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Message-ID: <fmb53bqjtrwkt7kfe72mbuezhel5yuvrnfge2p6hh3jknv5acf@ugwdhihbxrxn>
Date: Fri, 20 Jun 2025 23:26:29 +0200
From: Alejandro Colomar <alx@...nel.org>
To: libc-alpha@...rceware.org
Cc: 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>, Mark Harris <mark.hsj@...il.com>,
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
Subject: alx-0029r1 - Restore the traditional realloc(3) specification
Hi!
After the useful discussion with Eric and Paul, I've rewritten a draft
of a proposal I had for realloc(3) for C2y. Here it is (see below).
I'll present it here before presenting it to the C Committee (although
several members are CCd).
This time, I opted for an all-in-one change that puts us in the end
goal, since some people were concerned that step-by-step might be less
feasible. Also, the wording is more consistent doing this at once, and
people know what to expect from the begining.
Have a lovely day!
Alex
---
Name
alx-0029r1 - 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.
- 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: Laurent Bercot <ska-dietlibc@...rnet.org>
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>
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.
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://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>
Description
Let's start by quoting the author of realloc(3).
On 2024-10-18 05:30, Douglas McIlroy wrote:
> The discussion has taken a turn that's astonishing to one who
> doesn't know the inside details of real compilers.
>
> Regardless of the behavior of malloc(0), one expects this
> theorem to hold:
>
> Given that p = malloc(n) is not NULL,
> that 0<=m<=n,
> and that malloc(m) could in some circumstance
> return a non-null pointer,
> then realloc(p,m) will return a non-null pointer.
>
> REALLOC_ZERO_BYTES_FREES flies in the face of this rational
> expectation about dynamic storage allocation. A diabolical
> invention.
>
> Doug
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.
None of the specifications have been good, and C23 finally gave
up and made it undefined 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.
Code written for platforms returning a null 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.
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);
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.
If the realloc(3) specification was changed to require that
realloc(p,0) returns non-null on success, and that realloc(p,0)
only fails when out-of-memory, and to require that it sets
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).
This makes handling of realloc(3) as straightforward as one
would expect, with only two states: success or error.
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 an .
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
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.
Design decisions
This change needs three changes, which can be applied both at
once, or in two 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.
The first step 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.
The second step would require changes to (at least) the
following implementations: AIX.
The third step would be to require that on error, errno is set
to ENOMEM.
This proposal has merged all steps into a single proposal.
This proposal also needs to add ENOMEM to the standard, since it
hasn't been standardized yet.
Future directions
This proposal, by specifying realloc(3) as-if by calling
free(3) and malloc(3), makes it 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
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.
Proposed wording
Based on N3550.
7.5 Errors <errno.h>
## Add ENOMEM in p2.
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,
+and the value of the macro <b>ENOMEM</b>
+is stored in <b>errno</b>.
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,
+and the value of the macro <b>ENOMEM</b>
+is stored in <b>errno</b>.
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,
+and the value of the macro <b>ENOMEM</b>
+is stored in <b>errno</b>.
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,
-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.
@@ 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
+is returned
+and the value of the macro <b>ENOMEM</b>
+is stored in <b>errno</b>.
--
<https://www.alejandro-colomar.es/>
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