pthread_cleanup_push_defer_np
PTHREAD_CLEANUP(3) PTHREAD_CLEANUP(3)
NAME
pthread_cleanup_push, pthread_cleanup_pop,
pthread_cleanup_push_defer_np, pthread_cleanup_pop_restore_np - install
and remove cleanup handlers
SYNOPSIS
#include <pthread.h>
void pthread_cleanup_push(void (*routine) (void *), void *arg);
void pthread_cleanup_pop(int execute);
void pthread_cleanup_push_defer_np(void (*routine) (void *), void
*arg);
void pthread_cleanup_pop_restore_np(int execute);
DESCRIPTION
Cleanup handlers are functions that get called when a thread termi-
nates, either by calling pthread_exit(3) or because of cancellation.
Cleanup handlers are installed and removed following a stack-like dis-
cipline.
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 cancelled 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 malloc(3) or close file
descriptors on thread termination.
pthread_cleanup_push installs the routine function with argument arg as
a cleanup handler. From this point on to the matching
pthread_cleanup_pop, the function routine will be called with arguments
arg when the thread terminates, either through pthread_exit(3) 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.
pthread_cleanup_pop removes the most recently installed cleanup han-
dler. If the execute argument is not 0, it also executes the handler,
by calling the routine function with arguments arg. If the execute
argument is 0, the handler is only removed but not executed.
Matching pairs of pthread_cleanup_push and pthread_cleanup_pop must
occur in the same function, at the same level of block nesting. Actu-
ally, pthread_cleanup_push and pthread_cleanup_pop are macros, and the
expansion of pthread_cleanup_push introduces an open brace { with the
matching closing brace } being introduced by the expansion of the
matching pthread_cleanup_pop.
pthread_cleanup_push_defer_np is a non-portable extension that combines
pthread_cleanup_push and pthread_setcanceltype(3). It pushes a cleanup
handler just as 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 ini-
tially in asynchronous cancellation mode.
pthread_cleanup_pop_restore_np pops a cleanup handler introduced by
pthread_cleanup_push_defer_np, and restores the cancellation type to
its value at the time pthread_cleanup_push_defer_np was called.
pthread_cleanup_push_defer_np and pthread_cleanup_pop_restore_np must
occur in matching pairs, at the same level of block nesting.
The following sequence
pthread_cleanup_push_defer_np(routine, arg);
pthread_cleanup_pop_defer_np(execute);
is functionally equivalent to (but more compact and more efficient
than)
{ int oldtype;
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
pthread_cleanup_push(routine, arg);
...
pthread_cleanup_pop(execute);
pthread_setcanceltype(oldtype, NULL);
}
RETURN VALUE
None.
ERRORS
None.
AUTHOR
Xavier Leroy <Xavier.Leroy@inria.fr>
SEE ALSO
pthread_exit(3), pthread_cancel(3), pthread_setcanceltype(3).
EXAMPLE
Here is how to lock a mutex mut in such a way that it will be unlocked
if the thread is canceled while mut is locked:
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
Equivalently, the last two lines can be replaced by
pthread_cleanup_pop(1);
Notice that the code above is safe only in deferred cancellation mode
(see pthread_setcanceltype(3)). In asynchronous cancellation mode, a
cancellation can occur between pthread_cleanup_push and
pthread_mutex_lock, or between pthread_mutex_unlock and
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 rea-
son 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:
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);
The code above can be rewritten in a more compact and more efficient
way, using the non-portable functions pthread_cleanup_push_defer_np and
pthread_cleanup_pop_restore_np:
pthread_cleanup_push_restore_np(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop_restore_np(1);
LinuxThreads PTHREAD_CLEANUP(3)
