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