pthread_testcancel - set
pthread_setcancelstate(int state, int *oldstate);
pthread_setcanceltype(int type, int *oldtype);
The pthread_setcancelstate() function atomically both sets
thread's cancelability state to the indicated state and, if
not NULL, returns the previous cancelability state at the
by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and
The pthread_setcanceltype() function atomically both sets
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
The cancelability state and type of any newly created
the thread in which main() was first invoked, are
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 cancelability
Cancelability States [Toc] [Back]
The cancelability state of a thread determines the action
taken upon receipt
of a cancellation request. The thread may control
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
Cancelability Type When cancelability is enabled and the
type is PTHREAD_CANCEL_ASYNCHRONOUS, new or pending
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 cancelability type has no immediate effect as all
cancellation requests are held pending; however,
is enabled again the new type will be in effect.
Cancellation Points [Toc] [Back]
Cancellation points will occur when a thread is executing
functions: close(), creat(), fcntl(), fsync(), msync(),
open(), pause(), pthread_cond_timedwait(),
pthread_join(), pthread_testcancel(), read(), sigwaitinfo(),
sigsuspend(), sigwait(), sleep(), system(), tcdrain(),
If successful, the pthread_setcancelstate() and
functions will return zero. Otherwise, an error number
shall be returned
to indicate the error.
The pthread_setcancelstate() and pthread_setcanceltype()
used to control the points at which a thread may be asynchronously cancelled.
For cancellation control to be usable in modular
rules must be followed.
For purposes of this discussion, consider an object to be a
of a procedure. It is a set of procedures and global
as a unit and called by clients not known by the object.
depend on other objects.
First, cancelability should only be disabled on entry to an
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
the client of an object that uses that object) has disabled
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 actions.
If an object is called in such a state and it enables cancelability
and a cancellation request is pending for that thread,
thread will be cancelled, contrary to the wish of the client
Second, the cancelability type may be explicitly set to either deferred
or asynchronous upon entry to an object. But as with the
state, on exit from an object that cancelability type should
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
The function pthread_setcanceltype() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_DEFERRED or
pthread_testcancel() conforms to ISO/IEC 9945-1:1996
OpenBSD 3.6 January 17, 1999
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