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NAME

       alloca - allocate memory that is automatically freed

SYNOPSIS

       #include <alloca.h>

       void *alloca(size_t size);

DESCRIPTION

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

RETURN VALUE

       The  alloca()  function  returns  a  pointer  to  the  beginning of the
       allocated space.  If the  allocation  causes  stack  overflow,  program
       behavior is undefined.

CONFORMING TO

       This function is not in POSIX.1-2001.

       There  is  evidence  that  the  alloca() function appeared in 32V, PWB,
       PWB.2, 3BSD, and 4BSD.  There is a man page for it  in  4.3BSD.   Linux
       uses the GNU version.

NOTES

       The  alloca() function is machine- and compiler-dependent.  For certain
       applications, its use can improve efficiency compared  to  the  use  of
       malloc(3)  plus free(3).  In certain cases, it can also simplify memory
       deallocation in applications  that  use  longjmp(3)  or  siglongjmp(3).
       Otherwise, its use is discouraged.

       Because  the  space allocated by alloca() is allocated within the stack
       frame, that space is automatically freed  if  the  function  return  is
       jumped over by a call to longjmp(3) or siglongjmp(3).

       Do not attempt to free(3) space allocated by alloca()!

   Notes on the GNU Version
       Normally,  gcc(1) translates calls to alloca() with inlined code.  This
       is  not  done  when  either  the  -ansi,  -std=c89,  -std=c99,  or  the
       -fno-builtin  option  is  given  (and  the  header  <alloca.h>  is  not
       included).  But beware!  By default the  glibc  version  of  <stdlib.h>
       includes <alloca.h> and that contains the line:

           #define alloca(size)   __builtin_alloca (size)

       with messy consequences if one has a private version of this function.

       The  fact  that the code is inlined means that it is impossible to take
       the address of this function, or to change its behavior by linking with
       a different library.

       The  inlined  code often consists of a single instruction adjusting the
       stack pointer, and does not check for stack overflow.  Thus,  there  is
       no NULL error return.

BUGS

       There  is  no  error  indication if the stack frame cannot be extended.
       (However, after a failed allocation, the program is likely to receive a
       SIGSEGV signal if it attempts to access the unallocated space.)

       On many systems alloca() cannot be used inside the list of arguments of
       a function call, because the stack space  reserved  by  alloca()  would
       appear  on  the  stack  in  the  middle  of  the space for the function
       arguments.

SEE ALSO

       brk(2), longjmp(3), malloc(3)

COLOPHON

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