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Message-ID: <CAMdZqKGoCO6xmzq1=+Oa_xe4emeKRxY6FEaN3cY6mW6mUb6ggw@mail.gmail.com>
Date: Thu, 26 Jun 2025 17:44:49 -0700
From: Mark Harris <mark.hsj@...il.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: libc-alpha@...rceware.org, bug-gnulib@....org, musl@...ts.openwall.com,
наб <nabijaczleweli@...ijaczleweli.xyz>,
Douglas McIlroy <douglas.mcilroy@...tmouth.edu>, Paul Eggert <eggert@...ucla.edu>,
Robert Seacord <rcseacord@...il.com>, Elliott Hughes <enh@...gle.com>, Bruno Haible <bruno@...sp.org>,
JeanHeyd Meneide <phdofthehouse@...il.com>, Rich Felker <dalias@...c.org>,
Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>, Joseph Myers <josmyers@...hat.com>,
Florian Weimer <fweimer@...hat.com>, Laurent Bercot <ska-dietlibc@...rnet.org>,
Andreas Schwab <schwab@...e.de>, Thorsten Glaser <tg@...bsd.de>, Eric Blake <eblake@...hat.com>,
Vincent Lefevre <vincent@...c17.net>, Collin Funk <collin.funk1@...il.com>,
Wilco Dijkstra <Wilco.Dijkstra@....com>, DJ Delorie <dj@...hat.com>,
Cristian Rodríguez <cristian@...riguez.im>,
Siddhesh Poyarekar <siddhesh@...plt.org>, Sam James <sam@...too.org>, Mark Wielaard <mark@...mp.org>,
"Maciej W. Rozycki" <macro@...hat.com>, Martin Uecker <ma.uecker@...il.com>,
Christopher Bazley <chris.bazley.wg14@...il.com>, eskil@...ession.se,
Daniel Krügler <daniel.kruegler@...glemail.com>,
Kees Cook <keescook@...omium.org>, Valdis Klētnieks <valdis.kletnieks@...edu>
Subject: Re: alx-0029r5 - Restore the traditional realloc(3) specification
Alejandro Colomar wrote:
>
> Hi!
>
> Here's a new revision addressing the suggestions by Eric in v4. I've
> added a new subsection in the Rationale explaining why not go the other
> way around, as people keep suggesting that every now and then.
>
> I'll submit this version to the C Committee today, since there seems to
> be more consensus now, and the recent iterations have seen only minor
> wording improvements, but no major changes. Of course, we can continue
> improving the paper, though. Please suggest any improvements you may
> consider appropriate.
>
> It would be good to have explicit replies by glibc maintainers about it,
> so that the C Committee understands better what the maintainers think
> about it. I've got word from some committee members that if I can
> convince the maintainers, they'll vote for standardizing it. So, it
> would be great it people could emit 'Acked-by:' tags, or otherwise
> explain their position.
>
>
> Have a lovely day!
> Alex
>
> ---
> Name
> alx-0029r5 - Restore the traditional realloc(3) specification
>
> Principles
> - Uphold the character of the language
> - Keep the language small and simple
> - Facilitate portability
> - Avoid ambiguities
> - Pay attention to performance
> - Codify existing practice to address evident deficiencies.
> - Do not prefer any implementation over others
> - Ease migration to newer language editions
> - Avoid quiet changes
> - Enable secure programming
>
> Category
> Remove UB.
>
> Author
> Alejandro Colomar <alx@...nel.org>
>
> Cc: <bug-gnulib@....org>
> Cc: <musl@...ts.openwall.com>
> Cc: <libc-alpha@...rceware.org>
> Cc: наб <nabijaczleweli@...ijaczleweli.xyz>
> Cc: Douglas McIlroy <douglas.mcilroy@...tmouth.edu>
> Cc: Paul Eggert <eggert@...ucla.edu>
> Cc: Robert Seacord <rcseacord@...il.com>
> Cc: Elliott Hughes <enh@...gle.com>
> Cc: Bruno Haible <bruno@...sp.org>
> Cc: JeanHeyd Meneide <phdofthehouse@...il.com>
> Cc: Rich Felker <dalias@...c.org>
> Cc: Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>
> Cc: Joseph Myers <josmyers@...hat.com>
> Cc: Florian Weimer <fweimer@...hat.com>
> Cc: Andreas Schwab <schwab@...e.de>
> Cc: Thorsten Glaser <tg@...bsd.de>
> Cc: Eric Blake <eblake@...hat.com>
> Cc: Vincent Lefevre <vincent@...c17.net>
> Cc: Mark Harris <mark.hsj@...il.com>
> Cc: Collin Funk <collin.funk1@...il.com>
> Cc: Wilco Dijkstra <Wilco.Dijkstra@....com>
> Cc: DJ Delorie <dj@...hat.com>
> Cc: Cristian Rodríguez <cristian@...riguez.im>
> Cc: Siddhesh Poyarekar <siddhesh@...plt.org>
> Cc: Sam James <sam@...too.org>
> Cc: Mark Wielaard <mark@...mp.org>
> Cc: "Maciej W. Rozycki" <macro@...hat.com>
> Cc: Martin Uecker <ma.uecker@...il.com>
> Cc: Christopher Bazley <chris.bazley.wg14@...il.com>
> Cc: <eskil@...ession.se>
> Cc: Daniel Krügler <daniel.kruegler@...glemail.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Valdis Klētnieks <valdis.kletnieks@...edu>
>
> History
> <https://www.alejandro-colomar.es/src/alx/alx/wg14/alx-0029.git/>
>
> r0 (2025-06-17):
> - Initial draft.
>
> r1 (2025-06-20):
> - Full rewrite after the recent glibc discussion.
>
> r2 (2025-06-21):
> - Remove CC. Add CC.
> - wfix.
> - Drop quote.
> - Add a few more principles
> - Clarify why ENOMEM is used in this proposal, and make it
> optional.
> - Mention exceptional leak in code checking (size != 0).
> - Clarify that part of the description of realloc can be
> editorially removed after this change.
>
> r3 (2025-06-23):
> - Fix diff missing line.
> - Remove ENOMEM from the proposal.
> - Clarify that ENOMEM should be retained by platforms already
> using it.
> - Add mention that LLVM's address sanitizer will catch the leak
> mentioned in r2.
> - Add links to real bugs (including an RCE bug).
>
> r4 (2025-06-24):
> - Use a better link for the Whatsapp RCE.
> - s/Description/Rationale/
> - wfix
> - Mention that glibc <2.1.1 had the BSD behavior.
> - Add footnote that realloc(3) may fail while shrinking.
>
> r5 (2025-06-26):
> - It was glibc 2.1.1 that broke it, not glibc 2.2.
> - wfix
> - Mention in the footnote that the pointer may change.
> - Document why not go the other way around. It was explained
> several times during discussion, but people keep suggesting
> it.
>
> See also
> <https://nabijaczleweli.xyz/content/blogn_t/017-malloc0.html>
> <https://sourceware.org/pipermail/libc-alpha/1999-April/000956.html>
> <https://inbox.sourceware.org/libc-alpha/20241019014002.3684656-1-siddhesh@sourceware.org/T/#u>
> <https://inbox.sourceware.org/libc-alpha/qukfe5yxycbl5v7ooskvqdnm3au3orohbx4babfltegi47iyly@or6dgf7akeqv/T/#u>
> <https://github.com/bminor/glibc/commit/7c2b945e1fd64e0a5a4dbd6ae6592a7314dcd4b5>
> <https://github.com/llvm/llvm-project/issues/113065>
> <https://www.austingroupbugs.net/view.php?id=400>
> <https://www.austingroupbugs.net/view.php?id=526>
> <https://www.austingroupbugs.net/view.php?id=688>
> <https://sourceware.org/bugzilla/show_bug.cgi?id=12547>
> <https://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_400.htm>
> <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n868.htm>
> <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2438.htm>
> <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2464.pdf>
> <https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/functions/realloc.html>
> <https://pubs.opengroup.org/onlinepubs/9699919799.2013edition/functions/realloc.html>
> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120744>
> <https://lore.kernel.org/lkml/20220213182443.4037039-1-keescook@chromium.org/>
> <https://awakened1712.github.io/hacking/hacking-whatsapp-gif-rce/>
> <https://gbhackers.com/whatsapp-double-free-vulnerability/>
>
> Rationale
> The specification of realloc(3) has been problematic since the
> very first standards, even before ISO C. The wording has
> changed significantly, trying to forcedly permit implementations
> to return a null pointer when the requested size is zero. This
> originated from the intent of banning zero-sized objects from
> the language in C89, but that never worked well in
> retrospective, as we can see from the fallout.
I support the outcome that you are trying to achieve, however I think
the proposal could use a better explanation upfront as to what problem
this is attempting to solve. Calling a well-established function like
realloc "problematic" without explaining what is problematic about it
isn't helpful. This states that the wording has changed
significantly, but if that was the problem then a proposal to change
the wording again is clearly not addressing that.
Also I'm pretty sure that this did not originate from the intent of
banning zero-sized objects from the language. If there was an actual
effort to ban the possibility of zero-sized objects then it would have
been easier for them to simply define malloc(0) and realloc(p, 0) to
always be an error. It seems far more likely that the committee was
just trying to document the current behavior that programmers could
rely on, without unnecessarily making current implementations
nonconforming.
>
> None of the specifications have been good, and C23 finally gave
> up and made it undefined behavior.
>
> The problem is not only theoretical. Programmers don't know how
> to use realloc(3) correctly, and have written weird code in
> their attempts. This has resulted in a lot of non-sensical code
> in configure scripts[1], and even bugs in actual programs[2].
>
> [1] <https://codesearch.debian.net/search?q=%5Cbrealloc%5B+%5Ct%5D*%5B%28%5D%5B%5E%2C%5D*%2C%5B+%5Ct%5D0%5B%29%5D&literal=0>
It's not clear what you are claiming is nonsensical here or why. You
think that configure checks are in general nonsensical, or what is it
about these checks in particular that is nonsensical?
> [2] <https://lore.kernel.org/lkml/20220213182443.4037039-1-keescook@chromium.org/>
>
> In some cases, this non-sensical code has resulted in RCEs[3].
>
> [3] <https://awakened1712.github.io/hacking/hacking-whatsapp-gif-rce/>
This is using reallocarray(), which comes from OpenBSD, and is not in
ISO C at all. In OpenBSD it already had the desired behavior for size
0; an implementation of the OpenBSD reallocarray() that does not
correctly match its behavior is just buggy. POSIX picked it up after
Glibc and Bionic already created an implementation that did not match
the OpenBSD behavior.
>
> However, this doesn't need to be like that. The traditional
> implementation of realloc(3), present in Unix V7, inherited by
> the BSDs, and currently available in range of systems, including
> musl libc, doesn't have any issues. glibc --which uses an
> independent implemention rather than a Unix derivative-- also
> had this behavior originally; it changed to the current behavior
> in 1999 (glibc 2.1.1), only for compatibility with C89, even
> though ironically C99 was released soon after and removed the
> text that glibc was trying to comply to, and introduced some new
> text that was very confusing, and one of its interpretations
> would make the new glibc behavior non-conforming.
s/in range/in a range/
s/implemention/implementation/
s/comply to/comply with/
Assuming that you are referring to Seventh Edition Unix from 1979, it
would be better to spell it out or write V7 Unix, to distinguish it
from UNIX V7 (https://unix.org/unixv7.html) which refers to Issue 7 of
the standard commonly known as POSIX.
Your claim that this 1979 implementation of realloc()[1] "doesn't have
any issues" is grossly incongruent with reality. This realloc(p, n)
first calls free(p), then malloc(n), and finally copies the data if
the pointer has changed; at the time this was safe because there was
no multithreading and free() and malloc() did not modify the contents
of the memory block, however if realloc() failed it returned NULL with
p freed (but you could still read the data from the freed block at
least until the next allocation). Also this implementation of
realloc() did not accept p == NULL; that would crash. However, a
documented bonus feature not found in modern realloc() was that p
could be "a block freed since the last call of malloc, realloc or
calloc".[2]
[1] https://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/libc/gen/malloc.c
[2] https://man.cat-v.org/unix_7th/3/malloc
Is this really the "traditional realloc(3) specification" that the
title of this proposal is suggesting be restored? I suggest being
more specific as to what you actually want.
>
> Code written for platforms returning a null pointer can be
> migrated to platforms returning non-null, without significant
> issues.
>
> There are two kinds of code that call realloc(p,0). One
> hard-codes the 0, and is used as a replacement of free(p). This
> code ignores the return value, since it's unimportant. This
> code currently produces a leak of 0 bytes plus associated
> metadata on platforms such as musl libc, where it returns a
> non-null pointer. However, assuming that there are programs
> written with the knowledge that they won't ever be run on such
> platforms, we should take care of that, and make sure they don't
> leak. A way of accomplishing this would be to recommend
> implementations to issue a diagnostic when realloc(3) is called
> with a hardcoded zero. This is only an informal recommendation
> made by this proposal, as this is a matter of QoI, and the
> standard shouldn't say anything about it. This would prevent
> this class of minor leaks.
It used to be a common thing for a C library or function to allow
overriding the allocator that it uses internally with your own custom
allocator. Rather than having to specify multiple function pointers
for your own custom malloc, free, realloc, etc. sometimes it would
just take one function pointer for your custom realloc, with the
default being libc realloc. The library would allocate memory using
(*customalloc)(NULL, n), free it using (*customalloc)(p, 0), and
realloc using (*customalloc)(p, n). This simplified the interface for
overriding the allocator, and if it stored the custom allocator in
each object then it only had to store one function pointer. The point
is that I would expect calls to realloc that intend to only free
memory are more likely to be indirect calls than direct calls that are
easily diagnosed at compile time or with a simple search; for a direct
call it is easier to just call free. That said, I haven't seen this
kind of interface for years.
Both Glibc and Bionic already annotate realloc with
__attribute__((__warn_unused_result__)), so that should already warn
about direct calls to realloc(p, 0) that don't use the result
(indicating that the caller was likely intending to only free the
memory and not get another block in return). I would consider that
existing warning to be better than warning on any realloc(p, 0),
because it avoids false positives when the caller is already handling
any pointer that may be returned.
>
> Moreover, in glibc, realloc(p,0) may return non-null, in the
> case where p is NULL, so code must already take that into
> account, and thus code that simply takes realloc(p,0) as a
> synonym of free(p) is already leaky, as free(NULL) is a no-op,
> but realloc(NULL,0) allocates 0 bytes.
>
> The other kind of code is in algorithms that realloc(3) an
> arbitrary size, which might eventually be zero. This gets more
> complex.
>
> Here's the code that should be written for AIX or glibc:
>
> errno = 0;
> new = realloc(old, size);
> if (new == NULL) {
> if (errno == ENOMEM)
> free(old);
> goto fail;
> }
> ...
> free(new);
That is ridiculous; I've never seen anyone else check errno to
determine whether realloc has failed. It is far easier to just ensure
that size is non-zero. Yes it would be even easier if you didn't need
to ensure that, which is why I support your proposal, but it doesn't
have anything to do with errno.
The primary benefit of malloc and realloc setting errno to ENOMEM on
failure is that if you are writing a function that reports errors in
errno you can just return -1 on malloc or realloc failure, or if you
are calling other functions that report errors in errno you can share
a common error handling path with them that logs the error from errno
and don't need a custom error handling path for logging memory
allocation failures.
>
> Failing to check for ENOMEM in these platforms before freeing
> the old pointer would result in a double-free. If the program
> decides to continue using the old pointer instead of freeing it,
> it would result in a use-after-free.
>
> In the platforms where realloc(p,0) returns non-null, such as
> the BSDs or musl libc, it is simpler to handle it:
>
> new = realloc(old, size);
> if (new == NULL) { // errno is ENOMEM
> free(old);
> goto fail;
> }
> ...
> free(new);
>
> Whenever the result is a null pointer, these platforms are
> reporting an ENOMEM error, and thus it is superfluous to check
> errno there.
>
> Most code is written in this way, even if run on platforms
> returning a null pointer. This is because most programmers are
> just unaware of this problem. Part of the reason is also that
> returning a non-null pointer with zero bytes is the natural
> extension of the behavior, which is what programmers intuitively
> expect from libc; that is, if realloc(p,3) allocates 3 bytes,
> r(p,2) allocates two bytes, and r(p,1) allocates one byte, it is
> natural by induction to expect that r(p,0) will allocate zero
> bytes. Most algorithms naturally extend to 0 just fine, and
> special casing 0 is artificial.
>
> If the realloc(3) specification were changed to require that
> realloc(p,0) returns non-null on success, and that realloc(p,0)
> only fails when out-of-memory (and assuming the implementations
> will continue setting errno to ENOMEM), then code written for
> AIX or glibc would continue working just fine, since the errno
> check would be redundant with the null check. Simply, the
> conditional (errno == ENOMEM) would always be true when
> (new == NULL).
>
> Then, there are non-POSIX platforms that don't set ENOMEM. In
> those platforms, code might do this:
>
> new = realloc(old, size);
> if (new == NULL) {
> if (size != 0)
> free(old);
> goto fail;
> }
> ...
> free(new);
>
> That code would continue working with this proposal, except for
> a very rare corner case, in which it would leak. In the normal
> case, (size != 0) would never be true under (new == NULL),
This is backwards.
s/never // ?
> because a reallocation of 0 bytes would almost always succeed,
> and thus not return a null pointer under this proposal.
> However, in some cases, the system might not find space even for
> the small metadata needed for a 0-byte allocation. In such
> case, the (size != 0) conditional would prevent deallocating
> 'old', and thus cause a memory leak. This case is exceptional
> enough that it shouldn't stop us from fixing realloc(3).
> Anyway, on an out-of-memory case, the program is likely to
> terminate rather soon, so the issue is even less likely to have
> an impact on any existing programs. Also, LLVM's address
> sanitizer will soon able to catch such a leak:
> <https://github.com/llvm/llvm-project/issues/113065>
>
> This proposal makes handling of realloc(3) as straightforward as
> one would expect, with only two states: success or error. There
> are no in-between states.
>
> The resulting wording in the standard is also much simpler, as
> it doesn't need to define so many special cases.
>
> For consistency, all the other allocation functions are updated
> to both return a null pointer on error, and use consistent
> wording.
>
> Why not go the other way around?
> Some people keep asking why not go the other way around: why not
> force the BSDs and musl to return a null pointer if size is 0.
> This would result in double-free and use-after-free bugs, which
> can result in RCE vulnerabilities (remote code execution), which
> is clearly unacceptable.
>
> Consider this code, which is the usual code for calling
> realloc(3) in such systems:
>
> new = realloc(old, size);
> if (new == NULL) {
> free(old);
> goto fail;
> }
> ...
> free(new);
>
> If realoc(p,0) would return a null pointer and free the old
s/realoc/realloc/
> block, then the third line would be a double-free bug.
>
> Prior art
> gnulib
> gnulib provides the realloc-posix module, which aims to wrap the
> system realloc(3) and reallocarray(3) functions so that they
> behave in a POSIX-complying manner.
>
> It previously behaved like glibc. After I reported that it was
> non-conforming to POSIX, we discussed the best way forward,
> which we agreed was the same direction that this paper is
> proposing now for C2y. The implementation was changed in
>
> gnulib.git d884e6fc4a60 (2024-11-04; "realloc-posix: realloc (..., 0) now returns nonnull")
>
> There have been no regression reports since then, as we
> expected.
>
> Unix V7, BSD
s/Unix V7/Seventh Edition Unix/
> The proposed behavior is the one endorsed by Doug McIlroy, the
> author of the original implementation of realloc(3) in Unix V7,
> and also present in the BSDs.
>
> glibc <= 2.1
> glibc was implemented originally to return non-null. It was
> only in 1999, and purely to comply with the standards --with no
> requests by users to do so--, that the glibc maintainers decided
> to switch to the current behavior.
>
> Design decisions
> This change needs two changes, which can be applied all at once,
> or in separate steps.
>
> The first step would make realloc(p,s) be consistent with
> free(p) and malloc(s), including when p is a null pointer, when
> s is zero, and also when both corner cases happen at the same
> time. This change would already turn the implementations where
> malloc(0) returns non-null into the end goal we have. This
> would require changes to (at least) the following
> implementations: glibc, Bionic, Windows.
>
> The second step would be to require that malloc(0) returns a
> non-null pointer. This would require changes to (at least) the
> following implementations: AIX.
I appreciate that you no longer claim that these are the ONLY
implementations that would require changes.
>
> This proposal has merged all steps into a single proposal.
>
> Future directions
> This proposal, by specifying realloc(3) as-if by calling
> free(3) and malloc(3), makes redundant several mentions of
> realloc(3) next to either free(3) or malloc(3) in the standard.
> We could remove them in this proposal, or clean up that in a
> separate (mostly editorial) proposal. Let's keep it for a
> future proposal for now.
>
> Caveats
> n?n:1
> Code written today should be careful, in case it can run on
> older systems that are not fixed to comply with this stricter
> specification. Thus, code written today should call realloc(3)
> similar to this:
>
> realloc(p, n?n:1);
>
> When all existing implementations are fixed to comply with this
> stricter specification, that workaround can be removed.
>
> ENOMEM
> Existing implementations that set errno to ENOMEM must continue
> doing so when the input pointer is not freed. If they didn't,
> code that is currently portable to all POSIX systems
>
> errno = 0;
> new = realloc(old, size);
> if (new == NULL) {
> if (errno == ENOMEM)
> free(old);
> goto fail;
> }
> ...
> free(new);
>
> would leak on error.
>
> Since it is currently impossible to write code today that is
> portable to arbitrary C17 systems, this is not an issue in
> ISO C.
Current code that calls realloc in a manner that is portable to
arbitrary C17 or C23 systems will already ensure that the size
argument is non-zero (for example, using the n?n:1 method above).
Such code is very possible and requires no changes.
>
> - New code written for C2y will only need to check for
> NULL to detect errors.
>
> - Code written for specific C17 and older platforms
> that don't set errno will continue to work for those
> specific platforms.
>
> - Code written for POSIX.1-2024 and older platforms
> will continue working on POSIX C2y platforms,
> assuming that POSIX will continue mandating ENOMEM.
>
> - Code written for POSIX.1-2024 and older will not be
> able to be run on non-POSIX C2y platforms, but that
> could be expected.
I don't see how this is relevant to this proposal.
>
> The only important thing is that platforms that did set ENOMEM
> should continue setting it, to avoid introducing leaks.
>
> Proposed wording
> Based on N3550.
>
> 7.25.4.1 Memory management functions :: General
> @@ p1
> ...
> If the size of the space requested is zero,
> -the behavior is implementation-defined:
> -either
> -a null pointer is returned to indicate the error,
> -or
> the behavior is as if the size were some nonzero value,
> except that the returned pointer shall not be used
> to access an object.
>
> 7.25.4.2 The aligned_alloc function
> @@ Returns, p3
> The <b>aligned_alloc</b> function returns
> -either
> -a null pointer
> -or
> -a pointer to the allocated space.
> +a pointer to the allocated space
> +on success.
> +If
> +the space cannot be allocated,
> +a null pointer is returned.
>
> 7.25.4.3 The calloc function
> @@ Returns, p3
> The <b>calloc</b> function returns
> -either
> a pointer to the allocated space
> +on success.
> -or a null pointer
> -if
> +If
> the space cannot be allocated
> or if the product <tt>nmemb * size</tt>
> -would wraparound <b>size_t</b>.
> +would wraparound <b>size_t</b>,
> +a null pointer is returned.
>
> 7.25.4.7 The malloc function
> @@ Returns, p3
> The <b>malloc</b> function returns
> -either
> -a null pointer
> -or
> -a pointer to the allocated space.
> +a pointer to the allocated space
> +on success.
> +If
> +the space cannot be allocated,
> +a null pointer is returned.
>
> 7.25.4.8 The realloc function
> @@ Description, p2
> The <b>realloc</b> function
> deallocates the old object pointed to by <tt>ptr</tt>
> +as if by a call to <b>free</b>,
> and returns a pointer to a new object
> -that has the size specified by <tt>size</tt>.
> +that has the size specified by <tt>size</tt>
> +as if by a call to <b>malloc</b>.
> The contents of the new object
> shall be the same as that of the old object prior to deallocation,
> up to the lesser of the new and old sizes.
> Any bytes in the new object
> beyond the size of the old object
> have unspecified values.
>
> @@ p3
> If <tt>ptr</tt> is a null pointer,
> the <b>realloc</b> function behaves
> like the <b>malloc</b> function for the specified size.
> Otherwise,
> if <tt>ptr</tt> does not match a pointer
> earlier returned by a memory management function,
> or
> if the space has been deallocated
> by a call to the <b>free</b> or <b>realloc</b> function,
> ## We can probably remove all of the above, because of the
> ## behavior now being defined as-if by calls to malloc(3) and
> ## free(3). But let's do that editorially in a separate change.
> -or
> -if the size is zero,
> ## We're defining the behavior.
> the behavior is undefined.
> If
> -memory for the new object is not allocated,
> +the space cannot be allocated,
> ## Editorial; for consistency with the wording of the other functions.
> the old object is not deallocated
> and its value is unchanged.
> +XXX)
>
> @@ New footnote XXX
> +XXX)
> +While atypical,
> +<b>realloc</b> may fail
> +or return a different pointer
> +for a call that shrinks the block of memory.
>
> @@ Returns, p4
> The <b>realloc</b> function returns
> a pointer to the new object
> (which can have the same value
> -as a pointer to the old object),
> +as a pointer to the old object)
> +on success.
> -or
> +If
> +space cannot be allocated,
> a null pointer
> -if the new object has not been allocated.
> +is returned.
>
The actual changes seem quite reasonable, for those that make it this far.
- Mark
> --
> <https://www.alejandro-colomar.es/>
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