Originální popis anglicky:
pthread_cleanup_pop, pthread_cleanup_push - establish cancellation handlers
Návod, kniha: POSIX Programmer's Manual
#include <pthread.h>
void pthread_cleanup_pop(int
execute);
void pthread_cleanup_push(void (*
routine)(void*),
void *arg);
The
pthread_cleanup_pop() function shall remove the routine at the top of
the calling thread's cancellation cleanup stack and optionally invoke it (if
execute is non-zero).
The
pthread_cleanup_push() function shall push the specified cancellation
cleanup handler
routine onto the calling thread's cancellation cleanup
stack. The cancellation cleanup handler shall be popped from the cancellation
cleanup stack and invoked with the argument
arg when:
- *
- The thread exits (that is, calls
pthread_exit()).
- *
- The thread acts upon a cancellation request.
- *
- The thread calls pthread_cleanup_pop() with a
non-zero execute argument.
These functions may be implemented as macros. The application shall ensure that
they appear as statements, and in pairs within the same lexical scope (that
is, the
pthread_cleanup_push() macro may be thought to expand to a
token list whose first token is
'{' with
pthread_cleanup_pop()
expanding to a token list whose last token is the corresponding
'}' ).
The effect of calling
longjmp() or
siglongjmp() is undefined if
there have been any calls to
pthread_cleanup_push() or
pthread_cleanup_pop() made without the matching call since the jump
buffer was filled. The effect of calling
longjmp() or
siglongjmp() from inside a cancellation cleanup handler is also
undefined unless the jump buffer was also filled in the cancellation cleanup
handler.
The
pthread_cleanup_push() and
pthread_cleanup_pop() functions
shall not return a value.
No errors are defined.
These functions shall not return an error code of [EINTR].
The following sections are informative.
The following is an example using thread primitives to implement a cancelable,
writers-priority read-write lock:
typedef struct {
pthread_mutex_t lock;
pthread_cond_t rcond,
wcond;
int lock_count; /* < 0 .. Held by writer. */
/* > 0 .. Held by lock_count readers. */
/* = 0 .. Held by nobody. */
int waiting_writers; /* Count of waiting writers. */
} rwlock;
void
waiting_reader_cleanup(void *arg)
{
rwlock *l;
l = (rwlock *) arg;
pthread_mutex_unlock(&l->lock);
}
void
lock_for_read(rwlock *l)
{
pthread_mutex_lock(&l->lock);
pthread_cleanup_push(waiting_reader_cleanup, l);
while ((l->lock_count < 0) && (l->waiting_writers != 0))
pthread_cond_wait(&l->rcond, &l->lock);
l->lock_count++;
/*
* Note the pthread_cleanup_pop executes
* waiting_reader_cleanup.
*/
pthread_cleanup_pop(1);
}
void
release_read_lock(rwlock *l)
{
pthread_mutex_lock(&l->lock);
if (--l->lock_count == 0)
pthread_cond_signal(&l->wcond);
pthread_mutex_unlock(l);
}
void
waiting_writer_cleanup(void *arg)
{
rwlock *l;
l = (rwlock *) arg;
if ((--l->waiting_writers == 0) && (l->lock_count >= 0)) {
/*
* This only happens if we have been canceled.
*/
pthread_cond_broadcast(&l->wcond);
}
pthread_mutex_unlock(&l->lock);
}
void
lock_for_write(rwlock *l)
{
pthread_mutex_lock(&l->lock);
l->waiting_writers++;
pthread_cleanup_push(waiting_writer_cleanup, l);
while (l->lock_count != 0)
pthread_cond_wait(&l->wcond, &l->lock);
l->lock_count = -1;
/*
* Note the pthread_cleanup_pop executes
* waiting_writer_cleanup.
*/
pthread_cleanup_pop(1);
}
void
release_write_lock(rwlock *l)
{
pthread_mutex_lock(&l->lock);
l->lock_count = 0;
if (l->waiting_writers == 0)
pthread_cond_broadcast(&l->rcond)
else
pthread_cond_signal(&l->wcond);
pthread_mutex_unlock(&l->lock);
}
/*
* This function is called to initialize the read/write lock.
*/
void
initialize_rwlock(rwlock *l)
{
pthread_mutex_init(&l->lock, pthread_mutexattr_default);
pthread_cond_init(&l->wcond, pthread_condattr_default);
pthread_cond_init(&l->rcond, pthread_condattr_default);
l->lock_count = 0;
l->waiting_writers = 0;
}
reader_thread()
{
lock_for_read(&lock);
pthread_cleanup_push(release_read_lock, &lock);
/*
* Thread has read lock.
*/
pthread_cleanup_pop(1);
}
writer_thread()
{
lock_for_write(&lock);
pthread_cleanup_push(release_write_lock, &lock);
/*
* Thread has write lock.
*/
pthread_cleanup_pop(1);
}
The two routines that push and pop cancellation cleanup handlers,
pthread_cleanup_push() and
pthread_cleanup_pop(), can be thought
of as left and right parentheses. They always need to be matched.
The restriction that the two routines that push and pop cancellation cleanup
handlers,
pthread_cleanup_push() and
pthread_cleanup_pop(), have
to appear in the same lexical scope allows for efficient macro or compiler
implementations and efficient storage management. A sample implementation of
these routines as macros might look like this:
#define pthread_cleanup_push(rtn,arg) { \
struct _pthread_handler_rec __cleanup_handler, **__head; \
__cleanup_handler.rtn = rtn; \
__cleanup_handler.arg = arg; \
(void) pthread_getspecific(_pthread_handler_key, &__head); \
__cleanup_handler.next = *__head; \
*__head = &__cleanup_handler;
#define pthread_cleanup_pop(ex) \
*__head = __cleanup_handler.next; \
if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \
}
A more ambitious implementation of these routines might do even better by
allowing the compiler to note that the cancellation cleanup handler is a
constant and can be expanded inline.
This volume of IEEE Std 1003.1-2001 currently leaves unspecified
the effect of calling
longjmp() from a signal handler executing in a
POSIX System Interfaces function. If an implementation wants to allow this and
give the programmer reasonable behavior, the
longjmp() function has to
call all cancellation cleanup handlers that have been pushed but not popped
since the time
setjmp() was called.
Consider a multi-threaded function called by a thread that uses signals. If a
signal were delivered to a signal handler during the operation of
qsort() and that handler were to call
longjmp() (which, in turn,
did
not call the cancellation cleanup handlers) the helper threads
created by the
qsort() function would not be canceled. Instead, they
would continue to execute and write into the argument array even though the
array might have been popped off the stack.
Note that the specified cleanup handling mechanism is especially tied to the C
language and, while the requirement for a uniform mechanism for expressing
cleanup is language-independent, the mechanism used in other languages may be
quite different. In addition, this mechanism is really only necessary due to
the lack of a real exception mechanism in the C language, which would be the
ideal solution.
There is no notion of a cancellation cleanup-safe function. If an application
has no cancellation points in its signal handlers, blocks any signal whose
handler may have cancellation points while calling async-unsafe functions, or
disables cancellation while calling async-unsafe functions, all functions may
be safely called from cancellation cleanup routines.
None.
pthread_cancel() ,
pthread_setcancelstate() , the Base Definitions
volume of IEEE Std 1003.1-2001,
<pthread.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
