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pthread_attr_setguardsize(3) [netbsd man page]

PTHREAD_ATTR_GETGUARDSIZE(3)				   BSD Library Functions Manual 			      PTHREAD_ATTR_GETGUARDSIZE(3)

NAME
pthread_attr_getguardsize -- get and set thread guard size LIBRARY
POSIX Threads Library (libpthread, -lpthread) 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 _SC_PAGESIZE, the system page size. 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 IEEE Std 1003.1-2008 (``POSIX.1''). BSD
July 7, 2010 BSD

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PTHREAD_ATTR_SETGUARDSIZE(3)				     Linux Programmer's Manual				      PTHREAD_ATTR_SETGUARDSIZE(3)

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 struc- tures 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 3.27 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/. Linux 2008-10-24 PTHREAD_ATTR_SETGUARDSIZE(3)
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