Date: Fri, 13 Mar 2026 13:10:41 +0000 From: Aymeric Wibo <obiwac@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 4da237aee328 - main - alloca.3: Add entry about defining VLAs in same block as alloca() to BUGS Message-ID: <69b40cd1.27eba.de1cc3f@gitrepo.freebsd.org>
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The branch main has been updated by obiwac: URL: https://cgit.FreeBSD.org/src/commit/?id=4da237aee328f368cd85b659854c4556a39f15ef commit 4da237aee328f368cd85b659854c4556a39f15ef Author: Aymeric Wibo <obiwac@FreeBSD.org> AuthorDate: 2026-02-19 14:47:06 +0000 Commit: Aymeric Wibo <obiwac@FreeBSD.org> CommitDate: 2026-03-13 13:08:31 +0000 alloca.3: Add entry about defining VLAs in same block as alloca() to BUGS Refer to alloca() as a (builtin) function or macro, as it could be defined as either depending on the compiler. Paragraph about bug comes from Darwin's libc, and example added to illustrate it. Reviewed by: bnovkov Approved by: bnovkov MFC after: 3 days Obtained from: https://github.com/apple-oss-distributions/libc (partially) Sponsored by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D55370 --- share/man/man3/alloca.3 | 40 +++++++++++++++++++++++++++++----------- 1 file changed, 29 insertions(+), 11 deletions(-) diff --git a/share/man/man3/alloca.3 b/share/man/man3/alloca.3 index fd88014dbb33..9e905d4487f3 100644 --- a/share/man/man3/alloca.3 +++ b/share/man/man3/alloca.3 @@ -1,5 +1,6 @@ .\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. +.\" Copyright (c) 2026 Aymeric Wibo <obiwac@freebsd.org> .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions @@ -25,7 +26,7 @@ .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" -.Dd April 29, 2025 +.Dd February 19, 2026 .Dt ALLOCA 3 .Os .Sh NAME @@ -38,16 +39,14 @@ .Sh DESCRIPTION The .Fn alloca -function -allocates +function or macro allocates .Fa size bytes of space in the stack frame of the caller. This temporary space is automatically freed on return. .Sh RETURN VALUES -The .Fn alloca -function returns a pointer to the beginning of the allocated space. +returns a pointer to the beginning of the allocated space. .Sh SEE ALSO .Xr brk 2 , .Xr calloc 3 , @@ -55,24 +54,20 @@ function returns a pointer to the beginning of the allocated space. .Xr malloc 3 , .Xr realloc 3 .Sh HISTORY -The .Fn alloca -function appeared in +appeared in .At 32v . .\" .Bx ?? . .\" The function appeared in 32v, pwb and pwb.2 and in 3bsd 4bsd .\" The first man page (or link to a man page that I can find at the .\" moment is 4.3... .Sh BUGS -The .Fn alloca -function is machine and compiler dependent; its use is discouraged. .Pp -The .Fn alloca -function is slightly unsafe because it cannot ensure that the pointer +is slightly unsafe because it cannot ensure that the pointer returned points to a valid and usable block of memory. The allocation made may exceed the bounds of the stack, or even go further into other objects in memory, and @@ -81,3 +76,26 @@ cannot determine such an error. Avoid .Fn alloca with large unbounded allocations. +.Pp +The use of C99 variable-length arrays and +.Fn alloca +in the same function will cause the lifetime of +.Fn alloca Ns 's +storage to be limited to the block containing the +.Fn alloca . +For example, in the following snippet, +.Va p Ns 's +lifetime does not extend outside of the block, whereas it would've if +.Va vla +hadn't been defined or had been defined as a fixed-length array: +.Bd -literal -offset indent +char *p; +{ + const int n = 100; + int vla[n]; + p = alloca(32); + strcpy(p, "Hello, world!"); + printf("Inside: %s\\n", p); /* Valid. */ +} +printf("Outside: %s\\n", p); /* Undefined. */ +.Edhome | help
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