
@item Archive_Builder_Append_Option @tab list @tab - @tab -
@item Archive_Indexer @tab list @tab - @tab -
@item Archive_Suffix @tab string @tab - @tab -
@item Library_Partial_Linker @tab list @tab - @tab -
@item Shared_Library_Prefix @tab string @tab - @tab -
@item Shared_Library_Suffix @tab string @tab - @tab -
@item Symbolic_Link_Supported @tab string @tab - @tab -
@item Library_Major_Minor_Id_Supported @tab string @tab - @tab -
@item Library_Auto_Init_Supported @tab string @tab - @tab -
@item Shared_Library_Minimum_Switches @tab list @tab - @tab -
@item Library_Version_Switches @tab list @tab - @tab -
@item Library_Install_Name_Option @tab string @tab - @tab -
@item Runtime_Library_Dir @tab string @tab - @tab insensitive
@item Runtime_Source_Dir @tab string @tab - @tab insensitive
@item Driver @tab string @tab Compiler,Binder,Linker @tab insensitive (language)
@item Required_Switches @tab list @tab Compiler,Binder,Linker @tab insensitive (language)
@item Leading_Required_Switches @tab list @tab Compiler @tab insensitive (language)
@item Trailing_Required_Switches @tab list @tab Compiler @tab insensitive (language)
@item Pic_Options @tab list @tab Compiler @tab insensitive (language)
@item Path_Syntax @tab string @tab Compiler @tab insensitive (language)
@item Object_File_Suffix @tab string @tab Compiler @tab insensitive (language)
@item Object_File_Switches @tab list @tab Compiler @tab insensitive (language)
@item Multi_Unit_Switches @tab list @tab Compiler @tab insensitive (language)
@item Multi_Unit_Object_Separator @tab string @tab Compiler @tab insensitive (language)
@item Mapping_File_Switches @tab list @tab Compiler @tab insensitive (language)
@item Mapping_Spec_Suffix @tab string @tab Compiler @tab insensitive (language)
@item Mapping_body_Suffix @tab string @tab Compiler @tab insensitive (language)
@item Config_File_Switches @tab list @tab Compiler @tab insensitive (language)
@item Config_Body_File_Name @tab string @tab Compiler @tab insensitive (language)
@item Config_Body_File_Name_Index @tab string @tab Compiler @tab insensitive (language)
@item Config_Body_File_Name_Pattern @tab string @tab Compiler @tab insensitive (language)
@item Config_Spec_File_Name @tab string @tab Compiler @tab insensitive (language)
@item Config_Spec_File_Name_Index @tab string @tab Compiler @tab insensitive (language)
@item Config_Spec_File_Name_Pattern @tab string @tab Compiler @tab insensitive (language)
@item Config_File_Unique @tab string @tab Compiler @tab insensitive (language)
@item Dependency_Switches @tab list @tab Compiler @tab insensitive (language)
@item Dependency_Driver @tab list @tab Compiler @tab insensitive (language)
@item Include_Switches @tab list @tab Compiler @tab insensitive (language)
@item Include_Path @tab string @tab Compiler @tab insensitive (language)
@item Include_Path_File @tab string @tab Compiler @tab insensitive (language)
@item Prefix @tab string @tab Binder @tab insensitive (language)
@item Objects_Path @tab string @tab Binder @tab insensitive (language)
@item Objects_Path_File @tab string @tab Binder @tab insensitive (language)
@item Linker_Options @tab list @tab Linker @tab -
@item Leading_Switches @tab list @tab Linker @tab -
@item Map_File_Options @tab string @tab Linker @tab -
@item Executable_Switches @tab list @tab Linker @tab -
@item Lib_Dir_Switch @tab string @tab Linker @tab -
@item Lib_Name_Switch @tab string @tab Linker @tab -
@item Max_Command_Line_Length @tab string @tab Linker @tab -
@item Response_File_Format @tab string @tab Linker @tab -
@item Response_File_Switches @tab list @tab Linker @tab -
@end multitable

@c ---------------------------------------------
@node Case Statements
@subsection Case Statements
@c ---------------------------------------------

@noindent
A @b{case} statement is used in a project file to effect conditional
behavior. Through this statement, you can set the value of attributes
and variables depending on the value previously assigned to a typed
variable.

All choices in a choice list must be distinct. Unlike Ada, the choice
lists of all alternatives do not need to include all values of the type.
An @code{others} choice must appear last in the list of alternatives.

The syntax of a @code{case} construction is based on the Ada case statement
(although the @code{null} statement for empty alternatives is optional).

The case expression must be a typed string variable, whose value is often
given by an external reference (@pxref{External Values}).

Each alternative starts with the reserved word @code{when}, either a list of
literal strings separated by the @code{"|"} character or the reserved word
@code{others}, and the @code{"=>"} token.
Each literal string must belong to the string type that is the type of the
case variable.
After each @code{=>}, there are zero or more statements.  The only
statements allowed in a case construction are other case statements,
attribute declarations and variable declarations. String type declarations and
package declarations are not allowed. Variable declarations are restricted to
variables that have already been declared before the case construction.

@smallexample
case_statement ::=
  @i{case} @i{<typed_variable_>}name @i{is} @{case_item@} @i{end case} ;

case_item ::=
  @i{when} discrete_choice_list =>
    @{case_statement
      | attribute_declaration
      | variable_declaration
      | empty_declaration@}

discrete_choice_list ::= string_literal @{| string_literal@} | @i{others}
@end smallexample

@noindent
Here is a typical example:

@smallexample @c projectfile
@group
project MyProj is
   type OS_Type is ("GNU/Linux", "Unix", "NT", "VMS");
   OS : OS_Type := external ("OS", "GNU/Linux");

   package Compiler is
     case OS is
       when "GNU/Linux" | "Unix" =>
         for Switches ("Ada") use ("-gnath");
       when "NT" =>
         for Switches ("Ada") use ("-gnatP");
       when others =>
         null;
     end case;
   end Compiler;
end MyProj;
@end group
@end smallexample

@c ---------------------------------------------
@node Tools Supporting Project Files
@chapter Tools Supporting Project Files
@c ---------------------------------------------

@noindent

@menu
* gnatmake and Project Files::
* The GNAT Driver and Project Files::
* The Development Environments::
@end menu

@c ---------------------------------------------
@node gnatmake and Project Files
@section gnatmake and Project Files
@c ---------------------------------------------

@noindent
This section covers several topics related to @command{gnatmake} and
project files: defining ^switches^switches^ for @command{gnatmake}
and for the tools that it invokes; specifying configuration pragmas;
the use of the @code{Main} attribute; building and rebuilding library project
files.

@menu
* Switches Related to Project Files::
* Switches and Project Files::
* Specifying Configuration Pragmas::
* Project Files and Main Subprograms::
* Library Project Files::
@end menu

@c ---------------------------------------------
@node Switches Related to Project Files
@subsection Switches Related to Project Files
@c ---------------------------------------------

@noindent
The following switches are used by GNAT tools that support project files:

@table @option

@item ^-P^/PROJECT_FILE=^@var{project}
@cindex @option{^-P^/PROJECT_FILE^} (any project-aware tool)
Indicates the name of a project file. This project file will be parsed with
the verbosity indicated by @option{^-vP^MESSAGE_PROJECT_FILES=^@emph{x}},
if any, and using the external references indicated
by @option{^-X^/EXTERNAL_REFERENCE^} switches, if any.
@ifclear vms
There may zero, one or more spaces between @option{-P} and @var{project}.
@end ifclear

There must be only one @option{^-P^/PROJECT_FILE^} switch on the command line.

Since the Project Manager parses the project file only after all the switches
on the command line are checked, the order of the switches
@option{^-P^/PROJECT_FILE^},
@option{^-vP^/MESSAGES_PROJECT_FILE=^@emph{x}}
or @option{^-X^/EXTERNAL_REFERENCE^} is not significant.

@item ^-X^/EXTERNAL_REFERENCE=^@var{name=value}
@cindex @option{^-X^/EXTERNAL_REFERENCE^} (any project-aware tool)
Indicates that external variable @var{name} has the value @var{value}.
The Project Manager will use this value for occurrences of
@code{external(name)} when parsing the project file.

@ifclear vms
If @var{name} or @var{value} includes a space, then @var{name=value} should be
put between quotes.
@smallexample
  -XOS=NT
  -X"user=John Doe"
@end smallexample
@end ifclear

Several @option{^-X^/EXTERNAL_REFERENCE^} switches can be used simultaneously.
If several @option{^-X^/EXTERNAL_REFERENCE^} switches specify the same
@var{name}, only the last one is used.

An external variable specified with a @option{^-X^/EXTERNAL_REFERENCE^} switch
takes precedence over the value of the same name in the environment.

@item ^-vP^/MESSAGES_PROJECT_FILE=^@emph{x}
@cindex @option{^-vP^/MESSAGES_PROJECT_FILE^} (any project-aware tool)
Indicates the verbosity of the parsing of GNAT project files.

@ifclear vms
@option{-vP0} means Default;
@option{-vP1} means Medium;
@option{-vP2} means High.
@end ifclear

@ifset vms
There are three possible options for this qualifier: DEFAULT, MEDIUM and
HIGH.
@end ifset

The default is ^Default^DEFAULT^: no output for syntactically correct
project files.
If several @option{^-vP^/MESSAGES_PROJECT_FILE=^@emph{x}} switches are present,
only the last one is used.

@item ^-aP^/ADD_PROJECT_SEARCH_DIR=^<dir>
@cindex @option{^-aP^/ADD_PROJECT_SEARCH_DIR=^} (any project-aware tool)
Add directory <dir> at the beginning of the project search path, in order,
after the current working directory.

@ifclear vms
@item -eL
@cindex @option{-eL} (any project-aware tool)
Follow all symbolic links when processing project files.
@end ifclear

@item ^--subdirs^/SUBDIRS^=<subdir>
@cindex @option{^--subdirs^/SUBDIRS^=} (gnatmake and gnatclean)
This switch is recognized by gnatmake and gnatclean. It indicate that the real
directories (except the source directories) are the subdirectories <subdir>
of the directories specified in the project files. This applies in particular
to object directories, library directories and exec directories. If the
subdirectories do not exist, they are created automatically.

@end table

@c ---------------------------------------------
@node Switches and Project Files
@subsection Switches and Project Files
@c ---------------------------------------------

@noindent
@ifset vms
It is not currently possible to specify VMS style qualifiers in the project
files; only Unix style ^switches^switches^ may be specified.
@end ifset

For each of the packages @code{Builder}, @code{Compiler}, @code{Binder}, and
@code{Linker}, you can specify a @code{^Default_Switches^Default_Switches^}
attribute, a @code{Switches} attribute, or both;
as their names imply, these ^switch^switch^-related
attributes affect the ^switches^switches^ that are used for each of these GNAT
components when
@command{gnatmake} is invoked.  As will be explained below, these
component-specific ^switches^switches^ precede
the ^switches^switches^ provided on the @command{gnatmake} command line.

The @code{^Default_Switches^Default_Switches^} attribute is an attribute
indexed by language name (case insensitive) whose value is a string list.
For example:

@smallexample @c projectfile
@group
package Compiler is
  for ^Default_Switches^Default_Switches^ ("Ada")
      use ("^-gnaty^-gnaty^",
           "^-v^-v^");
end Compiler;
@end group
@end smallexample

@noindent
The @code{Switches} attribute is indexed on a file name (which may or may
not be case sensitive, depending
on the operating system) whose value is a string list.  For example:

@smallexample @c projectfile
@group
package Builder is
   for Switches ("main1.adb")
       use ("^-O2^-O2^");
   for Switches ("main2.adb")
       use ("^-g^-g^");
end Builder;
@end group
@end smallexample

@noindent
For the @code{Builder} package, the file names must designate source files
for main subprograms.  For the @code{Binder} and @code{Linker} packages, the
file names must designate @file{ALI} or source files for main subprograms.
In each case just the file name without an explicit extension is acceptable.

For each tool used in a program build (@command{gnatmake}, the compiler, the
binder, and the linker), the corresponding package @dfn{contributes} a set of
^switches^switches^ for each file on which the tool is invoked, based on the
^switch^switch^-related attributes defined in the package.
In particular, the ^switches^switches^
that each of these packages contributes for a given file @var{f} comprise:

@itemize @bullet
@item the value of attribute @code{Switches (@var{f})},
  if it is specified in the package for the given file,
@item otherwise, the value of @code{^Default_Switches^Default_Switches^ ("Ada")},
  if it is specified in the package.

@end itemize

@noindent
If neither of these attributes is defined in the package, then the package does
not contribute any ^switches^switches^ for the given file.

When @command{gnatmake} is invoked on a file, the ^switches^switches^ comprise
two sets, in the following order: those contributed for the file
by the @code{Builder} package;
and the switches passed on the command line.

When @command{gnatmake} invokes a tool (compiler, binder, linker) on a file,
the ^switches^switches^ passed to the tool comprise three sets,
in the following order:

@enumerate
@item
the applicable ^switches^switches^ contributed for the file
by the @code{Builder} package in the project file supplied on the command line;

@item
those contributed for the file by the package (in the relevant project file --
see below) corresponding to the tool; and

@item
the applicable switches passed on the command line.
@end enumerate

The term @emph{applicable ^switches^switches^} reflects the fact that
@command{gnatmake} ^switches^switches^ may or may not be passed to individual
tools, depending on the individual ^switch^switch^.

@command{gnatmake} may invoke the compiler on source files from different
projects. The Project Manager will use the appropriate project file to
determine the @code{Compiler} package for each source file being compiled.
Likewise for the @code{Binder} and @code{Linker} packages.

As an example, consider the following package in a project file:

@smallexample @c projectfile
@group
project Proj1 is
   package Compiler is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-g^-g^");
      for Switches ("a.adb")
          use ("^-O1^-O1^");
      for Switches ("b.adb")
          use ("^-O2^-O2^",
               "^-gnaty^-gnaty^");
   end Compiler;
end Proj1;
@end group
@end smallexample

@noindent
If @command{gnatmake} is invoked with this project file, and it needs to
compile, say, the files @file{a.adb}, @file{b.adb}, and @file{c.adb}, then
@file{a.adb} will be compiled with the ^switch^switch^
@option{^-O1^-O1^},
@file{b.adb} with ^switches^switches^
@option{^-O2^-O2^}
and @option{^-gnaty^-gnaty^},
and @file{c.adb} with @option{^-g^-g^}.

The following example illustrates the ordering of the ^switches^switches^
contributed by different packages:

@smallexample @c projectfile
@group
project Proj2 is
   package Builder is
      for Switches ("main.adb")
          use ("^-g^-g^",
               "^-O1^-)1^",
               "^-f^-f^");
   end Builder;
@end group

@group
   package Compiler is
      for Switches ("main.adb")
          use ("^-O2^-O2^");
   end Compiler;
end Proj2;
@end group
@end smallexample

@noindent
If you issue the command:

@smallexample
    gnatmake ^-Pproj2^/PROJECT_FILE=PROJ2^ -O0 main
@end smallexample

@noindent
then the compiler will be invoked on @file{main.adb} with the following
sequence of ^switches^switches^

@smallexample
   ^-g -O1 -O2 -O0^-g -O1 -O2 -O0^
@end smallexample

@noindent
with the last @option{^-O^-O^}
^switch^switch^ having precedence over the earlier ones;
several other ^switches^switches^
(such as @option{^-c^-c^}) are added implicitly.

The ^switches^switches^
@option{^-g^-g^}
and @option{^-O1^-O1^} are contributed by package
@code{Builder},  @option{^-O2^-O2^} is contributed
by the package @code{Compiler}
and @option{^-O0^-O0^} comes from the command line.

The @option{^-g^-g^}
^switch^switch^ will also be passed in the invocation of
@command{Gnatlink.}

A final example illustrates switch contributions from packages in different
project files:

@smallexample @c projectfile
@group
project Proj3 is
   for Source_Files use ("pack.ads", "pack.adb");
   package Compiler is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-gnata^-gnata^");
   end Compiler;
end Proj3;
@end group

@group
with "Proj3";
project Proj4 is
   for Source_Files use ("foo_main.adb", "bar_main.adb");
   package Builder is
      for Switches ("foo_main.adb")
          use ("^-s^-s^",
               "^-g^-g^");
   end Builder;
end Proj4;
@end group

@group
-- Ada source file:
with Pack;
procedure Foo_Main is
   @dots{}
end Foo_Main;
@end group
@end smallexample

@noindent
If the command is
@smallexample
gnatmake ^-PProj4^/PROJECT_FILE=PROJ4^ foo_main.adb -cargs -gnato
@end smallexample

@noindent
then the ^switches^switches^ passed to the compiler for @file{foo_main.adb} are
@option{^-g^-g^} (contributed by the package @code{Proj4.Builder}) and
@option{^-gnato^-gnato^} (passed on the command line).
When the imported package @code{Pack} is compiled, the ^switches^switches^ used
are @option{^-g^-g^} from @code{Proj4.Builder},
@option{^-gnata^-gnata^} (contributed from package @code{Proj3.Compiler},
and @option{^-gnato^-gnato^} from the command line.

When using @command{gnatmake} with project files, some ^switches^switches^ or
arguments may be expressed as relative paths. As the working directory where
compilation occurs may change, these relative paths are converted to absolute
paths. For the ^switches^switches^ found in a project file, the relative paths
are relative to the project file directory, for the switches on the command
line, they are relative to the directory where @command{gnatmake} is invoked.
The ^switches^switches^ for which this occurs are:
^-I^-I^,
^-A^-A^,
^-L^-L^,
^-aO^-aO^,
^-aL^-aL^,
^-aI^-aI^, as well as all arguments that are not switches (arguments to
^switch^switch^
^-o^-o^, object files specified in package @code{Linker} or after
-largs on the command line). The exception to this rule is the ^switch^switch^
^--RTS=^--RTS=^ for which a relative path argument is never converted.

@c ---------------------------------------------
@node Specifying Configuration Pragmas
@subsection Specifying Configuration Pragmas
@c ---------------------------------------------

@noindent
When using @command{gnatmake} with project files, if there exists a file
@file{gnat.adc} that contains configuration pragmas, this file will be
ignored.

Configuration pragmas can be defined by means of the following attributes in
project files: @code{Global_Configuration_Pragmas} in package @code{Builder}
and @code{Local_Configuration_Pragmas} in package @code{Compiler}.

Both these attributes are single string attributes. Their values is the path
name of a file containing configuration pragmas. If a path name is relative,
then it is relative to the project directory of the project file where the
attribute is defined.

When compiling a source, the configuration pragmas used are, in order,
those listed in the file designated by attribute
@code{Global_Configuration_Pragmas} in package @code{Builder} of the main
project file, if it is specified, and those listed in the file designated by
attribute @code{Local_Configuration_Pragmas} in package @code{Compiler} of
the project file of the source, if it exists.

@c ---------------------------------------------
@node Project Files and Main Subprograms
@subsection Project Files and Main Subprograms
@c ---------------------------------------------

@noindent
When using a project file, you can invoke @command{gnatmake}
with one or several main subprograms, by specifying their source files on the
command line.

@smallexample
    gnatmake ^-P^/PROJECT_FILE=^prj main1.adb main2.adb main3.adb
@end smallexample

@noindent
Each of these needs to be a source file of the same project, except
when the switch ^-u^/UNIQUE^ is used.

When ^-u^/UNIQUE^ is not used, all the mains need to be sources of the
same project, one of the project in the tree rooted at the project specified
on the command line. The package @code{Builder} of this common project, the
"main project" is the one that is considered by @command{gnatmake}.

When ^-u^/UNIQUE^ is used, the specified source files may be in projects
imported directly or indirectly by the project specified on the command line.
Note that if such a source file is not part of the project specified on the
command line, the ^switches^switches^ found in package @code{Builder} of the
project specified on the command line, if any, that are transmitted
to the compiler will still be used, not those found in the project file of
the source file.

When using a project file, you can also invoke @command{gnatmake} without
explicitly specifying any main, and the effect depends on whether you have
defined the @code{Main} attribute.  This attribute has a string list value,
where each element in the list is the name of a source file (the file
extension is optional) that contains a unit that can be a main subprogram.

If the @code{Main} attribute is defined in a project file as a non-empty
string list and the switch @option{^-u^/UNIQUE^} is not used on the command
line, then invoking @command{gnatmake} with this project file but without any
main on the command line is equivalent to invoking @command{gnatmake} with all
the file names in the @code{Main} attribute on the command line.

Example:
@smallexample @c projectfile
@group
   project Prj is
      for Main use ("main1.adb", "main2.adb", "main3.adb");
   end Prj;
@end group
@end smallexample

@noindent
With this project file, @code{"gnatmake ^-Pprj^/PROJECT_FILE=PRJ^"}
is equivalent to
@code{"gnatmake ^-Pprj^/PROJECT_FILE=PRJ^ main1.adb main2.adb main3.adb"}.

When the project attribute @code{Main} is not specified, or is specified
as an empty string list, or when the switch @option{-u} is used on the command
line, then invoking @command{gnatmake} with no main on the command line will
result in all immediate sources of the project file being checked, and
potentially recompiled. Depending on the presence of the switch @option{-u},
sources from other project files on which the immediate sources of the main
project file depend are also checked and potentially recompiled. In other
words, the @option{-u} switch is applied to all of the immediate sources of the
main project file.

When no main is specified on the command line and attribute @code{Main} exists
and includes several mains, or when several mains are specified on the
command line, the default ^switches^switches^ in package @code{Builder} will
be used for all mains, even if there are specific ^switches^switches^
specified for one or several mains.

But the ^switches^switches^ from package @code{Binder} or @code{Linker} will be
the specific ^switches^switches^ for each main, if they are specified.

@c ---------------------------------------------
@node Library Project Files
@subsection Library Project Files
@c ---------------------------------------------

@noindent
When @command{gnatmake} is invoked with a main project file that is a library
project file, it is not allowed to specify one or more mains on the command
line.

When a library project file is specified, switches ^-b^/ACTION=BIND^ and
^-l^/ACTION=LINK^ have special meanings.

@itemize @bullet
@item ^-b^/ACTION=BIND^ is only allowed for stand-alone libraries. It indicates
  to @command{gnatmake} that @command{gnatbind} should be invoked for the
  library.

@item ^-l^/ACTION=LINK^ may be used for all library projects. It indicates
  to @command{gnatmake} that the binder generated file should be compiled
  (in the case of a stand-alone library) and that the library should be built.
@end itemize

@c ---------------------------------------------
@node The GNAT Driver and Project Files
@section The GNAT Driver and Project Files
@c ---------------------------------------------

@noindent
A number of GNAT tools, other than @command{^gnatmake^gnatmake^}
can benefit from project files:
(@command{^gnatbind^gnatbind^},
@command{^gnatcheck^gnatcheck^},
@command{^gnatclean^gnatclean^},
@command{^gnatelim^gnatelim^},
@command{^gnatfind^gnatfind^},
@command{^gnatlink^gnatlink^},
@command{^gnatls^gnatls^},
@command{^gnatmetric^gnatmetric^},
@command{^gnatpp^gnatpp^},
@command{^gnatstub^gnatstub^},
and @command{^gnatxref^gnatxref^}). However, none of these tools can be invoked
directly with a project file switch (@option{^-P^/PROJECT_FILE=^}).
They must be invoked through the @command{gnat} driver.

The @command{gnat} driver is a wrapper that accepts a number of commands and
calls the corresponding tool. It was designed initially for VMS platforms (to
convert VMS qualifiers to Unix-style switches), but it is now available on all
GNAT platforms.

On non-VMS platforms, the @command{gnat} driver accepts the following commands
(case insensitive):

@itemize @bullet
@item BIND to invoke @command{^gnatbind^gnatbind^}
@item CHOP to invoke @command{^gnatchop^gnatchop^}
@item CLEAN to invoke @command{^gnatclean^gnatclean^}
@item COMP or COMPILE to invoke the compiler
@item ELIM to invoke @command{^gnatelim^gnatelim^}
@item FIND to invoke @command{^gnatfind^gnatfind^}
@item KR or KRUNCH to invoke @command{^gnatkr^gnatkr^}
@item LINK to invoke @command{^gnatlink^gnatlink^}
@item LS or LIST to invoke @command{^gnatls^gnatls^}
@item MAKE to invoke @command{^gnatmake^gnatmake^}
@item NAME to invoke @command{^gnatname^gnatname^}
@item PREP or PREPROCESS to invoke @command{^gnatprep^gnatprep^}
@item PP or PRETTY to invoke @command{^gnatpp^gnatpp^}
@item METRIC to invoke @command{^gnatmetric^gnatmetric^}
@item STUB to invoke @command{^gnatstub^gnatstub^}
@item XREF to invoke @command{^gnatxref^gnatxref^}

@end itemize

@noindent
(note that the compiler is invoked using the command
@command{^gnatmake -f -u -c^gnatmake -f -u -c^}).

On non-VMS platforms, between @command{gnat} and the command, two
special switches may be used:

@itemize @bullet
@item @command{-v} to display the invocation of the tool.
@item @command{-dn} to prevent the @command{gnat} driver from removing
  the temporary files it has created. These temporary files are
  configuration files and temporary file list files.

@end itemize

@noindent
The command may be followed by switches and arguments for the invoked
tool.

@smallexample
  gnat bind -C main.ali
  gnat ls -a main
  gnat chop foo.txt
@end smallexample

@noindent
Switches may also be put in text files, one switch per line, and the text
files may be specified with their path name preceded by '@@'.

@smallexample
   gnat bind @@args.txt main.ali
@end smallexample

@noindent
In addition, for commands BIND, COMP or COMPILE, FIND, ELIM, LS or LIST, LINK,
METRIC, PP or PRETTY, STUB and XREF, the project file related switches
(@option{^-P^/PROJECT_FILE^},
@option{^-X^/EXTERNAL_REFERENCE^} and
@option{^-vP^/MESSAGES_PROJECT_FILE=^x}) may be used in addition to
the switches of the invoking tool.

When GNAT PP or GNAT PRETTY is used with a project file, but with no source
specified on the command line, it invokes @command{^gnatpp^gnatpp^} with all
the immediate sources of the specified project file.

When GNAT METRIC is used with a project file, but with no source
specified on the command line, it invokes @command{^gnatmetric^gnatmetric^}
with all the immediate sources of the specified project file and with
@option{^-d^/DIRECTORY^} with the parameter pointing to the object directory
of the project.

In addition, when GNAT PP, GNAT PRETTY or GNAT METRIC is used with
a project file, no source is specified on the command line and
switch ^-U^/ALL_PROJECTS^ is specified on the command line, then
the underlying tool (^gnatpp^gnatpp^ or
^gnatmetric^gnatmetric^) is invoked for all sources of all projects,
not only for the immediate sources of the main project.
@ifclear vms
(-U stands for Universal or Union of the project files of the project tree)
@end ifclear

For each of the following commands, there is optionally a corresponding
package in the main project.

@itemize @bullet
@item package @code{Binder} for command BIND (invoking @code{^gnatbind^gnatbind^})

@item package @code{Check} for command CHECK (invoking
  @code{^gnatcheck^gnatcheck^})

@item package @code{Compiler} for command COMP or COMPILE (invoking the compiler)

@item package @code{Cross_Reference} for command XREF (invoking
  @code{^gnatxref^gnatxref^})

@item package @code{Eliminate} for command ELIM (invoking
  @code{^gnatelim^gnatelim^})

@item package @code{Finder} for command FIND (invoking @code{^gnatfind^gnatfind^})

@item package @code{Gnatls} for command LS or LIST (invoking @code{^gnatls^gnatls^})

@item package @code{Gnatstub} for command STUB
  (invoking @code{^gnatstub^gnatstub^})

@item package @code{Linker} for command LINK (invoking @code{^gnatlink^gnatlink^})

@item package @code{Check} for command CHECK
  (invoking @code{^gnatcheck^gnatcheck^})

@item package @code{Metrics} for command METRIC
  (invoking @code{^gnatmetric^gnatmetric^})

@item package @code{Pretty_Printer} for command PP or PRETTY
  (invoking @code{^gnatpp^gnatpp^})

@end itemize

@noindent
Package @code{Gnatls} has a unique attribute @code{Switches},
a simple variable with a string list value. It contains ^switches^switches^
for the invocation of @code{^gnatls^gnatls^}.

@smallexample @c projectfile
@group
project Proj1 is
   package gnatls is
      for Switches
          use ("^-a^-a^",
               "^-v^-v^");
   end gnatls;
end Proj1;
@end group
@end smallexample

@noindent
All other packages have two attribute @code{Switches} and
@code{^Default_Switches^Default_Switches^}.

@code{Switches} is an indexed attribute, indexed by the
source file name, that has a string list value: the ^switches^switches^ to be
used when the tool corresponding to the package is invoked for the specific
source file.

@code{^Default_Switches^Default_Switches^} is an attribute,
indexed by  the programming language that has a string list value.
@code{^Default_Switches^Default_Switches^ ("Ada")} contains the
^switches^switches^ for the invocation of the tool corresponding
to the package, except if a specific @code{Switches} attribute
is specified for the source file.

@smallexample @c projectfile
@group
project Proj is

   for Source_Dirs use ("**");

   package gnatls is
      for Switches use
          ("^-a^-a^",
           "^-v^-v^");
   end gnatls;
@end group
@group

   package Compiler is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-gnatv^-gnatv^",
               "^-gnatwa^-gnatwa^");
   end Binder;
@end group
@group

   package Binder is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-C^-C^",
               "^-e^-e^");
   end Binder;
@end group
@group

   package Linker is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-C^-C^");
      for Switches ("main.adb")
          use ("^-C^-C^",
               "^-v^-v^",
               "^-v^-v^");
   end Linker;
@end group
@group

   package Finder is
      for ^Default_Switches^Default_Switches^ ("Ada")
           use ("^-a^-a^",
                "^-f^-f^");
   end Finder;
@end group
@group

   package Cross_Reference is
      for ^Default_Switches^Default_Switches^ ("Ada")
          use ("^-a^-a^",
               "^-f^-f^",
               "^-d^-d^",
               "^-u^-u^");
   end Cross_Reference;
end Proj;
@end group
@end smallexample

@noindent
With the above project file, commands such as

@smallexample
   ^gnat comp -Pproj main^GNAT COMP /PROJECT_FILE=PROJ MAIN^
   ^gnat ls -Pproj main^GNAT LIST /PROJECT_FILE=PROJ MAIN^
   ^gnat xref -Pproj main^GNAT XREF /PROJECT_FILE=PROJ MAIN^
   ^gnat bind -Pproj main.ali^GNAT BIND /PROJECT_FILE=PROJ MAIN.ALI^
   ^gnat link -Pproj main.ali^GNAT LINK /PROJECT_FILE=PROJ MAIN.ALI^
@end smallexample

@noindent
will set up the environment properly and invoke the tool with the switches
found in the package corresponding to the tool:
@code{^Default_Switches^Default_Switches^ ("Ada")} for all tools,
except @code{Switches ("main.adb")}
for @code{^gnatlink^gnatlink^}.
It is also possible to invoke some of the tools,
(@code{^gnatcheck^gnatcheck^},
@code{^gnatmetric^gnatmetric^},
and @code{^gnatpp^gnatpp^})
on a set of project units thanks to the combination of the switches
@option{-P}, @option{-U} and possibly the main unit when one is interested
in its closure. For instance,
@smallexample
gnat metric -Pproj
@end smallexample

@noindent
will compute the metrics for all the immediate units of project
@code{proj}.
@smallexample
gnat metric -Pproj -U
@end smallexample

@noindent
will compute the metrics for all the units of the closure of projects
rooted at @code{proj}.
@smallexample
gnat metric -Pproj -U main_unit
@end smallexample

@noindent
will compute the metrics for the closure of units rooted at
@code{main_unit}. This last possibility relies implicitly
on @command{gnatbind}'s option @option{-R}. But if the argument files for the
tool invoked by the @command{gnat} driver are explicitly  specified
either directly or through the tool @option{-files} option, then the tool
is called only for these explicitly specified files.

@c ---------------------------------------------
@node The Development Environments
@section The Development Environments
@c ---------------------------------------------

@noindent
See the appropriate manuals for more details. These environments will
store a number of settings in the project itself, when they are meant
to be shared by the whole team working on the project. Here are the
attributes defined in the package @b{IDE} in projects.

@table @code
@item Remote_Host
This is a simple attribute. Its value is a string that designates the remote
host in a cross-compilation environment, to be used for remote compilation and
debugging. This field should not be specified when running on the local
machine.

@item Program_Host
This is a simple attribute. Its value is a string that specifies the
name of IP address of the embedded target in a cross-compilation environment,
on which the program should execute.

@item Communication_Protocol
This is a simple string attribute. Its value is the name of the protocol
to use to communicate with the target in a cross-compilation environment,
e.g.@: @code{"wtx"} or @code{"vxworks"}.

@item Compiler_Command
This is an associative array attribute, whose domain is a language name. Its
value is  string that denotes the command to be used to invoke the compiler.
The value of @code{Compiler_Command ("Ada")} is expected to be compatible with
gnatmake, in particular in the handling of switches.

@item Debugger_Command
This is simple attribute, Its value is a string that specifies the name of
the debugger to be used, such as gdb, powerpc-wrs-vxworks-gdb or gdb-4.

@item Default_Switches
This is an associative array attribute. Its indexes are the name of the
external tools that the GNAT Programming System (GPS) is supporting. Its
value is a list of switches to use when invoking that tool.

@item  Gnatlist
This is a simple attribute.  Its value is a string that specifies the name
of the @command{gnatls} utility to be used to retrieve information about the
predefined path; e.g., @code{"gnatls"}, @code{"powerpc-wrs-vxworks-gnatls"}.

@item VCS_Kind
This is a simple attribute. Its value is a string used to specify the
Version Control System (VCS) to be used for this project, e.g.@: CVS, RCS
ClearCase or Perforce.

@item Gnat
This is a simple attribute. Its value is a string that specifies the name
of the @command{gnat} utility to be used when executing various tools from
GPS, in particular @code{"gnat pp"}, @code{"gnat stub"},@dots{}

@item VCS_File_Check
This is a simple attribute. Its value is a string that specifies the
command used by the VCS to check the validity of a file, either
when the user explicitly asks for a check, or as a sanity check before
doing the check-in.

@item VCS_Log_Check
This is a simple attribute. Its value is a string that specifies
the command used by the VCS to check the validity of a log file.

@item VCS_Repository_Root
The VCS repository root path. This is used to create tags or branches
of the repository. For subversion the value should be the @code{URL}
as specified to check-out the working copy of the repository.

@item VCS_Patch_Root
The local root directory to use for building patch file. All patch chunks
will be relative to this path. The root project directory is used if
this value is not defined.

@end table