pthread_setcancelstatepthread_setcanceltypepthread_testcancel
- set cancelability state
LIBRARY
Lb libpthread
SYNOPSIS
#include <pthread.h> int
pthread_setcancelstate (int state int *oldstate); int
pthread_setcanceltype (int type int *oldtype); void
pthread_testcancel (void);
DESCRIPTION
The
pthread_setcancelstate ();
function atomically both sets the calling thread's cancelability state
to the indicated
Fa state
and, if
Fa oldstate
is not
NULL
returns the previous cancelability state at the location referenced by
Fa oldstate .
Legal values for
Fa 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
Fa type
and, if
Fa oldtype
is not
NULL
returns the previous cancelability type at the location referenced by
Fa oldtype .
Legal values for
Fa 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
respectively.
The
pthread_testcancel ();
function creates a cancellation point in the calling thread.
The
pthread_testcancel ();
function has no effect if cancelability is disabled.
Cancelability States
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:
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
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
Cancellation points will occur when a thread is executing the following
functions:
close (,);
creat (,);
fcntl (,);
fsync (,);
msync (,);
nanosleep (,);
open (,);
pause (,);
pthread_cond_timedwait (,);
pthread_cond_wait (,);
pthread_join (,);
pthread_testcancel (,);
read (,);
sigwaitinfo (,);
sigsuspend (,);
sigwait (,);
sleep (,);
system (,);
tcdrain (,);
wait (,);
waitpid (,);
write (.);
RETURN VALUES
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 does not 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 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.
ERRORS
The function
pthread_setcancelstate ();
may fail with:
Bq Er EINVAL
The specified state is not
PTHREAD_CANCEL_ENABLE
or
PTHREAD_CANCEL_DISABLE
The function
pthread_setcanceltype ();
may fail with:
Bq Er EINVAL
The specified state is not
PTHREAD_CANCEL_DEFERRED
or
PTHREAD_CANCEL_ASYNCHRONOUS
SEE ALSO
pthread_cancel3
STANDARDS
The
pthread_testcancel ();
function conforms to
St -p1003.1-96 .
AUTHORS
This manual page was written by
An David Leonard Aq d@openbsd.org
for the
Ox implementation of
pthread_cancel3.
