ALLOCA(3)		   Linux Programmers Manual		    ALLOCA(3)



NAME
       alloca - allocate memory that is automatically freed

SYNOPSIS
       #include 

       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 allo
       cated 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    is  not
       included).  But beware!	By default the	glibc  version	of  
       includes  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  argu
       ments.

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

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



GNU				  2008-01-24			     ALLOCA(3)