PTHREAD_CLEANUP_POP(3P)

💖 PTHREAD_CLEANUP_POP(3P) 💖

okay so this is a page from the POSIX Programmer's Manual, and it's the one about pthread_cleanup_pop (and its partner in crime pthread_cleanup_push). grab a snack, bestie, we're getting into cancellation handlers ✨

---

🌸 PROLOG 🌸

Real talk first: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go consult the corresponding Linux manual page for details of Linux behavior), or honestly the interface may not be implemented on Linux at all. 💀

---

🎀 NAME 🎀

pthread_cleanup_pop, pthread_cleanup_push — establish cancellation handlers. that's it, that's the whole vibe.

---

✨ SYNOPSIS ✨

#include <pthread.h>

void pthread_cleanup_pop(int execute);
void pthread_cleanup_push(void (*routine)(void*), void *arg);

---

🦋 DESCRIPTION 🦋

So pthread_cleanup_pop() removes the routine sitting at the top of the calling thread's cancellation cleanup stack, and optionally invokes it — it actually runs it if execute is non-zero. yes bestie, the flag is doing something. 💅

And pthread_cleanup_push() pushes the specified cancellation cleanup handler routine onto the calling thread's cancellation cleanup stack. That handler gets popped off 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.

Now, the iconic chaotic detail: it is unspecified whether pthread_cleanup_push() and pthread_cleanup_pop() are macros or functions. And if a macro definition gets suppressed so you can access an actual function, or if a program defines an external identifier with any of these names, the behavior is undefined. 🙈

The application shall ensure that they appear as statements, and in pairs within the same lexical scope — like, you can think of the pthread_cleanup_push() macro as expanding to a token list whose first token is '{', with pthread_cleanup_pop() expanding to a token list whose last token is the corresponding '}'. they're literally brackets, no cap.

⚠️ Also 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. And 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.

⚠️ And the effect of using return, break, continue, and goto to prematurely leave a code block described by a pair of pthread_cleanup_push() and pthread_cleanup_pop() function calls is undefined too. do not do it, i am begging. 😭

---

⭐ RETURN VALUE ⭐

The pthread_cleanup_push() and pthread_cleanup_pop() functions shall not return a value. nothing. void. slay.

---

💜 ERRORS 💜

No errors are defined. (iconic behavior honestly.)

These functions shall not return an error code of [EINTR].

The following sections are informative.

---

🍓 EXAMPLES 🍓

Here's 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->lock);
}

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. If the
        * lock is not held by a writer, there may be readers who
        * were blocked because waiting_writers was positive; they
        * can now be unblocked.
        */
        pthread_cond_broadcast(&l->rcond);
    }
    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);
}

---

🌈 APPLICATION USAGE 🌈

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. always. periodt. 💅

---

👑 RATIONALE 👑

So 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 is what 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 POSIX.1‐2017 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. chaos. actual chaos. 😭

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. (we know, bestie. we know. 🫶)

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. iykyk 💅

---

🧚 FUTURE DIRECTIONS 🧚

None.

---

💫 SEE ALSO 💫

• pthread_cancel()
• pthread_setcancelstate()
• The Base Definitions volume of POSIX.1‐2017, <pthread.h>

---

💖 COPYRIGHT 💖

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

---

and that's the whole page, bestie — brought to you by IEEE/The Open Group, 2017, page PTHREAD_CLEANUP_POP(3P). go match your parens, stay safe, and remember: push and pop are a pair, not a situationship. 🎀✨💅