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NAME
pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancellation clean-up
handlers
SYNOPSIS
#include <pthread.h>
void pthread_cleanup_push(void (*routine)(void *),
void *arg);
void pthread_cleanup_pop(int execute);
Compile and link with -pthread.
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 pthread_cleanup_push() function pushes routine onto the top of the stack of clean-up
handlers. When routine is later invoked, it will be given arg as its argument.
The pthread_cleanup_pop() function removes the routine at the top of the stack of clean-up
handlers, and optionally executes it if execute is nonzero.
A cancellation clean-up handler is popped from the stack and executed in the following
circumstances:
1. 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.
2. When a thread terminates by calling pthread_exit(3), all clean-up handlers are executed
as described in the preceding point. (Clean-up handlers are not called if the thread
terminates by performing a return from the thread start function.)
3. When a thread calls pthread_cleanup_pop() with a nonzero execute argument, the top-most
clean-up handler is popped and executed.
POSIX.1 permits pthread_cleanup_push() and pthread_cleanup_pop() to be implemented as
macros that expand to text containing '{' and '}', 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 established
only during the execution of a specified section of code.)
Calling longjmp(3) (siglongjmp(3)) produces undefined results if any call has been made to
pthread_cleanup_push() or pthread_cleanup_pop() without the matching call of the pair
since the jump buffer was filled by setjmp(3) (sigsetjmp(3)). Likewise, calling
longjmp(3) (siglongjmp(3)) from inside a clean-up handler produces undefined results
unless the jump buffer was also filled by setjmp(3) (sigsetjmp(3)) inside the handler.
RETURN VALUE
These functions do not return a value.
ERRORS
There are no errors.
CONFORMING TO
POSIX.1-2001.
NOTES
On Linux, the pthread_cleanup_push() and pthread_cleanup_pop() functions are implemented
as macros that expand to text containing '{' and '}', respectively. This means that
variables declared within the scope of paired calls to these functions will be visible
within only that scope.
POSIX.1 says that the effect of using return, break, continue, or goto to prematurely
leave a block bracketed pthread_cleanup_push() and pthread_cleanup_pop() is undefined.
Portable applications should avoid doing this.
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
pthread_cleanup_push() and pthread_cleanup_pop(). This loop increments a global variable,
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 return).
In the following shell session, the main thread sends a cancellation request to the other
thread:
$ ./a.out
New thread started
cnt = 0
cnt = 1
Canceling thread
Called clean-up handler
Thread was canceled; cnt = 0
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 cnt to 0.
In the next run, the main program sets a global variable that causes other thread to
terminate normally:
$ ./a.out x
New thread started
cnt = 0
cnt = 1
Thread terminated normally; cnt = 2
From the above, we see that the clean-up handler was not executed (because cleanup_pop_arg
was 0), and therefore the value of 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 nonzero value for cleanup_pop_arg:
$ ./a.out x 1
New thread started
cnt = 0
cnt = 1
Called clean-up handler
Thread terminated normally; cnt = 0
In the above, we see that although the thread was not canceled, the clean-up handler was
executed, because the argument given to pthread_cleanup_pop() was nonzero.
Program source
#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, NULL);
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);
}
SEE ALSO
pthread_cancel(3), pthread_cleanup_push_defer_np(3), pthread_setcancelstate(3),
pthread_testcancel(3), pthreads(7)
COLOPHON
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