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_attr_setstack(3), pthread_attr_setstacksize(3), pthread_create(3), pthreads(7)

COLOPHON

       This  page is part of release 3.44 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)