OSF DCE Application Development Guide--Introduction and Style Guide

OSF DCE Application Development Guide—Introduction and Style Guide

2.3.3.1 Cancelability State

A thread’s cancelability state is determined by the combination of two substates: general

cancelability and asynchronous cancelability. These substates can be set to either

CANCEL_ON or CANCEL_OFF by calls to the routines pthread_setcancel( ) and

pthread_setasynccancel( ) respectively. A thread’s cancelability state is determined by

its general and asynchronous cancelability substates, as shown in Table 2-2.

TABLE 2-2. Cancelability State

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General Asynchronous Cancelability

Cancelability Cancelability State

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CANCEL_OFF CANCEL_OFF disabled

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CANCEL_OFF CANCEL_ON disabled

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CANCEL_ON CANCEL_OFF deferred

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CANCEL_ON CANCEL_ON asynchronous

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One awkwardness introduced by this mechanism for setting cancelability state is that

threads cannot easily determine their current cancelability state, although

pthread_setcancel() and pthread_setasynccancel( ) return the previous substates.

When a thread is created, the default cancelability state is deferred (general

cancelability set to CANCEL_ON, asynchronous cancelability set to CANCEL_OFF).

A thread that needs to discover its current cancelability state should explicitly maintain

this state in some place where it can be easily queried.

2.3.3.2 Cancellation Points

Applications need to be aware of where cancellation may actually occur when

cancelability state is set to deferred. Cancellation points are points inside certain

functions where a thread must act upon any pending cancellation request when

cancelability state is deferred if the function would block indeﬁnitely. If cancelability

state is asynchronous, then every point is a cancellation point; that is, the thread may be

canceled at any time.

If cancelability state is deferred then cancellation may occur at the following points:

• While waiting on a condition variable; that is, within pthread_cond_wait() or

pthread_cond_timedwait( ).

• While awaiting the termination of another thread (within pthread_join( ).)

• When pthread_testcancel( ) is called.

• When sigwait( ) is called.

• When a thread is waiting within pthread_delay_np( ) (not a portable routine).

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