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

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