PTHREAD_TESTCANCEL(3) BSD Library Functions Manual PTHREAD_TESTCANCEL(3)
pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel -- set cancelability state
POSIX Threads Library (libpthread, -lpthread)
pthread_setcancelstate(int state, int *oldstate);
pthread_setcanceltype(int type, int *oldtype);
The pthread_setcancelstate() function atomically both sets the calling thread's cancelabil-
ity state to the indicated state and, if oldstate is not NULL, returns the previous cancela-
bility state at the location referenced 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 respec-
The pthread_testcancel() function creates a cancellation point in the calling thread. The
pthread_testcancel() function has no effect if cancelability is disabled.
The cancelability state of a thread determines the action taken upon receipt of a cancella-
tion 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, cancellation 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 enabled and the cancelability type is
PTHREAD_CANCEL_DEFERRED, cancellation requests are held pending until a cancellation
point (see below) is reached. If cancelability is disabled, the setting of the can-
celability type has no immediate effect as all cancellation requests are held pend-
ing; 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: accept(),
aio_suspend(), clock_nanosleep(), close(), connect(), creat(), fcntl(), fdatasync(),
fsync(), fsync_range(), kevent(), mq_receive(), mq_send(), mq_timedreceive(),
mq_timedsend(), msgrcv(), msgsnd(), msync(), nanosleep(), open(), pause(), poll(), pollts(),
pread(), pselect(), pthread_cond_timedwait(), pthread_cond_wait(), pthread_join(),
pthread_testcancel(), pwrite(), read(), readv(), recv(), recvfrom(), recvmsg(), select(),
sem_timedwait(), sem_wait(), send(), sendmsg(), sendto(), sigpause(), sigsuspend(),
sigtimedwait(), sigwait(), sigwaitinfo(), sleep(), system(), tcdrain(), usleep(), wait(),
wait4(), waitid(), waitpid(), write(), and writev().
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 canceled. 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 generalization of a procedure.
It is a set of procedures and global variables written 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 cancelability, it is because the client doesn't
want to have to worry about how to clean up if the thread is canceled while executing some
sequence of actions. If an object is called in such a state and it enables cancelability
and a cancellation request is pending for that thread, then the thread will be canceled,
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 cance-
lability 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 asyn-
The function pthread_setcancelstate() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_ENABLE or
The function pthread_setcanceltype() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_DEFERRED or
These functions conform to IEEE Std 1003.1-2001 (``POSIX.1'').
This man page was written by David Leonard <email@example.com> for the OpenBSD implementation of
BSD August 6, 2010 BSD