NAME
pthread_setcancelstate,
pthread_setcanceltype,
pthread_testcancel
—
set cancelability state
LIBRARY
POSIX Threads Library (libpthread, -lpthread)
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, if
oldstate is not
NULL
, 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, if
oldtype is not
NULL
, 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:
accept(),
aio_suspend(),
clock_nanosleep(),
close(),
connect(),
creat(),
fcntl(),
fdatasync(),
fsync(),
fsync_range(),
kevent(),
mq_receive(),
mq_send(),
mq_timedreceive(),
mq_timedsend(),
msgrcv(),
msgsnd(),
msync(),
nanosleep(),
open(),
pause(),
poll(),
pollts(),
pread(),
pselect(),
pthread_cond_timedwait(),
pthread_cond_wait(),
pthread_join(),
pthread_testcancel(),
pwrite(),
read(),
readv(),
recv(),
recvfrom(),
recvmsg(),
select(),
sem_timedwait(),
sem_wait(),
send(),
sendmsg(),
sendto(),
sigpause(),
sigsuspend(),
sigtimedwait(),
sigwait(),
sigwaitinfo(),
sleep(),
system(),
tcdrain(),
usleep(),
wait(),
wait4(),
waitid(),
waitpid(),
write(), and
writev().
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 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 cancelable.
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
These functions conform to
IEEE Std 1003.1-2001
(“POSIX.1”).
AUTHORS
This man page was written by
David Leonard
<
d@openbsd.org> for the
OpenBSD implementation of
pthread_cancel(3).