1.\" $FreeBSD$ 2.Dd June 11, 2013 3.Dt PTHREAD_TESTCANCEL 3 4.Os 5.Sh NAME 6.Nm pthread_setcancelstate , 7.Nm pthread_setcanceltype , 8.Nm pthread_testcancel 9.Nd set cancelability state 10.Sh LIBRARY 11.Lb libpthread 12.Sh SYNOPSIS 13.In pthread.h 14.Ft int 15.Fn pthread_setcancelstate "int state" "int *oldstate" 16.Ft int 17.Fn pthread_setcanceltype "int type" "int *oldtype" 18.Ft void 19.Fn pthread_testcancel "void" 20.Sh DESCRIPTION 21The 22.Fn pthread_setcancelstate 23function atomically both sets the calling thread's cancelability state 24to the indicated 25.Fa state 26and, if 27.Fa oldstate 28is not 29.Dv NULL , 30returns the previous cancelability state at the location referenced by 31.Fa oldstate . 32Legal values for 33.Fa state 34are 35.Dv PTHREAD_CANCEL_ENABLE 36and 37.Dv PTHREAD_CANCEL_DISABLE . 38.Pp 39The 40.Fn pthread_setcanceltype 41function atomically both sets the calling thread's cancelability type 42to the indicated 43.Fa type 44and, if 45.Fa oldtype 46is not 47.Dv NULL , 48returns the previous cancelability type at the location referenced by 49.Fa oldtype . 50Legal values for 51.Fa type 52are 53.Dv PTHREAD_CANCEL_DEFERRED 54and 55.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 56.Pp 57The cancelability state and type of any newly created threads, including the 58thread in which 59.Fn main 60was first invoked, are 61.Dv PTHREAD_CANCEL_ENABLE 62and 63.Dv PTHREAD_CANCEL_DEFERRED 64respectively. 65.Pp 66The 67.Fn pthread_testcancel 68function creates a cancellation point in the calling thread. 69The 70.Fn pthread_testcancel 71function has no effect if cancelability is disabled. 72.Pp 73.Ss Cancelability States 74The cancelability state of a thread determines the action taken upon 75receipt of a cancellation request. 76The thread may control cancellation in 77a 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 102functions: 103.Fn accept , 104.Fn accept4 , 105.Fn aio_suspend , 106.Fn connect , 107.Fn close , 108.Fn creat , 109.Fn fsync , 110.Fn mq_receive , 111.Fn mq_send , 112.Fn mq_timedreceive , 113.Fn mq_timedsend , 114.Fn msync , 115.Fn nanosleep , 116.Fn open , 117.Fn openat , 118.Fn pause , 119.Fn poll , 120.Fn pselect , 121.Fn pthread_cond_timedwait , 122.Fn pthread_cond_wait , 123.Fn pthread_join , 124.Fn pthread_testcancel , 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 sigtimedwait , 138.Fn sigwaitinfo , 139.Fn sigwait , 140.Fn sleep , 141.Fn system , 142.Fn tcdrain , 143.Fn usleep , 144.Fn wait , 145.Fn wait3 , 146.Fn wait4 , 147.Fn waitpid , 148.Fn write , 149.Fn writev . 150The 151.Fn fcntl 152function is a cancellation point if 153.Fa cmd 154is 155.Dv F_SETLKW . 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 163indicate the error. 164.Pp 165The 166.Fn pthread_setcancelstate 167and 168.Fn pthread_setcanceltype 169functions are used to control the points at which a thread may be 170asynchronously canceled. 171For cancellation control to be usable in modular 172fashion, some rules must be followed. 173.Pp 174For purposes of this discussion, consider an object to be a generalization 175of a procedure. 176It is a set of procedures and global variables written as 177a unit and called by clients not known by the object. 178Objects may depend 179on other objects. 180.Pp 181First, cancelability should only be disabled on entry to an object, never 182explicitly enabled. 183On exit from an object, the cancelability state should 184always be restored to its value on entry to the object. 185.Pp 186This follows from a modularity argument: if the client of an object (or the 187client of an object that uses that object) has disabled cancelability, it is 188because the client does not want to have to worry about how to clean up if the 189thread is canceled while executing some sequence of actions. 190If an object 191is called in such a state and it enables cancelability and a cancellation 192request is pending for that thread, then the thread will be canceled, 193contrary to the wish of the client that disabled. 194.Pp 195Second, the cancelability type may be explicitly set to either 196.Em deferred 197or 198.Em asynchronous 199upon entry to an object. 200But as with the cancelability state, on exit from 201an object that cancelability type should always be restored to its value on 202entry to the object. 203.Pp 204Finally, only functions that are cancel-safe may be called from a thread that 205is asynchronously cancelable. 206.Sh ERRORS 207The function 208.Fn pthread_setcancelstate 209may fail with: 210.Bl -tag -width Er 211.It Bq Er EINVAL 212The specified state is not 213.Dv PTHREAD_CANCEL_ENABLE 214or 215.Dv PTHREAD_CANCEL_DISABLE . 216.El 217.Pp 218The function 219.Fn pthread_setcanceltype 220may fail with: 221.Bl -tag -width Er 222.It Bq Er EINVAL 223The specified state is not 224.Dv PTHREAD_CANCEL_DEFERRED 225or 226.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 227.El 228.Sh SEE ALSO 229.Xr pthread_cancel 3 230.Sh STANDARDS 231The 232.Fn pthread_testcancel 233function conforms to 234.St -p1003.1-96 . 235The standard allows implementations to make many more functions 236cancellation points. 237.Sh AUTHORS 238This manual page was written by 239.An David Leonard Aq Mt d@openbsd.org 240for the 241.Ox 242implementation of 243.Xr pthread_cancel 3 . 244