258 lines
6.0 KiB
Groff
258 lines
6.0 KiB
Groff
.\" $FreeBSD$
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.Dd August 16, 2016
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.Dt PTHREAD_TESTCANCEL 3
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.Os
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.Sh NAME
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.Nm pthread_setcancelstate ,
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.Nm pthread_setcanceltype ,
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.Nm pthread_testcancel
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.Nd set cancelability state
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.Sh LIBRARY
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.Lb libpthread
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.Sh SYNOPSIS
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.In pthread.h
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.Ft int
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.Fn pthread_setcancelstate "int state" "int *oldstate"
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.Ft int
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.Fn pthread_setcanceltype "int type" "int *oldtype"
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.Ft void
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.Fn pthread_testcancel "void"
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.Sh DESCRIPTION
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The
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.Fn pthread_setcancelstate
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function atomically both sets the calling thread's cancelability state
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to the indicated
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.Fa state
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and, if
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.Fa oldstate
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is not
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.Dv NULL ,
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returns the previous cancelability state at the location referenced by
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.Fa oldstate .
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Legal values for
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.Fa state
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are
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.Dv PTHREAD_CANCEL_ENABLE
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and
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.Dv PTHREAD_CANCEL_DISABLE .
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.Pp
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The
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.Fn pthread_setcanceltype
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function atomically both sets the calling thread's cancelability type
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to the indicated
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.Fa type
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and, if
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.Fa oldtype
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is not
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.Dv NULL ,
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returns the previous cancelability type at the location referenced by
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.Fa oldtype .
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Legal values for
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.Fa type
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are
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.Dv PTHREAD_CANCEL_DEFERRED
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and
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.Dv PTHREAD_CANCEL_ASYNCHRONOUS .
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.Pp
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The cancelability state and type of any newly created threads, including the
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thread in which
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.Fn main
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was first invoked, are
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.Dv PTHREAD_CANCEL_ENABLE
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and
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.Dv PTHREAD_CANCEL_DEFERRED
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respectively.
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.Pp
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The
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.Fn pthread_testcancel
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function creates a cancellation point in the calling thread.
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The
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.Fn pthread_testcancel
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function has no effect if cancelability is disabled.
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.Pp
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.Ss Cancelability States
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The cancelability state of a thread determines the action taken upon
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receipt of a cancellation request.
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The thread may control cancellation in
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a number of ways.
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.Pp
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Each thread maintains its own
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.Dq cancelability state
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which may be encoded in two bits:
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.Bl -hang
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.It Em Cancelability Enable
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When cancelability is
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.Dv PTHREAD_CANCEL_DISABLE ,
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cancellation requests against the target thread are held pending.
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.It Em Cancelability Type
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When cancelability is enabled and the cancelability type is
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.Dv PTHREAD_CANCEL_ASYNCHRONOUS ,
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new or pending cancellation requests may be acted upon at any time.
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When cancelability is enabled and the cancelability type is
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.Dv PTHREAD_CANCEL_DEFERRED ,
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cancellation requests are held pending until a cancellation point (see
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below) is reached.
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If cancelability is disabled, the setting of the
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cancelability type has no immediate effect as all cancellation requests
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are held pending; however, once cancelability is enabled again the new
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type will be in effect.
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.El
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.Ss Cancellation Points
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Cancellation points will occur when a thread is executing the following
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functions:
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.Bl -tag -width "Fn pthread_cond_timedwait" -compact
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.It Fn accept
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.It Fn accept4
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.It Fn aio_suspend
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.It Fn connect
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.It Fn close
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.It Fn creat
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.It Fn fcntl
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The
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.Fn fcntl
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function is a cancellation point if
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.Fa cmd
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is
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.Dv F_SETLKW .
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.It Fn fdatasync
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.It Fn fsync
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.It Fn kevent
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The
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.Fn kevent
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function is a cancellation point if it is potentially blocking,
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i.e. when the
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.Fa nevents
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argument is non-zero.
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.It Fn mq_receive
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.It Fn mq_send
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.It Fn mq_timedreceive
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.It Fn mq_timedsend
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.It Fn msync
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.It Fn nanosleep
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.It Fn open
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.It Fn openat
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.It Fn pause
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.It Fn poll
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.It Fn ppoll
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.It Fn pselect
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.It Fn pthread_cond_timedwait
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.It Fn pthread_cond_wait
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.It Fn pthread_join
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.It Fn pthread_testcancel
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.It Fn read
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.It Fn readv
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.It Fn recv
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.It Fn recvfrom
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.It Fn recvmsg
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.It Fn select
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.It Fn sem_timedwait
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.It Fn sem_wait
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.It Fn send
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.It Fn sendmsg
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.It Fn sendto
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.It Fn sigsuspend
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.It Fn sigtimedwait
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.It Fn sigwaitinfo
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.It Fn sigwait
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.It Fn sleep
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.It Fn system
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.It Fn tcdrain
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.It Fn usleep
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.It Fn wait
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.It Fn wait3
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.It Fn wait4
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.It Fn wait6
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.It Fn waitid
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.It Fn waitpid
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.It Fn write
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.It Fn writev
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.El
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.Sh NOTES
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The
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.Fn pthread_setcancelstate
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and
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.Fn pthread_setcanceltype
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functions are used to control the points at which a thread may be
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asynchronously canceled.
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For cancellation control to be usable in modular
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fashion, some rules must be followed.
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.Pp
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For purposes of this discussion, consider an object to be a generalization
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of a procedure.
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It is a set of procedures and global variables written as
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a unit and called by clients not known by the object.
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Objects may depend
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on other objects.
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.Pp
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First, cancelability should only be disabled on entry to an object, never
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explicitly enabled.
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On exit from an object, the cancelability state should
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always be restored to its value on entry to the object.
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.Pp
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This follows from a modularity argument: if the client of an object (or the
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client of an object that uses that object) has disabled cancelability, it is
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because the client does not want to have to worry about how to clean up if the
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thread is canceled while executing some sequence of actions.
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If an object
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is called in such a state and it enables cancelability and a cancellation
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request is pending for that thread, then the thread will be canceled,
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contrary to the wish of the client that disabled.
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.Pp
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Second, the cancelability type may be explicitly set to either
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.Em deferred
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or
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.Em asynchronous
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upon entry to an object.
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But as with the cancelability state, on exit from
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an object that cancelability type should always be restored to its value on
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entry to the object.
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.Pp
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Finally, only functions that are cancel-safe may be called from a thread that
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is asynchronously cancelable.
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.Sh RETURN VALUES
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If successful, the
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.Fn pthread_setcancelstate
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and
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.Fn pthread_setcanceltype
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functions will return zero.
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Otherwise, an error number shall be returned to
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indicate the error.
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.Sh ERRORS
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The function
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.Fn pthread_setcancelstate
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may fail with:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The specified state is not
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.Dv PTHREAD_CANCEL_ENABLE
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or
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.Dv PTHREAD_CANCEL_DISABLE .
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.El
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.Pp
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The function
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.Fn pthread_setcanceltype
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may fail with:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The specified state is not
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.Dv PTHREAD_CANCEL_DEFERRED
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or
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.Dv PTHREAD_CANCEL_ASYNCHRONOUS .
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.El
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.Sh SEE ALSO
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.Xr pthread_cancel 3
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.Sh STANDARDS
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The
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.Fn pthread_testcancel
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function conforms to
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.St -p1003.1-96 .
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The standard allows implementations to make many more functions
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cancellation points.
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.Sh AUTHORS
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This manual page was written by
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.An David Leonard Aq Mt d@openbsd.org
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for the
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.Ox
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implementation of
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.Xr pthread_cancel 3 .
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