@node realloc
@section @code{realloc}
@findex realloc

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/realloc.html}

Gnulib module: realloc-posix

Portability problems fixed by Gnulib:
@itemize
@item
Upon failure, the function does not set @code{errno} to @code{ENOMEM} on
some platforms:
mingw, MSVC 14.

@item
On some platforms, @code{realloc (p, n)} can succeed even if @code{n}
exceeds @code{PTRDIFF_MAX}.  Although this behavior is arguably
allowed by POSIX it can lead to behavior not defined by POSIX later,
so @code{realloc-posix} does not allow going over the limit.
@end itemize

Without the @samp{realloc-gnu} module described below, it is not portable
to call @code{realloc} with a size of 0.  With a
NULL pointer argument, this is the same ambiguity as @code{malloc (0)}
on whether a unique zero-size object is created.  With a non-NULL
pointer argument @code{p}, C17 says that it is implementation-defined
whether @code{realloc (p, 0)} frees @code{p}.
Behavior varies on whether @code{realloc (p, 0)} always frees @code{p}
and successfully returns a null pointer, or always
fails and leaves @code{p} valid, or usually succeeds and returns a
unique zero-size object; a program not suspecting these variations in
semantics will leak memory (either the still-valid @code{p}, or the
non-NULL return value).

Extension: Gnulib provides a module @samp{realloc-gnu} that substitutes a
@code{realloc} implementation that behaves more like the glibc implementation.
It fixes these portability problems:

@itemize
@item
@code{realloc (NULL, 0)} returns @code{NULL} on success on some platforms:
AIX 7.2.

@item
On some platforms, @code{realloc (p, 0)} with non-null @code{p}
might not free @code{p}, or might clobber @code{errno},
or might not return @code{NULL}.
@end itemize