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

COLOPHON

       This page is part of release 3.54 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/.