PTHREAD_ATTR_GETGUARDSIZE(3P)				     POSIX Programmer's Manual				     PTHREAD_ATTR_GETGUARDSIZE(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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  guard-
       size 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 implementation-defined.

       If the stackaddr 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.

       The behavior is undefined if the value specified by the attr argument to pthread_attr_getguardsize()  or  pthread_attr_setguardsize()  does
       not refer to an initialized thread attributes object.

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

       These functions shall fail if:

       EINVAL The parameter guardsize is invalid.

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

       The following sections are informative.

EXAMPLES

   Retrieving the guardsize Attribute
       This example shows how to obtain the guardsize attribute of a thread attribute object.

	   #include <pthread.h>

	   pthread_attr_t thread_attr;
	   size_t  guardsize;
	   int	   rc;

	   /* code initializing thread_attr */
	   ...

	   rc = pthread_attr_getguardsize (&thread_attr, &guardsize);
	   if (rc != 0)  {
	       /* handle error */
	       ...
	   }
	   else {
	       if (guardsize > 0) {
	       /* a guard area of at least guardsize bytes is provided */
	       ...
	       }
	       else {
	       /* no guard area provided */
	       ...
	       }
	   }

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.

       The  default size of the guard area is left implementation-defined since on systems supporting very large page sizes, the overhead might be
       substantial if at least one guard page is required by default.

       If an implementation detects that the value specified by the attr argument to  pthread_attr_getguardsize()  or  pthread_attr_setguardsize()
       does not refer to an initialized thread attributes object, it is recommended that the function should fail and report an [EINVAL] error.

FUTURE DIRECTIONS

       None.

SEE ALSO

       The Base Definitions volume of POSIX.1-2008, <pthread.h>, <sys_mman.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C)  2013  by	the  Institute	of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 Stan-
       dard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE
/The Open Group						       2013					     PTHREAD_ATTR_GETGUARDSIZE(3P)