PTHREAD_TESTCANCEL(3) BSD Programmer's Manual PTHREAD_TESTCANCEL(3)
pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel - set cancelability state
#include <pthread.h> int pthread_setcancelstate(int state, int *oldstate); int pthread_setcanceltype(int type, int *oldtype); void pthread_testcancel(void);
The pthread_setcancelstate() function atomically both sets the calling thread's cancelability state to the indicated state and, if oldstate is not NULL, returns the previous cancelability state at the location refer- enced by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DISABLE. The pthread_setcanceltype() function atomically both sets the calling thread's cancelability type to the indicated type and, if oldtype is not NULL, returns the previous cancelability type at the location referenced by oldtype. Legal values for type are PTHREAD_CANCEL_DEFERRED and PTHREAD_CANCEL_ASYNCHRONOUS. The cancelability state and type of any newly created threads, including the thread in which main() was first invoked, are PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DEFERRED respectively. The pthread_testcancel() function creates a cancellation point in the calling thread. The pthread_testcancel() function has no effect if cance- lability is disabled.
The cancelability state of a thread determines the action taken upon re- ceipt of a cancellation request. The thread may control cancellation in a number of ways. Each thread maintains its own "cancelability state" which may be encoded in two bits: Cancelability Enable When cancelability is PTHREAD_CANCEL_DISABLE, can- cellation requests against the target thread are held pending. Cancelability Type When cancelability is enabled and the cancelability type is PTHREAD_CANCEL_ASYNCHRONOUS, new or pending cancellation requests may be acted upon at any time. When cancelability is en- abled and the cancelability type is PTHREAD_CANCEL_DEFERRED, can- cellation requests are held pending until a cancellation point (see below) is reached. If cancelability is disabled, the setting of the cancelability type has no immediate effect as all cancel- lation requests are held pending; however, once cancelability is enabled again the new type will be in effect.
Cancellation points will occur when a thread is executing the following functions: close(), creat(), fcntl(), fsync(), msync(), nanosleep(), open(), pause(), pthread_cond_timedwait(), pthread_cond_wait(), pthread_join(), pthread_testcancel(), read(), sigwaitinfo(), sig- suspend(), sigwait(), sleep(), system(), tcdrain(), wait(), waitpid(), write().
If successful, the pthread_setcancelstate() and pthread_setcanceltype() functions will return zero. Otherwise, an error number shall be returned to indicate the error. The pthread_setcancelstate() and pthread_setcanceltype() functions are used to control the points at which a thread may be asynchronously can- celled. For cancellation control to be usable in modular fashion, some rules must be followed. For purposes of this discussion, consider an object to be a generaliza- tion of a procedure. It is a set of procedures and global variables writ- ten as a unit and called by clients not known by the object. Objects may depend on other objects. First, cancelability should only be disabled on entry to an object, never explicitly enabled. On exit from an object, the cancelability state should always be restored to its value on entry to the object. This follows from a modularity argument: if the client of an object (or the client of an object that uses that object) has disabled cancelabili- ty, it is because the client doesn't want to have to worry about how to clean up if the thread is cancelled while executing some sequence of ac- tions. If an object is called in such a state and it enables cancelabili- ty and a cancellation request is pending for that thread, then the thread will be cancelled, contrary to the wish of the client that disabled. Second, the cancelability type may be explicitly set to either deferred or asynchronous upon entry to an object. But as with the cancelability state, on exit from an object that cancelability type should always be restored to its value on entry to the object. Finally, only functions that are cancel-safe may be called from a thread that is asynchronously cancelable.
The function pthread_setcancelstate() may fail with: [EINVAL] The specified state is not PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE. The function pthread_setcanceltype() may fail with: [EINVAL] The specified state is not PTHREAD_CANCEL_DEFERRED or PTHREAD_CANCEL_ASYNCHRONOUS.
pthread_testcancel() conforms to ISO/IEC 9945-1:1996 ("POSIX") MirOS BSD #10-current January 17, 1999 1
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