Re: [PATCH, BUG 211039] malloc.3: Document that realloc(p, 0) is specific to glibc and nonportable
From: Johannes Pfister <hidden>
Date: 2021-01-11 10:14:04
A more detailed notice is on realloc(3p).
Yes. But i think it will lead to bugs when there is a documentation that describes the behavior of realloc(), says realloc(ptr,0) will do free(ptr), says realloc() is conforming to POSIX.1-2001, POSIX.1-2008, C89, C99. But does not mention that the realloc(ptr,0) is not specified in this standards (except C89). And there are some distributions that do not include the realloc(3p) man page. On my Debian Buster (10) there is no realloc(3p) man page and man realloc goes to the malloc man page of the Linux Programmer's Manual. But maybe this is a problem of the distributions/Debian?
Thanks for the report, Johannes! Please review that your name is correct (I guessed it from the email).
Yes it is. Should i configure my name somewhere? Kind Regards Johannes Am Sa., 9. Jan. 2021 um 21:20 Uhr schrieb Alejandro Colomar [off-list ref]:
quoted hunk ↗ jump to hunk
A more detailed notice is on realloc(3p). ...... $ man 3p realloc \ |sed -n \ -e '/APPLICATION USAGE/,/^$/p' \ -e '/FUTURE DIRECTIONS/,/^$/p'; APPLICATION USAGE The description of realloc() has been modified from pre‐ vious versions of this standard to align with the ISO/IEC 9899:1999 standard. Previous versions explicitly permitted a call to realloc(p, 0) to free the space pointed to by p and return a null pointer. While this be‐ havior could be interpreted as permitted by this version of the standard, the C language committee have indicated that this interpretation is incorrect. Applications should assume that if realloc() returns a null pointer, the space pointed to by p has not been freed. Since this could lead to double-frees, implementations should also set errno if a null pointer actually indicates a failure, and applications should only free the space if errno was changed. FUTURE DIRECTIONS This standard defers to the ISO C standard. While that standard currently has language that might permit real‐ loc(p, 0), where p is not a null pointer, to free p while still returning a null pointer, the committee responsible for that standard is considering clarifying the language to explicitly prohibit that alternative. Bug: 211039 <https://bugzilla.kernel.org/show_bug.cgi?id=211039> Reported-by: Johannes Pfister <redacted> Cc: libc-alpha@sourceware.org Signed-off-by: Alejandro Colomar <redacted> --- Hi Johannes, Michael, Thanks for the report, Johannes! Please review that your name is correct (I guessed it from the email). Michael, please review the wording. Thanks, Alex man3/malloc.3 | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)diff --git a/man3/malloc.3 b/man3/malloc.3 index d8b4da62f..467e2438a 100644 --- a/man3/malloc.3 +++ b/man3/malloc.3@@ -149,7 +149,8 @@ is equal to zero, and .I ptr is not NULL, then the call is equivalent to -.IR free(ptr) . +.I free(ptr) +(this behavior is nonportable; see NOTES). Unless .I ptr is NULL, it must have been returned by an earlier call to@@ -375,6 +376,21 @@ The implementation is tunable via environment variables; see .BR mallopt (3) for details. +.SS Nonportable behavior +The behavior of +.BR realloc () +when +.I size +is equal to zero, +and +.I ptr +is not NULL, +is glibc specific; +other implementations may return NULL, and set +.IR errno . +Portable POSIX programs should avoid it. +See +.BR realloc (3p). .SH SEE ALSO .\" http://g.oswego.edu/dl/html/malloc.html .\" A Memory Allocator - by Doug Lea --2.30.0