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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);"
