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.Dd July 2, 2017
.Dt PTHREAD_ATTR_GETGUARDSIZE 3
.Os
.Sh NAME
.Nm pthread_attr_getguardsize ,
.Nm pthread_attr_setguardsize
.Nd get and set thread guard size
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_attr_getguardsize \
"const pthread_attr_t * restrict attr" "size_t * restrict guardsize"
.Ft int
.Fn pthread_attr_setguardsize "pthread_attr_t *attr" "size_t guardsize"
.Sh DESCRIPTION
The
.Fn pthread_attr_getguardsize
and
.Fn pthread_attr_setguardsize
functions get and set
.Fa guardsize
in the
.Fa attr
object.
If
.Fa guardsize
is larger than 0, the system reserves
an additional region of guarded memory of at least
.Fa guardsize
bytes at the end of the thread's stack for each new thread created by using
.Fa attr .
.Pp
The guarded area is understood to be pages of memory
that are protected from read and write access.
While the guarded area should be rounded by the system page size,
the actual default size is implementation-defined.
In
.Nx
the default
.Fa guardsize
is given by the
.Pa vm.thread_guard_size
.Xr sysctl 7 .
.Pp
The rationale behind
.Fa guardsize
is two-fold:
.Bl -enum -offset 2n
.It
On the one hand, it provides protection against overflow of the stack pointer.
If there is a guard area and a thread overflows its
stack pointer into this extra memory area, it should receive a
.Dv SIGSEGV
signal or experience other comparable fatal error condition.
Note that if a thread allocates large data structures on stack,
it may be necessary to raise the default
.Fa guardsize
in order to detect stack overflows.
.It
On the other hand, the overflow protection may waste system resources
if an application that creates a large number of threads knows that it
will never overflow the stack.
In this case it is possible to set
.Fa guardsize
to 0.
.El
.Pp
If
.Xr pthread_attr_setstack 3
or
.Xr pthread_attr_setstackaddr 3
is used to set the stack address attribute in
.Fa attr ,
the guard size attribute is ignored and no guard area will be allocated;
it is the responsibility of the application to handle the overflow conditions.
.Sh RETURN VALUES
If successful, both functions return 0.
Otherwise, an error number is returned to indicate the error.
.Sh ERRORS
No errors are defined for
.Fn pthread_attr_getguardsize .
.Pp
The
.Fn pthread_attr_setguardsize
may fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
There was insufficient memory.
.El
.Sh SEE ALSO
.Xr pthread_attr 3 ,
.Xr pthread_attr_setstack 3 ,
.Xr sysconf 3
.Sh STANDARDS
Both functions conform to
.St -p1003.1-2008 .
.Sh BUGS
Older versions of
.Nx ,
prior to 10.0, 9.4, and 8.3, incorrectly adjust the stack address by
the guard size in threads configured with
.Xr pthread_attr_setstack 3 ,
instead of ignoring the guard size in that case as
.Tn POSIX
prescribes
.Po
see
.Lk https://gnats.NetBSD.org/57721 "PR lib/57721"
.Pc .
.Pp
Even if you didn't set a nonzero guard size with
.Fn pthread_attr_setguardsize ,
the system will choose a nonzero default guard size.
.Pp
To work around this in applications that run on older and newer
versions of
.Nx ,
as well as on other operating systems, you can safely set the guard
size to zero:
.Dl "pthread_attr_setguardsize(&attr, 0);"