glibc/linuxthreads/man/pthread_cleanup_push.man

.TH PTHREAD_CLEANUP 3 LinuxThreads

.XREF pthread_cleanup_pop
.XREF pthread_cleanup_push_defer_np
.XREF pthread_cleanup_pop_restore_np

.SH NAME
pthread_cleanup_push, pthread_cleanup_pop, pthread_cleanup_push_defer_np, pthread_cleanup_pop_restore_np \- install and remove cleanup handlers

.SH 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);

.SH DESCRIPTION

Cleanup handlers are functions that get called when a thread
terminates, either by calling !pthread_exit!(3) 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 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
handler. 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.
Actually, !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 initially 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

.RS
.ft 3
.nf
.sp
pthread_cleanup_push_defer_np(routine, arg);
...
pthread_cleanup_pop_defer_np(execute);
.ft
.LP
.RE
.fi

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

.RS
.ft 3
.nf
.sp
{ int oldtype;
  pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
  pthread_cleanup_push(routine, arg);
  ...
  pthread_cleanup_pop(execute);
  pthread_setcanceltype(oldtype, NULL);
}
.ft
.LP
.RE
.fi

.SH "RETURN VALUE"

None.

.SH ERRORS

None.

.SH AUTHOR
Xavier Leroy <Xavier.Leroy@inria.fr>

.SH "SEE ALSO"
!pthread_exit!(3),
!pthread_cancel!(3),
!pthread_setcanceltype!(3).

.SH 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:

.RS
.ft 3
.nf
.sp
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
.ft
.LP
.RE
.fi

Equivalently, the last two lines can be replaced by

.RS
.ft 3
.nf
.sp
pthread_cleanup_pop(1);
.ft
.LP
.RE
.fi

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
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:

.RS
.ft 3
.nf
.sp
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);
.ft
.LP
.RE
.fi

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!:

.RS
.ft 3
.nf
.sp
pthread_cleanup_push_restore_np(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop_restore_np(1);
.ft
.LP
.RE
.fi