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PTHREAD_ATTR_SETGUARDSIZE(3Linux Programmer's ManuPTHREAD_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(const 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 speci-
     fied 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  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.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     allbox;  lbw28   lb   lb   l   l   l.    Interface Attribute Value   T{
     pthread_attr_setguardsize(),   pthread_attr_getguardsize()  T}   Thread
     safety  MT-Safe

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

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_get-
     guardsize() returns the guard size that was  set  by  pthread_attr_set-
     guardsize().)

     Setting  a  guard size of 0 may be useful to save memory in an applica-
     tion 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, allocat-
     ing extra space at the end of the stack for the guard area.

EXAMPLE

     See pthread_getattr_np(3).