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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 allo-
       cated space.  If the allocation causes stack overflow, program behavior
       is undefined.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +----------+---------------+---------+
       |Interface | Attribute     | Value   |
       +----------+---------------+---------+
       |alloca()  | Thread safety | MT-Safe |
       +----------+---------------+---------+
CONFORMING TO
       This function is not in POSIX.1.

       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).

       The space allocated by alloca() is not automatically deallocated if the
       pointer that refers to it simply goes out of scope.

       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  -std=c11
       option  is given and the header _alloca.h_ is not included.  Otherwise,
       (without an -ansi or -std=c* option) the glibc  version  of  _stdlib.h_
       includes _alloca.h_ and that contains the lines:

           #ifdef  __GNUC__
           #define alloca(size)   __builtin_alloca (size)
           #endif

       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 ap-
       pear  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 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-03-06                         ALLOCA(3)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON