PTHREAD_ATTR_GETGUARDSIZE(3)

NAME
pthread_attr_getguardsize, pthread_attr_setguardsize - get and set thread guard size

LIBRARY
libpthread

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() 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:

1.  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.

2.  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.

RETURN VALUES
If successful, both functions return 0. Otherwise, an error number is returned to indicate the error.

ERRORS
No errors are defined for pthread_attr_getguardsize().

The pthread_attr_setguardsize() may fail if:

[ENOMEM]
    There was insufficient memory.

SEE ALSO
pthread_attr(3), pthread_attr_setstack(3), sysconf(3)

STANDARDS
Both functions conform to p1003.1-2008.

BUGS
Older versions of NetBSD, prior to 10.0, 9.4, and 8.3, incorrectly adjust the stack address by the guard size in threads configured with pthread_attr_setstack(3), instead of ignoring the guard size in that case as POSIX prescribes ( see PR lib/57721: https://gnats.NetBSD.org/57721).

Even if you didn't set a nonzero guard size with pthread_attr_setguardsize(), the system will choose a nonzero default guard size.

To work around this in applications that run on older and newer versions of NetBSD, as well as on other operating systems, you can safely set the guard size to zero:
    pthread_attr_setguardsize(&attr, 0);