PTHREAD_TESTCANCEL(3) BSD Library Functions Manual PTHREAD_TESTCANCEL(3)
NAME
pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel -- set cancelability state
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 state and returns the previous cancelability state at the location referenced by
oldstate. Legal values for 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 type and returns the previous cancelability type at the location referenced by oldtype.
Legal values for 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 fash-ion, fashion,
ion, 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 doesn't 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 cance-lable. cancelable.
lable.
ERRORS
The function pthread_setcancelstate() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_ENABLE or PTHREAD_CANCEL_DISABLE.
The function pthread_setcanceltype() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_DEFERRED or PTHREAD_CANCEL_ASYNCHRONOUS.
SEE ALSO
pthread_cancel(3)
STANDARDS
pthread_testcancel() conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').
AUTHORS
This man page was written by David Leonard <d@openbsd.org> for the OpenBSD implementation of
pthread_cancel(3).
BSD January 17, 1999 BSD
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