PTHREAD_CANCEL(3) manual page

#### POSIX Threads for Windows – REFERENCE - Pthreads-w32

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## Name

pthread_cancel, pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - thread cancellation

## Synopsis

#include <pthread.h>

int pthread_cancel(pthread_t thread);

int pthread_setcancelstate(int state, int *oldstate);

int pthread_setcanceltype(int type, int *oldtype);

void pthread_testcancel(void);

## Description

Cancellation is the mechanism by which a thread can terminate the execution of another thread. More precisely, a thread can send a cancellation request to another thread. Depending on its settings, the target thread can then either ignore the request, honor it immediately, or defer it until it reaches a cancellation point.

When a thread eventually honors a cancellation request, it performs as if pthread_exit(PTHREAD_CANCELED) has been called at that point: all cleanup handlers are executed in reverse order, destructor functions for thread-specific data are called, and finally the thread stops executing with the return value PTHREAD_CANCELED. See pthread_exit(3) for more information.

pthread_cancel sends a cancellation request to the thread denoted by the thread argument.

pthread_setcancelstate changes the cancellation state for the calling thread -- that is, whether cancellation requests are ignored or not. The state argument is the new cancellation state: either PTHREAD_CANCEL_ENABLE to enable cancellation, or PTHREAD_CANCEL_DISABLE to disable cancellation (cancellation requests are ignored). If oldstate is not NULL, the previous cancellation state is stored in the location pointed to by oldstate, and can thus be restored later by another call to pthread_setcancelstate.

pthread_setcanceltype changes the type of responses to cancellation requests for the calling thread: asynchronous (immediate) or deferred. The type argument is the new cancellation type: either PTHREAD_CANCEL_ASYNCHRONOUS to cancel the calling thread as soon as the cancellation request is received, or PTHREAD_CANCEL_DEFERRED to keep the cancellation request pending until the next cancellation point. If oldtype is not NULL, the previous cancellation state is stored in the location pointed to by oldtype, and can thus be restored later by another call to pthread_setcanceltype.

Pthreads-w32 provides two levels of support for PTHREAD_CANCEL_ASYNCHRONOUS: full and partial. Full support requires an additional DLL and driver be installed on the Windows system (see the See Also section below) that allows blocked threads to be cancelled immediately. Partial support means that the target thread will not cancel until it resumes execution naturally. Partial support is provided if either the DLL or the driver are not automatically detected by the pthreads-w32 library at run-time.

Threads are always created by pthread_create(3) with cancellation enabled and deferred. That is, the initial cancellation state is PTHREAD_CANCEL_ENABLE and the initial type is PTHREAD_CANCEL_DEFERRED.

Cancellation points are those points in the program execution where a test for pending cancellation requests is performed and cancellation is executed if positive. The following POSIX threads functions are cancellation points:

pthread_join(3)
pthread_cond_wait(3)
pthread_cond_timedwait(3)
pthread_testcancel(3)
sem_wait(3)
sem_timedwait(3)
sigwait(3)

Pthreads-w32 provides two functions to enable additional cancellation points to be created in user functions that block on Win32 HANDLEs:

pthreadCancelableWait()
pthreadCancelableTimedWait()

All other POSIX threads functions are guaranteed not to be cancellation points. That is, they never perform cancellation in deferred cancellation mode.

pthread_testcancel does nothing except testing for pending cancellation and executing it. Its purpose is to introduce explicit checks for cancellation in long sequences of code that do not call cancellation point functions otherwise.

## Return Value

pthread_cancel, pthread_setcancelstate and pthread_setcanceltype return 0 on success and a non-zero error code on error.

## Errors

pthread_cancel returns the following error code on error:

ESRCH
  no thread could be found corresponding to that specified by the thread ID.

pthread_setcancelstate returns the following error code on error:

EINVAL
  the state argument is not

> PTHREAD_CANCEL_ENABLE nor PTHREAD_CANCEL_DISABLE

pthread_setcanceltype returns the following error code on error:

EINVAL
  the type argument is not

> PTHREAD_CANCEL_DEFERRED nor PTHREAD_CANCEL_ASYNCHRONOUS

## Author

Xavier Leroy <Xavier.Leroy@inria.fr>

Modified by Ross Johnson for use with Pthreads-w32.

## See Also

pthread_exit(3) , pthread_cleanup_push(3) , pthread_cleanup_pop(3) , Pthreads-w32 package README file 'Prerequisites' section.

## Bugs

POSIX specifies that a number of system calls (basically, all system calls that may block, such as read(2) , write(2) , wait(2) , etc.) and library functions that may call these system calls (e.g. fprintf(3) ) are cancellation points. Pthreads-win32 is not integrated enough with the C library to implement this, and thus none of the C library functions is a cancellation point.

A workaround for these calls is to temporarily switch to asynchronous cancellation (assuming full asynchronous cancellation support is installed). So, checking for cancellation during a read system call, for instance, can be achieved as follows:

    pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &oldCancelType);
    read(fd, buffer, length);
    pthread_setcanceltype(oldCancelType, NULL);

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