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| .TH PTHREAD_SETCANCELSTATE 3 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| pthread_setcancelstate, pthread_setcanceltype \- |
| set cancelability state and type |
| .SH SYNOPSIS |
| .nf |
| .B #include <pthread.h> |
| .PP |
| .BI "int pthread_setcancelstate(int " state ", int *" oldstate ); |
| .BI "int pthread_setcanceltype(int " type ", int *" oldtype ); |
| .PP |
| Compile and link with \fI\-pthread\fP. |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR pthread_setcancelstate () |
| sets the cancelability state of the calling thread to the value |
| given in |
| .IR state . |
| The previous cancelability state of the thread is returned |
| in the buffer pointed to by |
| .IR oldstate . |
| The |
| .I state |
| argument must have one of the following values: |
| .TP |
| .B PTHREAD_CANCEL_ENABLE |
| The thread is cancelable. |
| This is the default cancelability state in all new threads, |
| including the initial thread. |
| The thread's cancelability type determines when a cancelable thread |
| will respond to a cancellation request. |
| .TP |
| .B PTHREAD_CANCEL_DISABLE |
| The thread is not cancelable. |
| If a cancellation request is received, |
| it is blocked until cancelability is enabled. |
| .PP |
| The |
| .BR pthread_setcanceltype () |
| sets the cancelability type of the calling thread to the value |
| given in |
| .IR type . |
| The previous cancelability type of the thread is returned |
| in the buffer pointed to by |
| .IR oldtype . |
| The |
| .I type |
| argument must have one of the following values: |
| .TP |
| .B PTHREAD_CANCEL_DEFERRED |
| A cancellation request is deferred until the thread next calls |
| a function that is a cancellation point (see |
| .BR pthreads (7)). |
| This is the default cancelability type in all new threads, |
| including the initial thread. |
| .IP |
| Even with deferred cancellation, a |
| cancellation point in an asynchronous signal handler may still |
| be acted upon and the effect is as if it was an asynchronous |
| cancellation. |
| .TP |
| .B PTHREAD_CANCEL_ASYNCHRONOUS |
| The thread can be canceled at any time. |
| (Typically, |
| it will be canceled immediately upon receiving a cancellation request, |
| but the system doesn't guarantee this.) |
| .PP |
| The set-and-get operation performed by each of these functions |
| is atomic with respect to other threads in the process |
| calling the same function. |
| .SH RETURN VALUE |
| On success, these functions return 0; |
| on error, they return a nonzero error number. |
| .SH ERRORS |
| The |
| .BR pthread_setcancelstate () |
| can fail with the following error: |
| .TP |
| .B EINVAL |
| Invalid value for |
| .IR state . |
| .PP |
| The |
| .BR pthread_setcanceltype () |
| can fail with the following error: |
| .TP |
| .B EINVAL |
| Invalid value for |
| .IR type . |
| .\" .SH VERSIONS |
| .\" Available since glibc 2.0 |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR pthread_setcancelstate (), |
| .BR pthread_setcanceltype () |
| T} Thread safety T{ |
| MT-Safe |
| T} |
| T{ |
| .BR pthread_setcancelstate (), |
| .BR pthread_setcanceltype () |
| T} Async-cancel safety T{ |
| AC-Safe |
| T} |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .hy |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| For details of what happens when a thread is canceled, see |
| .BR pthread_cancel (3). |
| .PP |
| Briefly disabling cancelability is useful |
| if a thread performs some critical action |
| that must not be interrupted by a cancellation request. |
| Beware of disabling cancelability for long periods, |
| or around operations that may block for long periods, |
| since that will render the thread unresponsive to cancellation requests. |
| .SS Asynchronous cancelability |
| Setting the cancelability type to |
| .B PTHREAD_CANCEL_ASYNCHRONOUS |
| is rarely useful. |
| Since the thread could be canceled at |
| .I any |
| time, it cannot safely reserve resources (e.g., allocating memory with |
| .BR malloc (3)), |
| acquire mutexes, semaphores, or locks, and so on. |
| Reserving resources is unsafe because the application has no way of |
| knowing what the state of these resources is when the thread is canceled; |
| that is, did cancellation occur before the resources were reserved, |
| while they were reserved, or after they were released? |
| Furthermore, some internal data structures |
| (e.g., the linked list of free blocks managed by the |
| .BR malloc (3) |
| family of functions) may be left in an inconsistent state |
| if cancellation occurs in the middle of the function call. |
| Consequently, clean-up handlers cease to be useful. |
| .PP |
| Functions that can be safely asynchronously canceled are called |
| .IR "async-cancel-safe functions" . |
| POSIX.1-2001 and POSIX.1-2008 require only that |
| .BR pthread_cancel (3), |
| .BR pthread_setcancelstate (), |
| and |
| .BR pthread_setcanceltype () |
| be async-cancel-safe. |
| In general, other library functions |
| can't be safely called from an asynchronously cancelable thread. |
| .PP |
| One of the few circumstances in which asynchronous cancelability is useful |
| is for cancellation of a thread that is in a pure compute-bound loop. |
| .SS Portability notes |
| The Linux threading implementations permit the |
| .I oldstate |
| argument of |
| .BR pthread_setcancelstate () |
| to be NULL, in which case the information about the previous |
| cancelability state is not returned to the caller. |
| Many other implementations also permit a NULL |
| .I oldstat |
| argument, |
| .\" It looks like at least Solaris, FreeBSD and Tru64 support this. |
| but POSIX.1 does not specify this point, |
| so portable applications should always specify a non-NULL value in |
| .IR oldstate . |
| A precisely analogous set of statements applies for the |
| .I oldtype |
| argument of |
| .BR pthread_setcanceltype (). |
| .SH EXAMPLES |
| See |
| .BR pthread_cancel (3). |
| .SH SEE ALSO |
| .BR pthread_cancel (3), |
| .BR pthread_cleanup_push (3), |
| .BR pthread_testcancel (3), |
| .BR pthreads (7) |