NAME

       pthread_attr_setguardsize,  pthread_attr_getguardsize  -  set/get  guard size attribute in
       thread attributes object

LIBRARY

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS

       #include <pthread.h>

       int pthread_attr_setguardsize(pthread_attr_t *attr, size_t guardsize);
       int pthread_attr_getguardsize(const pthread_attr_t *restrict attr,
                                     size_t *restrict guardsize);

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  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).

ATTRIBUTES

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

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │pthread_attr_setguardsize(), pthread_attr_getguardsize()       │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       glibc 2.1.  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.

EXAMPLES

       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)