.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "PTHREAD_CLEANUP_POP" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_cleanup_pop 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
pthread_cleanup_pop, pthread_cleanup_push \- establish cancellation
handlers
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
void pthread_cleanup_pop(int\fP \fIexecute\fP\fB);
.br
void pthread_cleanup_push(void (*\fP\fIroutine\fP\fB)(void*), void
*\fP\fIarg\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpthread_cleanup_pop\fP() function shall remove the routine
at the top of the calling thread's cancellation cleanup stack
and optionally invoke it (if \fIexecute\fP is non-zero).
.LP
The \fIpthread_cleanup_push\fP() function shall push the specified
cancellation cleanup handler \fIroutine\fP 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 \fIarg\fP when:
.IP " *" 3
The thread exits (that is, calls \fIpthread_exit\fP()).
.LP
.IP " *" 3
The thread acts upon a cancellation request.
.LP
.IP " *" 3
The thread calls \fIpthread_cleanup_pop\fP() with a non-zero \fIexecute\fP
argument.
.LP
.LP
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 \fIpthread_cleanup_push\fP()
macro may be thought to expand to a token list whose first token
is \fB'{'\fP with \fIpthread_cleanup_pop\fP() expanding to a token
list whose last token is the corresponding \fB'}'\fP
).
.LP
The effect of calling \fIlongjmp\fP() or \fIsiglongjmp\fP() is undefined
if there have been any calls to \fIpthread_cleanup_push\fP()
or \fIpthread_cleanup_pop\fP() made without the matching call since
the jump buffer was filled. The effect of calling \fIlongjmp\fP()
or \fIsiglongjmp\fP() from inside a
cancellation cleanup handler is also undefined unless the jump buffer
was also filled in the cancellation cleanup handler.
.SH RETURN VALUE
.LP
The \fIpthread_cleanup_push\fP() and \fIpthread_cleanup_pop\fP() functions
shall not return a value.
.SH ERRORS
.LP
No errors are defined.
.LP
These functions shall not return an error code of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
The following is an example using thread primitives to implement a
cancelable, writers-priority read-write lock:
.sp
.RS
.nf

\fBtypedef 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;
.sp

void
waiting_reader_cleanup(void *arg)
{
    rwlock *l;
.sp

    l = (rwlock *) arg;
    pthread_mutex_unlock(&l->lock);
}
.sp

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);
}
.sp

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);
}
.sp

void
waiting_writer_cleanup(void *arg)
{
    rwlock *l;
.sp

    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);
}
.sp

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);
}
.sp

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);
}
.sp

/*
 * 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;
}
.sp

reader_thread()
{
    lock_for_read(&lock);
    pthread_cleanup_push(release_read_lock, &lock);
   /*
    * Thread has read lock.
    */
    pthread_cleanup_pop(1);
}
.sp

writer_thread()
{
    lock_for_write(&lock);
    pthread_cleanup_push(release_write_lock, &lock);
   /*
    * Thread has write lock.
    */
pthread_cleanup_pop(1);
}
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
The two routines that push and pop cancellation cleanup handlers,
\fIpthread_cleanup_push\fP() and
\fIpthread_cleanup_pop\fP(), can be thought of as left and right parentheses.
They always need to be matched.
.SH RATIONALE
.LP
The restriction that the two routines that push and pop cancellation
cleanup handlers, \fIpthread_cleanup_push\fP() and
\fIpthread_cleanup_pop\fP(), 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:
.sp
.RS
.nf

\fB#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;
.sp

#define pthread_cleanup_pop(ex) \\
    *__head = __cleanup_handler.next; \\
    if (ex) (*__cleanup_handler.rtn)(__cleanup_handler.arg); \\
}
\fP
.fi
.RE
.LP
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.
.LP
This volume of IEEE\ Std\ 1003.1-2001 currently leaves unspecified
the effect of calling \fIlongjmp\fP() 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 \fIlongjmp\fP() function has to call all cancellation
cleanup handlers that have been pushed but
not popped since the time \fIsetjmp\fP() was called.
.LP
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 \fIqsort\fP() and that handler were to call \fIlongjmp\fP()
(which, in turn, did \fInot\fP call the cancellation cleanup handlers)
the helper
threads created by the \fIqsort\fP() 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.
.LP
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.
.LP
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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpthread_cancel\fP(), \fIpthread_setcancelstate\fP(), the Base
Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<pthread.h>\fP
.SH COPYRIGHT
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 .