1.\" $OpenBSD: pthread_testcancel.3,v 1.16 2014/08/31 20:23:10 guenther Exp $ 2.\" 3.\" 4.\" David Leonard, 1999. Public Domain. 5.\" 6.Dd $Mdocdate: August 31 2014 $ 7.Dt PTHREAD_TESTCANCEL 3 8.Os 9.Sh NAME 10.Nm pthread_setcancelstate , 11.Nm pthread_setcanceltype , 12.Nm pthread_testcancel 13.Nd set cancelability state 14.Sh SYNOPSIS 15.In pthread.h 16.Ft int 17.Fn pthread_setcancelstate "int state" "int *oldstate" 18.Ft int 19.Fn pthread_setcanceltype "int type" "int *oldtype" 20.Ft void 21.Fn pthread_testcancel "void" 22.Sh DESCRIPTION 23The 24.Fn pthread_setcancelstate 25function atomically both sets the calling thread's cancelability state 26to the indicated 27.Fa state 28and, if 29.Fa oldstate 30is not 31.Dv NULL , 32returns the previous cancelability state at the location referenced by 33.Fa oldstate . 34Legal values for 35.Fa state 36are 37.Dv PTHREAD_CANCEL_ENABLE 38and 39.Dv PTHREAD_CANCEL_DISABLE . 40.Pp 41The 42.Fn pthread_setcanceltype 43function atomically both sets the calling thread's cancelability type 44to the indicated 45.Fa type 46and, if 47.Fa oldtype 48is not 49.Dv NULL , 50returns the previous cancelability type at the location referenced by 51.Fa oldtype . 52Legal values for 53.Fa type 54are 55.Dv PTHREAD_CANCEL_DEFERRED 56and 57.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 58.Pp 59The cancelability state and type of any newly created threads, including the 60thread in which 61.Fn main 62was first invoked, are 63.Dv PTHREAD_CANCEL_ENABLE 64and 65.Dv PTHREAD_CANCEL_DEFERRED 66respectively. 67.Pp 68The 69.Fn pthread_testcancel 70function creates a cancellation point in the calling thread. 71The 72.Fn pthread_testcancel 73function has no effect if cancelability is disabled. 74.Ss Cancelability States 75The cancelability state of a thread determines the action taken upon 76receipt of a cancellation request. 77The thread may control cancellation in a number of ways. 78.Pp 79Each thread maintains its own 80.Dq cancelability state 81which may be encoded in two bits: 82.Bl -hang 83.It Em Cancelability Enable 84When cancelability is 85.Dv PTHREAD_CANCEL_DISABLE , 86cancellation requests against the target thread are held pending. 87.It Em Cancelability Type 88When cancelability is enabled and the cancelability type is 89.Dv PTHREAD_CANCEL_ASYNCHRONOUS , 90new or pending cancellation requests may be acted upon at any time. 91When cancelability is enabled and the cancelability type is 92.Dv PTHREAD_CANCEL_DEFERRED , 93cancellation requests are held pending until a cancellation point (see 94below) is reached. 95If cancelability is disabled, the setting of the 96cancelability type has no immediate effect as all cancellation requests 97are held pending; however, once cancelability is enabled again the new 98type will be in effect. 99.El 100.Ss Cancellation Points 101Cancellation points will occur when a thread is executing the following 102base interfaces: 103.Fn accept , 104.Fn close , 105.Fn connect , 106.Fn creat , 107.Fn fcntl "F_SETLKW" , 108.Fn fdatasync , 109.Fn fsync , 110.Fn lockf , 111.Fn msgrcv , 112.Fn msgsnd , 113.Fn msync , 114.Fn nanosleep , 115.Fn open , 116.Fn openat , 117.Fn pause , 118.Fn poll , 119.Fn pread , 120.Fn pthread_cond_timedwait , 121.Fn pthread_cond_wait , 122.Fn pthread_join , 123.Fn pthread_testcancel , 124.Fn pwrite , 125.Fn read , 126.Fn readv , 127.Fn recv , 128.Fn recvfrom , 129.Fn recvmsg , 130.Fn select , 131.Fn sem_timedwait , 132.Fn sem_wait , 133.Fn send , 134.Fn sendmsg , 135.Fn sendto , 136.Fn sigsuspend , 137.Fn sigwait , 138.Fn sleep , 139.Fn system , 140.Fn tcdrain , 141.Fn wait , 142.Fn waitpid , 143.Fn write , 144.Fn writev . 145.Pp 146In addition, 147cancellation points will occur when a thread is executing the following 148extension interfaces: 149.Fn accept4 , 150.Fn closefrom , 151.Fn ppoll , 152.Fn preadv , 153.Fn pwritev , 154.Fn wait3 , 155.Fn wait4 . 156.Sh RETURN VALUES 157If successful, the 158.Fn pthread_setcancelstate 159and 160.Fn pthread_setcanceltype 161functions will return zero. 162Otherwise, an error number shall be returned to indicate the error. 163.Pp 164The 165.Fn pthread_setcancelstate 166and 167.Fn pthread_setcanceltype 168functions are used to control the points at which a thread may be 169asynchronously cancelled. 170For cancellation control to be usable in modular 171fashion, some rules must be followed. 172.Pp 173For purposes of this discussion, consider an object to be a generalization 174of a procedure. 175It is a set of procedures and global variables written as 176a unit and called by clients not known by the object. 177Objects may depend on other objects. 178.Pp 179First, cancelability should only be disabled on entry to an object, never 180explicitly enabled. 181On exit from an object, the cancelability state should 182always be restored to its value on entry to the object. 183.Pp 184This follows from a modularity argument: if the client of an object (or the 185client of an object that uses that object) has disabled cancelability, it is 186because the client doesn't want to have to worry about how to clean up if the 187thread is cancelled while executing some sequence of actions. 188If an object 189is called in such a state and it enables cancelability and a cancellation 190request is pending for that thread, then the thread will be cancelled, 191contrary to the wish of the client that disabled. 192.Pp 193Second, the cancelability type may be explicitly set to either 194.Em deferred 195or 196.Em asynchronous 197upon entry to an object. 198But as with the cancelability state, on exit from 199an object that cancelability type should always be restored to its value on 200entry to the object. 201.Pp 202Finally, only functions that are cancel-safe may be called from a thread that 203is asynchronously cancelable. 204.Sh ERRORS 205The function 206.Fn pthread_setcancelstate 207may fail with: 208.Bl -tag -width Er 209.It Bq Er EINVAL 210The specified state is not 211.Dv PTHREAD_CANCEL_ENABLE 212or 213.Dv PTHREAD_CANCEL_DISABLE . 214.El 215.Pp 216The function 217.Fn pthread_setcanceltype 218may fail with: 219.Bl -tag -width Er 220.It Bq Er EINVAL 221The specified state is not 222.Dv PTHREAD_CANCEL_DEFERRED 223or 224.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 225.El 226.Sh SEE ALSO 227.Xr pthread_cancel 3 228.Sh STANDARDS 229.Fn pthread_testcancel 230conforms to 231.St -p1003.1-96 232