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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding 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 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 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 Technology -- 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 Standard 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 .