@item Code Generation Options
@xref{Code Gen Options,,Options for Code Generation Conventions}.
@gccoptlist{
--fno-underscoring -fno-second-underscore @gol
+-ff2c -fno-underscoring -fsecond-underscore @gol
-fbounds-check -fmax-stack-var-size=@var{n} @gol
-fpackderived -frepack-arrays}
@end table
@table @gcctabopt
-@cindex -fno-underscoring option
-@cindex options, -fno-underscoring
+@cindex @option{-ff2c} option
+@cindex options, @option{-ff2c}
+@item -ff2c
+@cindex calling convention
+@cindex @command{f2c} calling convention
+@cindex @command{g77} calling convention
+@cindex libf2c calling convention
+Generate code designed to be compatible with code generated
+by @command{g77} and @command{f2c}.
+
+The calling conventions used by @command{g77} (originally implemented
+in @command{f2c}) require functions that return type
+default @code{REAL} to actually return the C type @code{double}, and
+functions that return type @code{COMPLEX} to return the values via an
+extra argument in the calling sequence that points to where to
+store the return value. Under the default GNU calling conventions, such
+functions simply return their results as they would in GNU
+C -- default @code{REAL} functions return the C type @code{float}, and
+@code{COMPLEX} functions return the GNU C type @code{complex}.
+Additionally, this option implies the @options{-fsecond-underscore}
+option, unless @options{-fno-second-underscore} is explicitly requested.
+
+This does not affect the generation of code that interfaces with
+the @command{libgfortran} library.
+
+@emph{Caution:} It is not a good idea to mix Fortran code compiled
+with @code{-ff2c} with code compiled with the default @code{-fno-f2c}
+calling conventions as, calling @code{COMPLEX} or default @code{REAL}
+functions between program parts which were compiled with different
+calling conventions will break at execution time.
+
+@emph{Caution:} This will break code which passes intrinsic functions
+of type default @code{REAL} or @code{COMPLEX} as actual arguments, as
+the library implementations use the @command{-fno-f2c} calling conventions.
+
+@cindex @option{-fno-underscoring option}
+@cindex options, @option{-fno-underscoring}
@item -fno-underscoring
@cindex underscore
@cindex symbol names, underscores
Do not transform names of entities specified in the Fortran
source file by appending underscores to them.
-With @option{-funderscoring} in effect, @command{gfortran} appends two
-underscores to names with underscores and one underscore to external names
-with no underscores. (@command{gfortran} also appends two underscores to
-internal names with underscores to avoid naming collisions with external
-names. The @option{-fno-second-underscore} option disables appending of the
-second underscore in all cases.)
+With @option{-funderscoring} in effect, @command{gfortran} appends one
+underscore to external names with no underscores.
This is done to ensure compatibility with code produced by many
-UNIX Fortran compilers, including @command{f2c} which perform the
-same transformations.
+UNIX Fortran compilers.
+
+@emph{Caution}: The default behavior of @command{gfortran} is
+incompatible with @command{f2c} and @command{g77}, please use the
+@option{-ff2c} and @option{-fsecond-underscore} options if you want
+object files compiled with @option{gfortran} to be compatible with
+object code created with these tools.
Use of @option{-fno-underscoring} is not recommended unless you are
experimenting with issues such as integration of (GNU) Fortran into
prevent accidental linking between procedures with incompatible
interfaces.
-@cindex -fno-second-underscore option
-@cindex options, -fno-second-underscore
-@item -fno-second-underscore
+@cindex @option{-fsecond-underscore option}
+@cindex options, @option{-fsecond-underscore}
+@item -fsecond-underscore
@cindex underscore
@cindex symbol names, underscores
@cindex transforming symbol names
@cindex symbol names, transforming
-Do not append a second underscore to names of entities specified
-in the Fortran source file.
+@cindex @command{f2c} calling convention
+@cindex @command{g77} calling convention
+@cindex libf2c calling convention
+By default, @command{gfortran} appends an underscore to external
+names. If this option is used @command{gfortran} appends two
+underscores to names with underscores and one underscore to external names
+with no underscores. (@command{gfortran} also appends two underscores to
+internal names with underscores to avoid naming collisions with external
+names.
This option has no effect if @option{-fno-underscoring} is
-in effect.
+in effect. It is implied by the @option{-ff2c} option.
Otherwise, with this option, an external name such as @samp{MAX_COUNT}
is implemented as a reference to the link-time external symbol
-@samp{max_count_}, instead of @samp{max_count__}.
+@samp{max_count__}, instead of @samp{max_count_}. This is required
+for compatibility with @command{g77} and @command{f2c}, and is implied
+by use of the @option{-ff2c} option.
@cindex -fbounds-check option
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