+@c For copying conditions, see the file gccint.texi.
+
+@node Host Config
+@chapter Host Configuration
+@cindex host configuration
+
+Most details about the machine and system on which the compiler is
+actually running are detected by the @command{configure} script. Some
+things are impossible for @command{configure} to detect; these are
+described in two ways, either by macros defined in a file named
+@file{xm-@var{machine}.h} or by hook functions in the file specified
+by the @var{out_host_hook_obj} variable in @file{config.gcc}. (The
+intention is that very few hosts will need a header file but nearly
+every fully supported host will need to override some hooks.)
+
+If you need to define only a few macros, and they have simple
+definitions, consider using the @code{xm_defines} variable in your
+@file{config.gcc} entry instead of creating a host configuration
+header. @xref{System Config}.
+
+@menu
+* Host Common:: Things every host probably needs implemented.
+* Filesystem:: Your host can't have the letter `a' in filenames?
+* Host Misc:: Rare configuration options for hosts.
+@end menu
+
+@node Host Common
+@section Host Common
+@cindex host hooks
+@cindex host functions
+
+Some things are just not portable, even between similar operating systems,
+and are too difficult for autoconf to detect. They get implemented using
+hook functions in the file specified by the @var{host_hook_obj}
+variable in @file{config.gcc}.
+
+@deftypefn {Host Hook} void HOST_HOOKS_EXTRA_SIGNALS (void)
+This host hook is used to set up handling for extra signals. The most
+common thing to do in this hook is to detect stack overflow.
+@end deftypefn
+
+@deftypefn {Host Hook} {void *} HOST_HOOKS_GT_PCH_GET_ADDRESS (size_t @
+ @var{size}, int @var{fd})
+This host hook returns the address of some space that is likely to be
+free in some subsequent invocation of the compiler. We intend to load
+the PCH data at this address such that the data need not be relocated.
+The area should be able to hold @var{size} bytes. If the host uses
+@code{mmap}, @var{fd} is an open file descriptor that can be used for
+probing.
+@end deftypefn
+
+@deftypefn {Host Hook} int HOST_HOOKS_GT_PCH_USE_ADDRESS (void * @var{address}, @
+ size_t @var{size}, int @var{fd}, size_t @var{offset})
+This host hook is called when a PCH file is about to be loaded.
+We want to load @var{size} bytes from @var{fd} at @var{offset}
+into memory at @var{address}. The given address will be the result of
+a previous invocation of @code{HOST_HOOKS_GT_PCH_GET_ADDRESS}.
+Return @minus{}1 if we couldn't allocate @var{size} bytes at @var{address}.
+Return 0 if the memory is allocated but the data is not loaded. Return 1
+if the hook has performed everything.
+
+If the implementation uses reserved address space, free any reserved
+space beyond @var{size}, regardless of the return value. If no PCH will
+be loaded, this hook may be called with @var{size} zero, in which case
+all reserved address space should be freed.
+
+Do not try to handle values of @var{address} that could not have been
+returned by this executable; just return @minus{}1. Such values usually
+indicate an out-of-date PCH file (built by some other GCC executable),
+and such a PCH file won't work.
+@end deftypefn
+
+@deftypefn {Host Hook} size_t HOST_HOOKS_GT_PCH_ALLOC_GRANULARITY (void);
+This host hook returns the alignment required for allocating virtual
+memory. Usually this is the same as getpagesize, but on some hosts the
+alignment for reserving memory differs from the pagesize for committing
+memory.
+@end deftypefn
+
+@node Filesystem
+@section Host Filesystem