man3/alloca.3

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  34 .\"     @(#)alloca.3    5.1 (Berkeley) 5/2/91
  35 .\"
  36 .\" Converted Mon Nov 29 11:05:55 1993 by Rik Faith <faith@cs.unc.edu>
  37 .\" Modified Tue Oct 22 23:41:56 1996 by Eric S. Raymond <esr@thyrsus.com>
  38 .\" Modified 2002-07-17, aeb
  39 .\" 2008-01-24, mtk:
  40 .\"     Various rewrites and additions (notes on longjmp() and SIGSEGV).
  41 .\"     Weaken warning against use of alloca() (as per Debian bug 461100).
  42 .\"
  43 .TH ALLOCA 3 2019-03-06 "GNU" "Linux Programmer's Manual"
  44 .SH NAME
  45 alloca \- allocate memory that is automatically freed
  46 .SH SYNOPSIS
  47 .B #include <alloca.h>
  48 .PP
  49 .BI "void *alloca(size_t " size );
  50 .SH DESCRIPTION
  51 The
  52 .BR alloca ()
  53 function allocates
  54 .I size
  55 bytes of space in the stack frame of the caller.
  56 This temporary space is
  57 automatically freed when the function that called
  58 .BR alloca ()
  59 returns to its caller.
  60 .SH RETURN VALUE
  61 The
  62 .BR alloca ()
  63 function returns a pointer to the beginning of the allocated space.
  64 If the allocation causes stack overflow, program behavior is undefined.
  65 .SH ATTRIBUTES
  66 For an explanation of the terms used in this section, see
  67 .BR attributes (7).
  68 .TS
  69 allbox;
  70 lb lb lb
  71 l l l.
  72 Interface       Attribute       Value
  73 T{
  74 .BR alloca ()
  75 T}      Thread safety   MT-Safe
  76 .TE
  77 .SH CONFORMING TO
  78 This function is not in POSIX.1.
  79 .PP
  80 There is evidence that the
  81 .BR alloca ()
  82 function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.
  83 There is a man page for it in 4.3BSD.
  84 Linux uses the GNU version.
  85 .SH NOTES
  86 The
  87 .BR alloca ()
  88 function is machine- and compiler-dependent.
  89 For certain applications,
  90 its use can improve efficiency compared to the use of
  91 .BR malloc (3)
  92 plus
  93 .BR free (3).
  94 In certain cases,
  95 it can also simplify memory deallocation in applications that use
  96 .BR longjmp (3)
  97 or
  98 .BR siglongjmp (3).
  99 Otherwise, its use is discouraged.
 100 .PP
 101 Because the space allocated by
 102 .BR alloca ()
 103 is allocated within the stack frame,
 104 that space is automatically freed if the function return
 105 is jumped over by a call to
 106 .BR longjmp (3)
 107 or
 108 .BR siglongjmp (3).
 109 .PP
 110 The space allocated by
 111 .BR alloca ()
 112 is
 113 .I not
 114 automatically deallocated if the pointer that refers to it
 115 simply goes out of scope.
 116 .PP
 117 Do not attempt to
 118 .BR free (3)
 119 space allocated by
 120 .BR alloca ()!
 121 .SS Notes on the GNU version
 122 Normally,
 123 .BR gcc (1)
 124 translates calls to
 125 .BR alloca ()
 126 with inlined code.
 127 This is not done when either the
 128 .IR "\-ansi" ,
 129 .IR "\-std=c89" ,
 130 .IR "\-std=c99" ,
 131 or the
 132 .IR "\-std=c11"
 133 option is given
 134 .BR and
 135 the header
 136 .I <alloca.h>
 137 is not included.
 138 Otherwise, (without an \-ansi or \-std=c* option) the glibc version of
 139 .I <stdlib.h>
 140 includes
 141 .I <alloca.h>
 142 and that contains the lines:
 143 .PP
 144 .in +4n
 145 .EX
 146 #ifdef  __GNUC__
 147 #define alloca(size)   __builtin_alloca (size)
 148 #endif
 149 .EE
 150 .in
 151 .PP
 152 with messy consequences if one has a private version of this function.
 153 .PP
 154 The fact that the code is inlined means that it is impossible
 155 to take the address of this function, or to change its behavior
 156 by linking with a different library.
 157 .PP
 158 The inlined code often consists of a single instruction adjusting
 159 the stack pointer, and does not check for stack overflow.
 160 Thus, there is no NULL error return.
 161 .SH BUGS
 162 There is no error indication if the stack frame cannot be extended.
 163 (However, after a failed allocation, the program is likely to receive a
 164 .B SIGSEGV
 165 signal if it attempts to access the unallocated space.)
 166 .PP
 167 On many systems
 168 .BR alloca ()
 169 cannot be used inside the list of arguments of a function call, because
 170 the stack space reserved by
 171 .BR alloca ()
 172 would appear on the stack in the middle of the space for the
 173 function arguments.
 174 .SH SEE ALSO
 175 .BR brk (2),
 176 .BR longjmp (3),
 177 .BR malloc (3)