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NAME

       alloca - allocate memory that is automatically freed

LIBRARY

       Standard C library (libc, -lc)

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).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │alloca()                                                                      │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       None.

HISTORY

       PWB, 32V.

NOTES

       The  alloca()  function  is  machine-  and compiler-dependent.  Because it allocates from the stack, it's
       faster than malloc(3) and 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()!

       By  necessity,  alloca()  is  a  compiler built-in, also known as __builtin_alloca().  By default, modern
       compilers automatically translate all uses of alloca() into  the  built-in,  but  this  is  forbidden  if
       standards  conformance is requested (-ansi, -std=c*), in which case <alloca.h> is required, lest a symbol
       dependency be emitted.

       The fact that alloca() is a built-in means it is impossible to take its address or to change its behavior
       by linking with a different library.

       Variable  length  arrays  (VLAs)  are part of the C99 standard, optional since C11, and can be used for a
       similar purpose.  However, they do not port to standard C++, and, being variables, live  in  their  block
       scope  and  don't have an allocator-like interface, making them unfit for implementing functionality like
       strdupa(3).

BUGS

       Due to the nature of the stack, it is impossible to check if the  allocation  would  overflow  the  space
       available,  and,  hence,  neither  is  indicating an error.  (However, the program is likely to receive a
       SIGSEGV signal if it attempts to access unavailable 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)