pthread_attr_getguardsize or pthread_attr_setguardsize Subroutines

Purpose

Gets or sets the thread guardsize attribute.

Library

Threads Library (libthreads.a)

Syntax

#include <pthread.h>

int pthread_attr_getguardsize (const pthread_attr_t *attr, size_t *guardsize );
int pthread_attr_setguardsize (pthread_attr_t *attr, size_t guardsize );

Description

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 results (possibly in a SIGSEGV signal being delivered to the thread).

The guardsize attribute is provided to the application for two reasons:

• Overflow protection can potentially result in wasted system resources. An application that creates a large number of threads, and which knows its threads will never overflow their stack, can save system resources by turning off guard areas.

• When threads allocate large data structures on the stack, large guard areas may be needed to detect stack overflow.

The pthread_attr_getguardsize function gets the guardsize attribute in the attr object. This attribute is returned in the guardsize parameter.

The pthread_attr_setguardsize function sets the guardsize attribute in the attr object. The new value of this attribute is obtained from the guardsize parameter. If guardsize is zero, a guard area will not be provided for threads created with attr. If guardsize is greater than zero, a guard area of at least size guardsize bytes is provided for each thread created with attr.

A conforming implementation is permitted to 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 will 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-dependent and may not be the same on all implementations.

If the stackaddr attribute has been set (that is, the caller is allocating and managing its own thread stacks), the guardsize attribute is ignored and no protection will 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 Values

If successful, the pthread_attr_getguardsize and pthread_attr_setsguardsize functions return zero. Otherwise, an error number is returned to indicate the error.

Error Codes

The pthread_attr_getguardsize and pthread_attr_setguardsize functions will fail if: