mirror of https://github.com/mkerrisk/man-pages
153 lines
4.9 KiB
Groff
153 lines
4.9 KiB
Groff
.\" Copyright (c) 1980, 1991 Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)alloca.3 5.1 (Berkeley) 5/2/91
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.\"
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.\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
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.\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 2002-07-17, aeb
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.\" 2008-01-24, mtk:
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.\" Various rewrites and additions (notes on longjmp() and SIGSEGV).
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.\" Weaken warning against use of alloca() (as per Debian bug 461100).
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.\"
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.TH ALLOCA 3 2008-01-24 "GNU" "Linux Programmer's Manual"
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.SH NAME
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alloca \- allocate memory that is automatically freed
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.SH SYNOPSIS
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.B #include <alloca.h>
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.sp
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.BI "void *alloca(size_t " size );
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.SH DESCRIPTION
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The
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.BR alloca ()
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function allocates
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.I size
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bytes of space in the stack frame of the caller.
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This temporary space is
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automatically freed when the function that called
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.BR alloca ()
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returns to its caller.
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.SH "RETURN VALUE"
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The
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.BR alloca ()
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function returns a pointer to the beginning of the allocated space.
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If the allocation causes stack overflow, program behavior is undefined.
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.SH "CONFORMING TO"
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This function is not in POSIX.1-2001.
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There is evidence that the
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.BR alloca ()
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function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.
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There is a man page for it in 4.3BSD.
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Linux uses the GNU version.
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.SH NOTES
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The
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.BR alloca ()
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function is machine- and compiler-dependent.
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For certain applications,
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its use can improve efficiency compared to the use of
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.BR malloc (3)
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plus
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.BR free (3).
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In certain cases,
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it can also simplify memory deallocation in applications that use
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.BR longjmp (3)
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or
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.BR siglongjmp (3).
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Otherwise, its use is discouraged.
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Because the space allocated by
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.BR alloca ()
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is allocated within the stack frame,
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that space is automatically freed if the function return
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is jumped over by a call to
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.BR longjmp (3)
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or
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.BR siglongjmp (3).
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Do not attempt to
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.BR free (3)
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space allocated by
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.BR alloca ()!
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.SS Notes on the GNU Version
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Normally,
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.BR gcc (1)
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translates calls to
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.BR alloca ()
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with inlined code.
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This is not done when either the
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.IR "\-ansi" ,
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.IR "\-std=c89" ,
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.IR "\-std=c99" ,
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or the
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.IR "\-fno\-builtin"
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option is given
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(and the header
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.I <alloca.h>
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is not included).
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But beware!
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By default the glibc version of
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.I <stdlib.h>
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includes
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.I <alloca.h>
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and that contains the line:
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.nf
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#define alloca(size) __builtin_alloca (size)
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.fi
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with messy consequences if one has a private version of this function.
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.LP
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The fact that the code is inlined means that it is impossible
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to take the address of this function, or to change its behavior
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by linking with a different library.
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.LP
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The inlined code often consists of a single instruction adjusting
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the stack pointer, and does not check for stack overflow.
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Thus, there is no NULL error return.
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.SH BUGS
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There is no error indication if the stack frame cannot be extended.
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(However, after a failed allocation, the program is likely to receive a
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.B SIGSEGV
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signal if it attempts to access the unallocated space.)
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On many systems
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.BR alloca ()
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cannot be used inside the list of arguments of a function call, because
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the stack space reserved by
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.BR alloca ()
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would appear on the stack in the middle of the space for the
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function arguments.
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.SH "SEE ALSO"
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.BR brk (2),
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.BR longjmp (3),
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.BR malloc (3)
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