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alloca(3)                  Library Functions Manual                  alloca(3)

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 allo-
       cated space.  If the allocation causes stack overflow, program behavior
       is undefined.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

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

STANDARDS
       This function is not in POSIX.1.

       alloca() originates from PWB and 32V, and appears in all their  deriva-
       tives.

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  discour-
       aged.

       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 trans-
       late all uses of alloca() into the built-in, but this is  forbidden  if
       standards conformance is requested (-ansi, -std=c*), in which case <al-
       loca.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  im-
       plementing functionality like strdupa(3).

BUGS
       Due  to the nature of the stack, it is impossible to check if the allo-
       cation would overflow the space available, and, hence, neither is indi-
       cating  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 ap-
       pear on the stack in the middle of the space  for  the  function  argu-
       ments.

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

Linux man-pages 6.03              2022-12-15                         alloca(3)

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