Any reviewer for pthread_cleanup_push(3)?
Cheers,
Michael
.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.\"
.TH PTHREAD_CLEANUP_PUSH 3 2008-11-14 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_cleanup_push, pthread_cleanup_pop \- push and pop
thread cancellation clean-up handlers
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.BI "void pthread_cleanup_push(void (*" routine ")(void *),"
.BI " void *" arg );
.BI "void pthread_cleanup_pop(int " execute );
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
These functions manipulate the calling thread's stack of
thread-cancellation clean-up handlers.
A clean-up handler is a function that is automatically executed
when a thread is canceled (or in various other circumstances
described below);
it might, for example, unlock a mutex so that
it becomes available to other threads in the process.
The
.BR pthread_cleanup_push ()
function pushes
.I routine
onto the top of the stack of clean-up handlers.
When
.I routine
is later invoked, it will be given
.I arg
as its argument.
The
.BR pthread_cleanup_pop ()
function removes the routine at the top of the stack of clean-up handlers,
and optionally executes it if
.I execute
is non-zero.
A cancellation clean-up handler is popped from the stack
and executed in the following circumstances:
.IP 1. 3
When a thread is canceled,
all of the stacked clean-up handlers are popped and executed in
the reverse of the order in which they were pushed onto the stack.
.IP 2.
When a thread terminates by calling
.BR pthread_exit (3),
all clean-up handlers are executed as described in the preceding point.
(Clean-up handlers are \fInot\fP called if the thread terminates by
performing a
.I return
from the thread start function.)
.IP 3.
When a thread calls
.BR pthread_cleanup_pop ()
with a non-zero
.I execute
argument, the top-most clean-up handler is popped and executed.
.PP
POSIX.1 permits
.BR pthread_cleanup_push ()
and
.BR pthread_cleanup_pop ()
to be implemented as macros that expand to text
containing \(aq\fB{\fP\(aq and \(aq\fB}\fP\(aq, respectively.
For this reason, the caller must ensure that calls to these
functions are paired within the same function,
and at the same lexical nesting level.
(In other words, a clean-up handler is only established
during the execution of a specified section of code.)
Calling
.BR longjmp (3)
.RB ( siglongjmp (3))
produces undefined results if any call has been made to
.BR pthread_cleanup_push ()
or
.BR pthread_cleanup_pop ()
without the matching call of the pair since the jump buffer
was filled by
.BR setjmp (3)
.RB ( sigsetjmp (3)).
Likewise, calling
.BR longjmp (3)
.RB ( siglongjmp (3))
from inside a clean-up handler produces undefined results
unless the jump buffer was also filled by
.BR setjmp (3)
.RB ( sigsetjmp (3))
inside the handler.
.SH RETURN VALUE
These functions do not return a value.
.SH ERRORS
There are no errors.
.\" SH VERSIONS
.\" Available since glibc 2.0
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
On Linux, the
.BR pthread_cleanup_push ()
and
.BR pthread_cleanup_pop ()
functions \fIare\fP implemented as macros that expand to text
containing \(aq\fB{\fP\(aq and \(aq\fB}\fP\(aq, respectively.
This means that variables declared within the scope of
paired calls to these functions will only be visible within that scope.
.SH EXAMPLE
The program below provides a simple example of the use of the functions
described in this page.
The program creates a thread that executes a loop bracketed by
.BR pthread_cleanup_push ()
and
.BR pthread_cleanup_pop ().
This loop executes a global variable,
.IR cnt ,
once each second.
Depending on what command-line arguments are supplied,
the main thread sends the other thread a cancellation request,
or sets a global variable that causes the other thread
to exit its loop and terminate normally (by doing a
.IR return ).
In the following shell session,
the main thread sends a cancellation request to the other thread:
.in +4n
.nf
$ \fB./a.out\fP
New thread started
cnt = 0
cnt = 1
Canceling thread
Called clean-up handler
Thread was canceled; cnt = 0
.fi
.in
>From the above, we see that the thread was canceled,
and that the cancellation clean-up handler was called
and it reset the value of the global variable
.I cnt
to 0.
In the next run, the main program sets a
global variable that causes other thread to terminate normally:
.in +4n
.nf
$ \fB./a.out x\fP
New thread started
cnt = 0
cnt = 1
Thread terminated normally; cnt = 2
.fi
.in
>From the above, we see that the clean-up handler was not executed (because
.I cleanup_pop_arg
was 0), and therefore the value of
.I cnt
was not reset.
In the next run, the main program sets a global variable that
causes the other thread to terminate normally,
and supplies a non-zero value for
.IR cleanup_pop_arg :
.in +4n
.nf
$ \fB./a.out x 1\fP
New thread started
cnt = 0
cnt = 1
Called clean-up handler
Thread terminated normally; cnt = 0
.fi
.in
In the above, we see that although the thread was not canceled,
the clean-up handler was executed, because the argument given to
.BR pthread_cleanup_pop ()
was non-zero.
.SS Program source
\&
.nf
#include <pthread.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#define handle_error_en(en, msg) \\
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
static int done = 0;
static int cleanup_pop_arg = 0;
static int cnt = 0;
static void
cleanup_handler(void *arg)
{
printf("Called clean\-up handler\\n");
cnt = 0;
}
static void *
thread_start(void *arg)
{
time_t start, curr;
printf("New thread started\\n");
pthread_cleanup_push(cleanup_handler, NULL);
curr = start = time(NULL);
while (!done) {
pthread_testcancel(); /* A cancellation point */
if (curr < time(NULL)) {
curr = time(NULL);
printf("cnt = %d\\n", cnt); /* A cancellation point */
cnt++;
}
}
pthread_cleanup_pop(cleanup_pop_arg);
return NULL;
}
int
main(int argc, char *argv[])
{
pthread_t thr;
int s;
void *res;
s = pthread_create(&thr, NULL, thread_start, (void *) 1);
if (s != 0)
handle_error_en(s, "pthread_create");
sleep(2); /* Allow new thread to run a while */
if (argc > 1) {
if (argc > 2)
cleanup_pop_arg = atoi(argv[2]);
done = 1;
} else {
printf("Canceling thread\\n");
s = pthread_cancel(thr);
if (s != 0)
handle_error_en(s, "pthread_cancel");
}
s = pthread_join(thr, &res);
if (s != 0)
handle_error_en(s, "pthread_join");
if (res == PTHREAD_CANCELED)
printf("Thread was canceled; cnt = %d\\n", cnt);
else
printf("Thread terminated normally; cnt = %d\\n", cnt);
exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR pthread_cancel (3),
.BR pthread_cleanup_push_defer_np (3),
.BR pthread_setcancelstate (3),
.BR pthread_testcancel (3),
.BR pthreads (7)
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