115 lines
4.2 KiB
Plaintext
115 lines
4.2 KiB
Plaintext
@node realloc
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@subsection @code{realloc}
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@findex realloc
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POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9799919799/functions/realloc.html}
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Gnulib module: realloc-posix
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@mindex realloc-posix
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Portability problems fixed by Gnulib:
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@itemize
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@item
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On some platforms, when the function fails it does not set @code{errno}:
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mingw, MSVC 14.
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@item
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On some platforms, when the function fails it might set @code{errno}
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to @code{EAGAIN} instead of to @code{ENOMEM}. Although POSIX allows
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@code{EAGAIN}, the @code{realloc-posix} module insists on @code{ENOMEM}
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which also conforms to POSIX and is GNU-compatible:
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Solaris 11.4.
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@item
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On some platforms, @code{realloc (p, n)} can succeed even if @code{n}
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exceeds @code{PTRDIFF_MAX}. Although this behavior is arguably
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allowed by POSIX it is not compatible with GNU and
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can lead to behavior not defined by POSIX later,
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so @code{realloc-posix} does not allow going over the limit.
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@item
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It is not portable to call @code{realloc} with a size of 0.
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With a null pointer argument, this is the same ambiguity as @code{malloc (0)}
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as to whether a successful call returns a null pointer or a pointer to a
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new zero-sized memory region. Although both behaviors conform to POSIX,
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@code{realloc-posix} insists on the latter behavior, which is
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compatible with GNU.
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With non-null @code{p}, behavior is a real mess for @code{realloc (p, 0)}.
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C23 says behavior is undefined.
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C89 through C17 say a successful call returns either a null pointer
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or a pointer to a new zero-sized memory region.
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POSIX.1-2017 extends C99 by saying that if a successful call
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returns a null pointer it
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must also set @code{errno} to an implementation-defined value,
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and POSIX.1-2024 extends C17 a bit further by saying that
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such a call must set @code{errno} to @code{EINVAL}.
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It is not known what POSIX.1-2024's successor will say,
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though presumably it will extend C23.
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In practice, successful @code{realloc (p, 0)} calls
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have one of the following behaviors:
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@enumerate
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@item
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Free @code{p}, do not change @code{errno}, and return a null pointer:
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glibc 2.33--2.40 by default, Android.
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@item
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Free @code{p}, possibly set @code{errno}, and return a null pointer:
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glibc 1--2.1 if specially built with @code{REALLOC_ZERO_BYTES_FREES=1},
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glibc 2.1.1--2.32 by default,
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mingw, MSVC 14.
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@item
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Free @code{p}, set @code{errno} to @code{EINVAL}, and return a null pointer:
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AIX 7.3 without @code{_LINUX_SOURCE_COMPAT}.
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@item
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Free @code{p} and return a pointer to a new region of size zero:
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AIX 7.3 with @code{_LINUX_SOURCE_COMPAT}, glibc 1--2.1 by default,
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glibc 2.1.1--2.40 if specially built with @code{REALLOC_ZERO_BYTES_FREES=0},
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musl libc, macOS, FreeBSD, NetBSD, OpenBSD, Solaris, Cygwin.
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@end enumerate
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@noindent
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Behaviors (3) and (4) conform to POSIX; behaviors (1) and (2) do not.
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The @code{realloc-posix} module insists on behavior (4) as it is more
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popular than (3) and is more useful in practice. Behavior (4) is the
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only one of the four in which returning a null pointer means failure,
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which is what non-expert programmers typically expect.
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A program not suspecting these variations in semantics will either:
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@itemize
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@item
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Leak memory (the still-valid @code{p}) if it unwisely does not check
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for @code{realloc} failure, when it assumes behavior (1), (2) or (3) but the
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system implements behavior (4).
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@item
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Falsely respond to memory exhaustion (if it wisely checks for @code{realloc}
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failure), or have double-free bugs (if it unwisely does not check),
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when it assumes behavior (4) but the system implements (1), (2) or (3).
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@end itemize
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@item
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When @code{realloc (p, 0)} sets errno to @code{EINVAL} and returns null,
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a POSIX.1-2024 application cannot tell whether the call has succeeded
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and freed @code{p}, or has failed without freeing @code{p}.
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This is only a theoretical problem, though, as the only practical
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implementation with this behavior is AIX without @code{_LINUX_SOURCE_COMPAT},
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which behaves this way only when the call succeeds.
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@end itemize
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Portability problems not fixed by Gnulib:
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@itemize
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@item
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When not growing an already-allocated region, i.e.,
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when @code{p} points to a region of size @code{psize} and @code{n <= psize},
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@code{realloc (p, n)} can fail and return a null pointer:
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glibc 2.40 and probably other platforms.
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@end itemize
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