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_set-
       guardsize().)

       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 3.25 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)