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 .