linuxthreads.texi

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If the value of @var{stacksize} is less than @code{PTHREAD_STACK_MIN},
or greater than the system's maximum stack size, or if the value of
@var{stackaddr} lacks the proper alignment, @code{pthread_attr_setstack}
will fail and return @code{EINVAL}.

@item guardsize
Change the minimum size in bytes of the guard area for the thread's
stack.  The default size is a single page.  If this value is set, it
will be rounded up to the nearest page size.  If the value is set to 0,
a guard area will not be created for this thread.  The space allocated
for the guard area is used to catch stack overflow.  Therefore, when
allocating large structures on the stack, a larger guard area may be
required to catch a stack overflow.

If the caller is managing their own stacks (if the @code{stackaddr}
attribute has been set), then the @code{guardsize} attribute is ignored.

If the value exceeds the @code{stacksize}, @code{pthread_atrr_setguardsize}
will fail and return @code{EINVAL}.
@end table

@node Cancellation
@section Cancellation

Cancellation is the mechanism by which a thread can terminate the
execution of another thread. More precisely, a thread can send a
cancellation request to another thread. Depending on its settings, the
target thread can then either ignore the request, honor it immediately,
or defer it till it reaches a cancellation point.  When threads are
first created by @code{pthread_create}, they always defer cancellation
requests.

When a thread eventually honors a cancellation request, it behaves as if
@code{pthread_exit(PTHREAD_CANCELED)} was called.  All cleanup handlers
are executed in reverse order, finalization functions for
thread-specific data are called, and finally the thread stops executing.
If the canceled thread was joinable, the return value
@code{PTHREAD_CANCELED} is provided to whichever thread calls
@var{pthread_join} on it. See @code{pthread_exit} for more information.

Cancellation points are the points where the thread checks for pending
cancellation requests and performs them.  The POSIX threads functions
@code{pthread_join}, @code{pthread_cond_wait},
@code{pthread_cond_timedwait}, @code{pthread_testcancel},
@code{sem_wait}, and @code{sigwait} are cancellation points.  In
addition, these system calls are cancellation points:

@multitable @columnfractions .33 .33 .33
@item @t{accept}	@tab @t{open}		@tab @t{sendmsg}
@item @t{close}		@tab @t{pause}		@tab @t{sendto}
@item @t{connect}	@tab @t{read}		@tab @t{system}
@item @t{fcntl}		@tab @t{recv}		@tab @t{tcdrain}
@item @t{fsync}		@tab @t{recvfrom}	@tab @t{wait}
@item @t{lseek}		@tab @t{recvmsg}	@tab @t{waitpid}
@item @t{msync}		@tab @t{send}		@tab @t{write}
@item @t{nanosleep}
@end multitable

@noindent
All library functions that call these functions (such as
@code{printf}) are also cancellation points.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcancelstate (int @var{state}, int *@var{oldstate})
@code{pthread_setcancelstate} changes the cancellation state for the
calling thread -- that is, whether cancellation requests are ignored or
not. The @var{state} argument is the new cancellation state: either
@code{PTHREAD_CANCEL_ENABLE} to enable cancellation, or
@code{PTHREAD_CANCEL_DISABLE} to disable cancellation (cancellation
requests are ignored).

If @var{oldstate} is not @code{NULL}, the previous cancellation state is
stored in the location pointed to by @var{oldstate}, and can thus be
restored later by another call to @code{pthread_setcancelstate}.

If the @var{state} argument is not @code{PTHREAD_CANCEL_ENABLE} or
@code{PTHREAD_CANCEL_DISABLE}, @code{pthread_setcancelstate} fails and
returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcanceltype (int @var{type}, int *@var{oldtype})
@code{pthread_setcanceltype} changes the type of responses to
cancellation requests for the calling thread: asynchronous (immediate)
or deferred.  The @var{type} argument is the new cancellation type:
either @code{PTHREAD_CANCEL_ASYNCHRONOUS} to cancel the calling thread
as soon as the cancellation request is received, or
@code{PTHREAD_CANCEL_DEFERRED} to keep the cancellation request pending
until the next cancellation point. If @var{oldtype} is not @code{NULL},
the previous cancellation state is stored in the location pointed to by
@var{oldtype}, and can thus be restored later by another call to
@code{pthread_setcanceltype}.

If the @var{type} argument is not @code{PTHREAD_CANCEL_DEFERRED} or
@code{PTHREAD_CANCEL_ASYNCHRONOUS}, @code{pthread_setcanceltype} fails
and returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_testcancel (@var{void})
@code{pthread_testcancel} does nothing except testing for pending
cancellation and executing it. Its purpose is to introduce explicit
checks for cancellation in long sequences of code that do not call
cancellation point functions otherwise.
@end deftypefun

@node Cleanup Handlers
@section Cleanup Handlers

Cleanup handlers are functions that get called when a thread terminates,
either by calling @code{pthread_exit} or because of
cancellation. Cleanup handlers are installed and removed following a
stack-like discipline.

The purpose of cleanup handlers is to free the resources that a thread
may hold at the time it terminates. In particular, if a thread exits or
is canceled while it owns a locked mutex, the mutex will remain locked
forever and prevent other threads from executing normally. The best way
to avoid this is, just before locking the mutex, to install a cleanup
handler whose effect is to unlock the mutex. Cleanup handlers can be
used similarly to free blocks allocated with @code{malloc} or close file
descriptors on thread termination.

Here is how to lock a mutex @var{mut} in such a way that it will be
unlocked if the thread is canceled while @var{mut} is locked:

@smallexample
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
@end smallexample

Equivalently, the last two lines can be replaced by

@smallexample
pthread_cleanup_pop(1);
@end smallexample

Notice that the code above is safe only in deferred cancellation mode
(see @code{pthread_setcanceltype}). In asynchronous cancellation mode, a
cancellation can occur between @code{pthread_cleanup_push} and
@code{pthread_mutex_lock}, or between @code{pthread_mutex_unlock} and
@code{pthread_cleanup_pop}, resulting in both cases in the thread trying
to unlock a mutex not locked by the current thread. This is the main
reason why asynchronous cancellation is difficult to use.

If the code above must also work in asynchronous cancellation mode,
then it must switch to deferred mode for locking and unlocking the
mutex:

@smallexample
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop(1);
pthread_setcanceltype(oldtype, NULL);
@end smallexample

The code above can be rewritten in a more compact and efficient way,
using the non-portable functions @code{pthread_cleanup_push_defer_np}
and @code{pthread_cleanup_pop_restore_np}:

@smallexample
pthread_cleanup_push_defer_np(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop_restore_np(1);
@end smallexample

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_push (void (*@var{routine}) (void *), void *@var{arg})

@code{pthread_cleanup_push} installs the @var{routine} function with
argument @var{arg} as a cleanup handler. From this point on to the
matching @code{pthread_cleanup_pop}, the function @var{routine} will be
called with arguments @var{arg} when the thread terminates, either
through @code{pthread_exit} or by cancellation. If several cleanup
handlers are active at that point, they are called in LIFO order: the
most recently installed handler is called first.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_pop (int @var{execute})
@code{pthread_cleanup_pop} removes the most recently installed cleanup
handler. If the @var{execute} argument is not 0, it also executes the
handler, by calling the @var{routine} function with arguments
@var{arg}. If the @var{execute} argument is 0, the handler is only
removed but not executed.
@end deftypefun

Matching pairs of @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} must occur in the same function, at the same
level of block nesting.  Actually, @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} are macros, and the expansion of
@code{pthread_cleanup_push} introduces an open brace @code{@{} with the
matching closing brace @code{@}} being introduced by the expansion of the
matching @code{pthread_cleanup_pop}.

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_push_defer_np (void (*@var{routine}) (void *), void *@var{arg})
@code{pthread_cleanup_push_defer_np} is a non-portable extension that
combines @code{pthread_cleanup_push} and @code{pthread_setcanceltype}.
It pushes a cleanup handler just as @code{pthread_cleanup_push} does,
but also saves the current cancellation type and sets it to deferred
cancellation. This ensures that the cleanup mechanism is effective even
if the thread was initially in asynchronous cancellation mode.
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_pop_restore_np (int @var{execute})
@code{pthread_cleanup_pop_restore_np} pops a cleanup handler introduced
by @code{pthread_cleanup_push_defer_np}, and restores the cancellation
type to its value at the time @code{pthread_cleanup_push_defer_np} was
called.
@end deftypefun

@code{pthread_cleanup_push_defer_np} and
@code{pthread_cleanup_pop_restore_np} must occur in matching pairs, at
the same level of block nesting.

The sequence

@smallexample
pthread_cleanup_push_defer_np(routine, arg);
...
pthread_cleanup_pop_defer_np(execute);
@end smallexample

@noindent
is functionally equivalent to (but more compact and efficient than)

@smallexample
@{
  int oldtype;
  pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
  pthread_cleanup_push(routine, arg);
  ...
  pthread_cleanup_pop(execute);
  pthread_setcanceltype(oldtype, NULL);
@}
@end smallexample


@node Mutexes
@section Mutexes

A mutex is a MUTual EXclusion device, and is useful for protecting
shared data structures from concurrent modifications, and implementing
critical sections and monitors.

A mutex has two possible states: unlocked (not owned by any thread),
and locked (owned by one thread). A mutex can never be owned by two
different threads simultaneously. A thread attempting to lock a mutex
that is already locked by another thread is suspended until the owning
thread unlocks the mutex first.

None of the mutex functions is a cancellation point, not even
@code{pthread_mutex_lock}, in spite of the fact that it can suspend a
thread for arbitrary durations. This way, the status of mutexes at
cancellation points is predictable, allowing cancellation handlers to
unlock precisely those mutexes that need to be unlocked before the
thread stops executing. Consequently, threads using deferred
cancellation should never hold a mutex for extended periods of time.

It is not safe to call mutex functions from a signal handler.  In
particular, calling @code{pthread_mutex_lock} or
@code{pthread_mutex_unlock} from a signal handler may deadlock the
calling thread.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_init (pthread_mutex_t *@var{mutex}, const pthread_mutexattr_t *@var{mutexattr})

@code{pthread_mutex_init} initializes the mutex object pointed to by
@var{mutex} according to the mutex attributes specified in @var{mutexattr}.
If @var{mutexattr} is @code{NULL}, default attributes are used instead.

The LinuxThreads implementation supports only one mutex attribute,
the @var{mutex type}, which is either ``fast'', ``recursive'', or
``error checking''. The type of a mutex determines whether
it can be locked again by a thread that already owns it.
The default type is ``fast''.

Variables of type @code{pthread_mutex_t} can also be initialized
statically, using the constants @code{PTHREAD_MUTEX_INITIALIZER} (for
timed mutexes), @code{PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP} (for
recursive mutexes), @code{PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP}
(for fast mutexes(, and @code{PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP}
(for error checking mutexes).

@code{pthread_mutex_init} always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_lock (pthread_mutex_t *mutex))
@code{pthread_mutex_lock} locks the given mutex. If the mutex is
currently unlocked, it becomes locked and owned by the calling thread,
and @code{pthread_mutex_lock} returns immediately. If the mutex is