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man:alloca

ALLOCA(3) Linux Programmer's Manual 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 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
     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  argu-
     ments.

SEE ALSO

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

COLOPHON

     This  page  is  part of release 4.16 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/.

GNU 2017-09-15 ALLOCA(3)

/data/webs/external/dokuwiki/data/pages/man/alloca.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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