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 .\"
  38 .TH ALLOCA 3  2002-07-17 "GNU" "Linux Programmer's Manual"
  39 .SH NAME
  40 alloca \- memory allocator
  41 .SH SYNOPSIS
  42 .B #include <alloca.h>
  43 .sp
  44 .BI "void *alloca(size_t " size );
  45 .SH DESCRIPTION
  46 The
  47 .B alloca
  48 function allocates
  49 .I size
  50 bytes of space in the stack frame of the caller.  This temporary space is
  51 automatically freed when the function that called
  52 .B alloca
  53 returns to its caller.
  54 .SH "RETURN VALUE"
  55 The
  56 .B alloca
  57 function returns a pointer to the beginning of the allocated space.
  58 If the allocation causes stack overflow, program behaviour is undefined.
  59 .SH "CONFORMING TO"
  60 There is evidence that the
  61 .B alloca
  62 function appeared in 32v, pwb, pwb.2, 3bsd, and 4bsd.  There is a man page
  63 for it in BSD 4.3.  Linux uses the GNU version.
  64 This function is not in POSIX or SUSv3.
  65 .SH "NOTES ON THE GNU VERSION"
  66 Normally,
  67 .B gcc
  68 translates calls to
  69 .B alloca
  70 by inlined code. This is not done when either the \-ansi or
  71 the \-fno\-builtin option is given. But beware! By default
  72 the glibc version of
  73 .I <stdlib.h>
  74 includes
  75 .I <alloca.h>
  76 and that contains the line
  77 .RS
  78 # define alloca(size)   __builtin_alloca (size)
  79 .RE
  80 with messy consequences if one has a private version of this function.
  81 .LP
  82 The fact that the code is inlined, means that it is impossible
  83 to take the address of this function, or to change its behaviour
  84 by linking with a different library.
  85 .LP
  86 The inlined code often consists of a single instruction adjusting
  87 the stack pointer, and does not check for stack overflow.
  88 Thus, there is no NULL error return.
  89 .SH BUGS
  90 The
  91 .B alloca
  92 function is machine and compiler dependent. On many systems
  93 its implementation is buggy. Its use is discouraged.
  94 .LP
  95 On many systems
  96 .B alloca
  97 cannot be used inside the list of arguments of a function call, because
  98 the stack space reserved by
  99 .B alloca
 100 would appear on the stack in the middle of the space for the
 101 function arguments.
 102 .SH "SEE ALSO"
 103 .BR brk (2),
 104 .BR pagesize (2),
 105 .BR calloc (3),
 106 .BR malloc (3),
 107 .BR realloc (3)