For review: pthread_cleanup_push.3

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Any reviewer for pthread_cleanup_push(3)?

Cheers,

Michael

.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@xxxxxxxxx>
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.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.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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