alloca

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.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(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 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 5.09 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)