PTHREAD_TESTCANCEL(3) BSD Library Functions Manual PTHREAD_TESTCANCEL(3)NAME
pthread_setcancelstate, pthread_setcanceltype, pthread_testcancel — set
POSIX Threads Library (libpthread, -lpthread)
pthread_setcancelstate(int state, int *oldstate);
pthread_setcanceltype(int type, int *oldtype);
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 refer‐
enced by oldstate. Legal values for state are PTHREAD_CANCEL_ENABLE and
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
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 can‐
celability is disabled.
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, can‐
cellation 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 set‐
ting of the cancelability type has no immediate effect as all
cancellation requests are held pending; however, once cancelabil‐
ity is enabled again the new type will be in effect.
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(),
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 can‐
celed. 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 generaliza‐
tion 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 cancelabil‐
ity, 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 cancela‐
bility and a cancellation request is pending for that thread, then the
thread will be canceled, contrary to the wish of the client that dis‐
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.
The function pthread_setcancelstate() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_ENABLE or
The function pthread_setcanceltype() may fail with:
[EINVAL] The specified state is not PTHREAD_CANCEL_DEFERRED or
The pthread_testcancel() function conforms to ISO/IEC 9945-1:1996
This manual page was written by David Leonard ⟨email@example.com⟩ for the
OpenBSD implementation of pthread_cancel(3).
BSD January 17, 1999 BSD