pthread_attr_setguardsize, pthread_attr_getguardsize - set/get guard size attribute in thread attributes object
POSIX threads library (libpthread, -lpthread)
#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);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.
On success, these functions return 0; on error, they return a nonzero error number.
See pthread_getattr_np(3).
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).
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| Thread safety | MT-Safe |
POSIX.1-2008.
glibc 2.1. POSIX.1-2001.
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.
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.
mmap(2), mprotect(2), pthread_attr_init(3), pthread_attr_setstack(3), pthread_attr_setstacksize(3), pthread_create(3), pthreads(7)