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 func‐

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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                              2019-03-06                        ALLOCA(3)

Pages that refer to this page: calloc(3)free(3)malloc(3)realloc(3)reallocarray(3)strdup(3)strdupa(3)strndup(3)strndupa(3)