PTHREAD_ATTR_GETGUARDSIZE(3)POSIX Programmer's ManuaPlTHREAD_ATTR_GETGUARDSIZE(3)



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

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.1-2017, <pthread.h>, <sys_mman.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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                  2017         PTHREAD_ATTR_GETGUARDSIZE(3)
