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Various rewrites and additions (including notes on longjmp() and SIGSEGV). 

Weaken warning against use of alloca(), as per http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=461100.
This commit is contained in:
Michael Kerrisk 2008-01-24 08:25:10 +00:00
parent 2dec881bfe
commit 6c3be22150
1 changed files with 45 additions and 19 deletions
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64
man3/alloca.3
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@ -34,8 +34,11 @@
.\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2002-07-17, aeb
.\" 2008-01-24, mtk:
.\" Various rewrites and additions (notes on longjmp() and SIGSEGV).
.\" Weaken warning against use of alloca() (as per Debian bug 461100).
.\"
.TH ALLOCA 3 2002-07-17 "GNU" "Linux Programmer's Manual"
.TH ALLOCA 3 2008-01-24 "GNU" "Linux Programmer's Manual"
.SH NAME
alloca \- memory allocator
.SH SYNOPSIS
@ -58,21 +61,48 @@ The
function returns a pointer to the beginning of the allocated space.
If the allocation causes stack overflow, program behavior is undefined.
.SH "CONFORMING TO"
This function is not in POSIX.1-2001.
There is evidence that the
.BR alloca ()
function appeared in 32v, pwb, pwb.2, 3bsd, and 4bsd.
function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.
There is a man page for it in 4.3BSD.
Linux uses the GNU version.
This function is not in POSIX.1-2001.
.SH NOTES
The
.BR alloca ()
function is machine- and compiler-dependent.
Its use is discouraged.
Because the space allocated by
.BR alloca ()
is allocated within the stack frame,
that space is automatically freed if the function return
is jumped over by a call to
.BR longjmp (3)
or
.BR siglongjmp (3).
Do not attempt to
.BR free (3)
space allocated by
.BR alloca ()!
.SS Notes on the GNU Version
Normally,
.B gcc (1)
.BR gcc (1)
translates calls to
.BR alloca ()
by inlined code.
This is not done when either the \fI\-ansi\fP or
the \fI\-fno\-builtin\fP option is given.
with inlined code.
This is not done when either the
.IR "\-ansi" ,
.IR "\-std=c89" ,
.IR "\-std=c99" ,
or the
.IR "\-fno\-builtin"
option is given
(and the header
.I <alloca.h>
is not included).
But beware!
By default the glibc version of
.I <stdlib.h>
@ -86,7 +116,7 @@ and that contains the line:
.fi
with messy consequences if one has a private version of this function.
.LP
The fact that the code is inlined, means that it is impossible
The fact that the code is inlined means that it is impossible
to take the address of this function, or to change its behavior
by linking with a different library.
.LP
@ -94,13 +124,11 @@ The inlined code often consists of a single instruction adjusting
the stack pointer, and does not check for stack overflow.
Thus, there is no NULL error return.
.SH BUGS
The
.BR alloca ()
function is machine- and compiler- dependent.
On many systems
its implementation is buggy.
Its use is discouraged.
.LP
There is no error indication if the stack frame cannot be extended.
(However, after a failed allocation, the program is likely to receive a
.B SIGSEGV
signal if it attempts to access the unallocated space.)
On many systems
.BR alloca ()
cannot be used inside the list of arguments of a function call, because
@ -110,7 +138,5 @@ would appear on the stack in the middle of the space for the
function arguments.
.SH "SEE ALSO"
.BR brk (2),
.BR getpagesize (2),
.BR calloc (3),
.BR malloc (3),
.BR realloc (3)
.BR longjmp (3),
.BR malloc (3)