NAME
pthread_attr_setguardsize, pthread_attr_getguardsize - set/get guard
size attribute in thread attributes object
SYNOPSIS
#include <pthread.h>
int pthread_attr_setguardsize(pthread_attr_t *attr, size_t guardsize);
int pthread_attr_getguardsize(pthread_attr_t *attr, size_t *guardsize);
Compile and link with -pthread.
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.1-2001 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).
VERSIONS
These functions are provided by glibc since version 2.1.
CONFORMING TO
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.
EXAMPLE
See pthread_getattr_np(3).
SEE ALSO
mmap(2), mprotect(2), pthread_attr_init(3), pthread_create(3),
pthread_attr_setstack(3), pthread_attr_setstacksize(3), pthreads(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.