alloca(3) — Linux manual page


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

NAME         top

       alloca - allocate memory that is automatically freed

SYNOPSIS         top

       #include <alloca.h>

       void *alloca(size_t size);

DESCRIPTION         top

       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         top

       The alloca() function returns a pointer to the beginning of the
       allocated space.  If the allocation causes stack overflow,
       program behavior is undefined.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                             Attribute     Value   │
       │alloca()                              │ Thread safety │ MT-Safe │

CONFORMING TO         top

       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         top

       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)

       with messy consequences if one has a private version of this

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.13 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

GNU                            2021-03-22                      ALLOCA(3)

Pages that refer to this page: malloc(3)strdup(3)