NAME¶

pthread_attr_setguardsize, pthread_attr_getguardsize - set/get guard size attribute in thread attributes object

LIBRARY¶

POSIX threads library (libpthread, -lpthread)

SYNOPSIS¶

#include <pthread.h>

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

DESCRIPTION¶

The pthread_attr_setguardsize() function sets the guard size attribute of the thread attributes object referred to by attr to the value specified in guardsize.

If guardsize is greater than 0, then for each new thread created using attr the system allocates an additional region of at least guardsize bytes at the end of the thread's stack to act as the guard area for the stack (but see BUGS).

If guardsize is 0, then new threads created with attr will not have a guard area.

The default guard size is the same as the system page size.

If the stack address attribute has been set in attr (using pthread_attr_setstack(3) or pthread_attr_setstackaddr(3)), meaning that the caller is allocating the thread's stack, then the guard size attribute is ignored (i.e., no guard area is created by the system): it is the application's responsibility to handle stack overflow (perhaps by using mprotect(2) to manually define a guard area at the end of the stack that it has allocated).

The pthread_attr_getguardsize() function returns the guard size attribute of the thread attributes object referred to by attr in the buffer pointed to by guardsize.

RETURN VALUE¶

On success, these functions return 0; on error, they return a nonzero error number.

ERRORS¶

POSIX.1 documents an EINVAL error if attr or guardsize is invalid. On Linux these functions always succeed (but portable and future-proof applications should nevertheless handle a possible error return).

ATTRIBUTES¶

For an explanation of the terms used in this section, see attributes(7).

STANDARDS¶

POSIX.1-2008.

HISTORY¶

glibc 2.1. POSIX.1-2001.

NOTES¶

A guard area consists of virtual memory pages that are protected to prevent read and write access. If a thread overflows its stack into the guard area, then, on most hard architectures, it receives a SIGSEGV signal, thus notifying it of the overflow. Guard areas start on page boundaries, and the guard size is internally rounded up to the system page size when creating a thread. (Nevertheless, pthread_attr_getguardsize() returns the guard size that was set by pthread_attr_setguardsize().)

Setting a guard size of 0 may be useful to save memory in an application that creates many threads and knows that stack overflow can never occur.

Choosing a guard size larger than the default size may be necessary for detecting stack overflows if a thread allocates large data structures on the stack.

BUGS¶

As at glibc 2.8, the NPTL threading implementation includes the guard area within the stack size allocation, rather than allocating extra space at the end of the stack, as POSIX.1 requires. (This can result in an EINVAL error from pthread_create(3) if the guard size value is too large, leaving no space for the actual stack.)

The obsolete LinuxThreads implementation did the right thing, allocating extra space at the end of the stack for the guard area.

EXAMPLES¶

See pthread_getattr_np(3).

SEE ALSO¶

mmap(2), mprotect(2), pthread_attr_init(3), pthread_attr_setstack(3), pthread_attr_setstacksize(3), pthread_create(3), pthreads(7)