Open Source Software

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).

   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 4.09 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/.



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