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Document realloc (p, 0) gotchas

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* doc/autoconf.texi (Function Portability, Particular Functions):
Don’t recommend realloc-gnu as it doesn’t (and probably shouldn’t)
enforce glibc compatibility.  Document variance in glibc behavior.
This commit is contained in:
Paul Eggert 2024-10-26 16:29:49 -07:00
parent 53c23bb4e1
commit 36fa24859f
1 changed files with 6 additions and 7 deletions
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doc/autoconf.texi
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@ -4859,8 +4859,7 @@ The C standard says @code{realloc (NULL, 0)} is equivalent to
has succeeded if it returns a null pointer. If @code{ptr} is non-null,
the C standard says @code{realloc (ptr, 0)} has undefined behavior.
The @code{AC_FUNC_REALLOC} macro avoids some of these portability issues,
and the Gnulib module @code{realloc-gnu} avoids more of them.
The @code{AC_FUNC_REALLOC} macro avoids some of these portability issues.
@xref{Particular Functions}.
@item @code{signal} handler
@ -5489,9 +5488,8 @@ Gnulib's @code{obstack} module. @xref{Gnulib}.
@c @fuindex realloc
@prindex @code{realloc}
@caindex func_realloc_0_nonnull
If the @code{realloc} function is compatible with the GNU C
library @code{realloc} (i.e., @samp{realloc (NULL, 0)} returns a
valid pointer), define @code{HAVE_REALLOC} to 1. Otherwise define
If a successful call to @samp{realloc (NULL, 0)} returns a
non-null pointer, define @code{HAVE_REALLOC} to 1. Otherwise define
@code{HAVE_REALLOC} to 0, ask for an @code{AC_LIBOBJ} replacement for
@samp{realloc}, and define @code{realloc} to @code{rpl_realloc} so that
the native @code{realloc} is not used in the main project. See
@ -5500,8 +5498,9 @@ the native @code{realloc} is not used in the main project. See
The result of this macro is cached in the
@code{ac_cv_func_realloc_0_nonnull} variable.
If you don't want to maintain a @code{realloc.c} file in your package
manually, you can instead use the Gnulib module @code{realloc-gnu}.
This macro does not check compatibility with glibc @code{realloc (@var{p}, 0)}
when @var{p} is non-null, as glibc 1--2.1 behaves differently from glibc
2.1.1--2.40 (at least), and the C standard says behavior is undefined.
@end defmac
@defmac AC_FUNC_SELECT_ARGTYPES