NAME

       pthread_attr_getguardsize,  pthread_attr_setguardsize  -  get and set the thread guardsize
       attribute

SYNOPSIS

       #include <pthread.h>

       int pthread_attr_getguardsize(const pthread_attr_t *restrict attr,
              size_t *restrict guardsize);
       int pthread_attr_setguardsize(pthread_attr_t *attr,
              size_t guardsize);

DESCRIPTION

       The pthread_attr_getguardsize() function shall get the guardsize  attribute  in  the  attr
       object. This attribute shall be returned in the guardsize parameter.

       The  pthread_attr_setguardsize()  function  shall  set the guardsize attribute in the attr
       object. The new value of this attribute shall be obtained from the guardsize parameter. If
       guardsize  is  zero,  a guard area shall not be provided for threads created with attr. If
       guardsize is greater than zero, a guard area of at least size  guardsize  bytes  shall  be
       provided for each thread created with attr.

       The  guardsize  attribute  controls  the  size  of the guard area for the created thread's
       stack. The guardsize attribute provides protection against overflow of the stack  pointer.
       If  a  thread's stack is created with guard protection, the implementation allocates extra
       memory at the overflow end of the stack as a buffer against stack overflow  of  the  stack
       pointer. If an application overflows into this buffer an error shall result (possibly in a
       SIGSEGV signal being delivered to the thread).

       A conforming implementation may round up the value contained in guardsize to a multiple of
       the  configurable  system  variable  {PAGESIZE}  (see  <sys/mman.h>). If an implementation
       rounds  up  the  value  of  guardsize  to  a   multiple   of   {PAGESIZE},   a   call   to
       pthread_attr_getguardsize()  specifying  attr  shall  store in the guardsize parameter the
       guard size specified by the previous pthread_attr_setguardsize() function call.

       The default value of the guardsize attribute is {PAGESIZE} bytes.   The  actual  value  of
       {PAGESIZE} is implementation-defined.

       If  the  stackaddr  or stack attribute has been set (that is, the caller is allocating and
       managing its own  thread  stacks),  the  guardsize  attribute  shall  be  ignored  and  no
       protection  shall  be  provided  by  the  implementation.  It is the responsibility of the
       application to manage stack overflow along with stack allocation and  management  in  this
       case.

RETURN VALUE

       If  successful,  the pthread_attr_getguardsize() and pthread_attr_setguardsize() functions
       shall return zero; otherwise, an error number shall be returned to indicate the error.

ERRORS

       The pthread_attr_getguardsize() and pthread_attr_setguardsize() functions shall fail if:

       EINVAL The attribute attr is invalid.

       EINVAL The parameter guardsize is invalid.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       The guardsize attribute is provided to the application for two reasons:

        1. Overflow protection can potentially result in wasted system resources.  An application
           that  creates  a  large  number of threads, and which knows its threads never overflow
           their stack, can save system resources by turning off guard areas.

        2. When threads allocate large data structures on the stack, large  guard  areas  may  be
           needed to detect stack overflow.

FUTURE DIRECTIONS

       None.

SEE ALSO

       The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>, <sys/mman.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .