|PTHREAD_TESTCANCEL(3)||Library Functions Manual||PTHREAD_TESTCANCEL(3)|
pthread_testcancel — set
atomically both sets the calling thread's cancelability state to the
indicated state and, if oldstate
NULL, returns the previous cancelability
state at the location referenced by oldstate. Legal
values for state are
atomically both sets the calling thread's cancelability type to the
indicated type and, if oldtype
NULL, returns the previous cancelability type
at the location referenced by oldtype. Legal values
for type are
The cancelability state and type of any newly created threads,
including the thread in which
main() was first
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 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:
PTHREAD_CANCEL_DISABLE, cancellation requests against the target thread are held pending.
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 cancelability type has no immediate effect as all cancellation 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 base interfaces:
In addition, cancellation points will occur when a thread is
executing the following extension interfaces:
If successful, the
pthread_setcanceltype() functions will return zero.
Otherwise, an error number shall be returned to indicate the error.
pthread_setcanceltype() functions are used to
control the points at which a thread may be asynchronously cancelled. For
cancellation control to be usable in modular fashion, some rules must be
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 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, 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.
pthread_testcancel() conforms to
ISO/IEC 9945-1:1996 (“POSIX.1”)
|August 31, 2014||OpenBSD-current|