alloca(3)

1ALLOCA(3)                  Linux Programmer's Manual                 ALLOCA(3)
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NAME

6       alloca - allocate memory that is automatically freed
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SYNOPSIS

9       #include <alloca.h>
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11       void *alloca(size_t size);
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DESCRIPTION

14       The  alloca() function allocates size bytes of space in the stack frame
15       of the caller.  This temporary space is automatically  freed  when  the
16       function that called alloca() returns to its caller.
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RETURN VALUE

19       The  alloca()  function returns a pointer to the beginning of the allo‐
20       cated space.  If the allocation causes stack overflow, program behavior
21       is undefined.
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CONFORMING TO

24       This function is not in POSIX.1-2001.
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26       There  is  evidence  that  the  alloca() function appeared in 32V, PWB,
27       PWB.2, 3BSD, and 4BSD.  There is a man page for it  in  4.3BSD.   Linux
28       uses the GNU version.
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NOTES

31       The  alloca() function is machine- and compiler-dependent.  For certain
32       applications, its use can improve efficiency compared  to  the  use  of
33       malloc(3)  plus free(3).  In certain cases, it can also simplify memory
34       deallocation in applications  that  use  longjmp(3)  or  siglongjmp(3).
35       Otherwise, its use is discouraged.
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37       Because  the  space allocated by alloca() is allocated within the stack
38       frame, that space is automatically freed  if  the  function  return  is
39       jumped over by a call to longjmp(3) or siglongjmp(3).
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41       Do not attempt to free(3) space allocated by alloca()!
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43   Notes on the GNU Version
44       Normally,  gcc(1) translates calls to alloca() with inlined code.  This
45       is  not  done  when  either  the  -ansi,  -std=c89,  -std=c99,  or  the
46       -fno-builtin  option  is  given  (and  the  header  <alloca.h>  is  not
47       included).  But beware!  By default the  glibc  version  of  <stdlib.h>
48       includes <alloca.h> and that contains the line:
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50           #define alloca(size)   __builtin_alloca (size)
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52       with messy consequences if one has a private version of this function.
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54       The  fact  that the code is inlined means that it is impossible to take
55       the address of this function, or to change its behavior by linking with
56       a different library.
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58       The  inlined  code often consists of a single instruction adjusting the
59       stack pointer, and does not check for stack overflow.  Thus,  there  is
60       no NULL error return.
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BUGS

63       There  is  no  error  indication if the stack frame cannot be extended.
64       (However, after a failed allocation, the program is likely to receive a
65       SIGSEGV signal if it attempts to access the unallocated space.)
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67       On many systems alloca() cannot be used inside the list of arguments of
68       a function call, because the stack space  reserved  by  alloca()  would
69       appear  on  the stack in the middle of the space for the function argu‐
70       ments.
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COLOPHON

76       This page is part of release 3.22 of the Linux  man-pages  project.   A
77       description  of  the project, and information about reporting bugs, can
78       be found at http://www.kernel.org/doc/man-pages/.
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82GNU                               2008-01-24                         ALLOCA(3)