pthread_attr_getguardsize(3)

NAME

pthread_attr_getguardsize, pthread_attr_setguardsize — get and set thread guard size

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);

The pthread_attr_getguardsize() and pthread_attr_setguardsize() functions get and set guardsize in the attr object. If guardsize is larger than 0, the system reserves an additional region of guarded memory of at least guardsize bytes at the end of the thread's stack for each new thread created by using attr.

The guarded area is understood to be pages of memory that are protected from read and write access. While the guarded area should be rounded by the system page size, the actual default size is implementation-defined. In NetBSD the default guardsize is given by the vm.thread_guard_size sysctl(7).

The rationale behind guardsize is two-fold:

On the one hand, it provides protection against overflow of the stack pointer. If there is a guard area and a thread overflows its stack pointer into this extra memory area, it should receive a SIGSEGV signal or experience other comparable fatal error condition. Note that if a thread allocates large data structures on stack, it may be necessary to raise the default guardsize in order to detect stack overflows.
On the other hand, the overflow protection may waste system resources if an application that creates a large number of threads knows that it will never overflow the stack. In this case it is possible to set guardsize to 0.

If pthread_attr_setstack(3) or pthread_attr_setstackaddr(3) is used to set the stack address attribute in attr, the guard size attribute is ignored and no guard area will be allocated; it is the responsibility of the application to handle the overflow conditions.