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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  effi-
       ciency  compared to the use of malloc(3) plus free(3).  In certain cases, it can also simplify memory dealloca-
       tion 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-01-24                         ALLOCA(3)