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.

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.

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 .