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 automati-
       cally 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  over-
       flow, 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  com-
       pared  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  over-
       flow.  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)