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 \- allocate memory that is automatically freed
  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 For certain applications,
  76 its use can improve efficiency compared to the use of
  77 .BR malloc (3)
  78 plus
  79 .BR free (3).
  80 In certain cases,
  81 it can also simplify memory deallocation in applications that use
  82 .BR longjmp (3)
  83 or
  84 .BR siglongjmp (3).
  85 Otherwise, its use is discouraged.
  86
  87 Because the space allocated by
  88 .BR alloca ()
  89 is allocated within the stack frame,
  90 that space is automatically freed if the function return
  91 is jumped over by a call to
  92 .BR longjmp (3)
  93 or
  94 .BR siglongjmp (3).
  95
  96 Do not attempt to
  97 .BR free (3)
  98 space allocated by
  99 .BR alloca ()!
 100 .SS Notes on the GNU version
 101 Normally,
 102 .BR gcc (1)
 103 translates calls to
 104 .BR alloca ()
 105 with inlined code.
 106 This is not done when either the
 107 .IR "\-ansi" ,
 108 .IR "\-std=c89" ,
 109 .IR "\-std=c99" ,
 110 or the
 111 .IR "\-fno\-builtin"
 112 option is given
 113 (and the header
 114 .I <alloca.h>
 115 is not included).
 116 But beware!
 117 By default the glibc version of
 118 .I <stdlib.h>
 119 includes
 120 .I <alloca.h>
 121 and that contains the line:
 122 .nf
 123
 124     #define alloca(size)   __builtin_alloca (size)
 125
 126 .fi
 127 with messy consequences if one has a private version of this function.
 128 .LP
 129 The fact that the code is inlined means that it is impossible
 130 to take the address of this function, or to change its behavior
 131 by linking with a different library.
 132 .LP
 133 The inlined code often consists of a single instruction adjusting
 134 the stack pointer, and does not check for stack overflow.
 135 Thus, there is no NULL error return.
 136 .SH BUGS
 137 There is no error indication if the stack frame cannot be extended.
 138 (However, after a failed allocation, the program is likely to receive a
 139 .B SIGSEGV
 140 signal if it attempts to access the unallocated space.)
 141
 142 On many systems
 143 .BR alloca ()
 144 cannot be used inside the list of arguments of a function call, because
 145 the stack space reserved by
 146 .BR alloca ()
 147 would appear on the stack in the middle of the space for the
 148 function arguments.
 149 .SH SEE ALSO
 150 .BR brk (2),
 151 .BR longjmp (3),
 152 .BR malloc (3)