pthread_cleanup_push(3) Library Functions Manual pthread_cleanup_push(3)
NAME
pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cance-
lation clean-up handlers
LIBRARY
POSIX threads library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
void pthread_cleanup_push(void (*routine)(void *), void *arg);
void pthread_cleanup_pop(int execute);
DESCRIPTION
These functions manipulate the calling thread's stack of thread-cance-
lation clean-up handlers. A clean-up handler is a function that is au-
tomatically 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 cancelation clean-up handler is popped from the stack and executed in
the following circumstances:
• 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.
• 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.)
• When a thread calls pthread_cleanup_pop() with a nonzero execute ar-
gument, 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 '}', re-
spectively. 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 estab-
lished 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) (sig-
longjmp(3)) from inside a clean-up handler produces undefined results
unless the jump buffer was also filled by setjmp(3) (sigsetjmp(3)) in-
side the handler.
RETURN VALUE
These functions do not return a value.
ERRORS
There are no errors.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│pthread_cleanup_push(), │ Thread safety │ MT-Safe │
│pthread_cleanup_pop() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008.
NOTES
On Linux, the pthread_cleanup_push() and pthread_cleanup_pop() func-
tions 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.
EXAMPLES
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. Depend-
ing on what command-line arguments are supplied, the main thread sends
the other thread a cancelation request, or sets a global variable that
causes the other thread to exit its loop and terminate normally (by do-
ing a return).
In the following shell session, the main thread sends a cancelation re-
quest 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 can-
celation 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 (be-
cause 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 <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.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 curr;
printf("New thread started\n");
pthread_cleanup_push(cleanup_handler, NULL);
curr = time(NULL);
while (!done) {
pthread_testcancel(); /* A cancelation point */
if (curr < time(NULL)) {
curr = time(NULL);
printf("cnt = %d\n", cnt); /* A cancelation 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_setcancel-
state(3), pthread_testcancel(3), pthreads(7)
Linux man-pages 6.03 2023-02-05 pthread_cleanup_push(3)
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