man3/alloca.3

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  32 .\"     @(#)alloca.3    5.1 (Berkeley) 5/2/91
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  34 .\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
  35 .\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
  36 .\" Modified 2002-07-17, aeb
  37 .\" 2008-01-24, mtk:
  38 .\"     Various rewrites and additions (notes on longjmp() and SIGSEGV).
  39 .\"     Weaken warning against use of alloca() (as per Debian bug 461100).
  40 .\"
  41 .TH ALLOCA 3 2008-01-24 "GNU" "Linux Programmer's Manual"
  42 .SH NAME
  43 alloca \- memory allocator
  44 .SH SYNOPSIS
  45 .B #include <alloca.h>
  46 .sp
  47 .BI "void *alloca(size_t " size );
  48 .SH DESCRIPTION
  49 The
  50 .BR alloca ()
  51 function allocates
  52 .I size
  53 bytes of space in the stack frame of the caller.
  54 This temporary space is
  55 automatically freed when the function that called
  56 .BR alloca ()
  57 returns to its caller.
  58 .SH "RETURN VALUE"
  59 The
  60 .BR alloca ()
  61 function returns a pointer to the beginning of the allocated space.
  62 If the allocation causes stack overflow, program behavior is undefined.
  63 .SH "CONFORMING TO"
  64 This function is not in POSIX.1-2001.
  65
  66 There is evidence that the
  67 .BR alloca ()
  68 function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.
  69 There is a man page for it in 4.3BSD.
  70 Linux uses the GNU version.
  71 .SH NOTES
  72 The
  73 .BR alloca ()
  74 function is machine- and compiler-dependent.
  75 Its use is discouraged.
  76
  77 Because the space allocated by
  78 .BR alloca ()
  79 is allocated within the stack frame,
  80 that space is automatically freed if the function return
  81 is jumped over by a call to
  82 .BR longjmp (3)
  83 or
  84 .BR siglongjmp (3).
  85
  86 Do not attempt to
  87 .BR free (3)
  88 space allocated by
  89 .BR alloca ()!
  90 .SS Notes on the GNU Version
  91 Normally,
  92 .BR gcc (1)
  93 translates calls to
  94 .BR alloca ()
  95 with inlined code.
  96 This is not done when either the
  97 .IR "\-ansi" ,
  98 .IR "\-std=c89" ,
  99 .IR "\-std=c99" ,
 100 or the
 101 .IR "\-fno\-builtin"
 102 option is given
 103 (and the header
 104 .I <alloca.h>
 105 is not included).
 106 But beware!
 107 By default the glibc version of
 108 .I <stdlib.h>
 109 includes
 110 .I <alloca.h>
 111 and that contains the line:
 112 .nf
 113
 114     #define alloca(size)   __builtin_alloca (size)
 115
 116 .fi
 117 with messy consequences if one has a private version of this function.
 118 .LP
 119 The fact that the code is inlined means that it is impossible
 120 to take the address of this function, or to change its behavior
 121 by linking with a different library.
 122 .LP
 123 The inlined code often consists of a single instruction adjusting
 124 the stack pointer, and does not check for stack overflow.
 125 Thus, there is no NULL error return.
 126 .SH BUGS
 127 There is no error indication if the stack frame cannot be extended.
 128 (However, after a failed allocation, the program is likely to receive a
 129 .B SIGSEGV
 130 signal if it attempts to access the unallocated space.)
 131
 132 On many systems
 133 .BR alloca ()
 134 cannot be used inside the list of arguments of a function call, because
 135 the stack space reserved by
 136 .BR alloca ()
 137 would appear on the stack in the middle of the space for the
 138 function arguments.
 139 .SH "SEE ALSO"
 140 .BR brk (2),
 141 .BR longjmp (3),
 142 .BR malloc (3)