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

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

       This  page is part of release 3.25 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)