1.\" $FreeBSD$ 2.Dd March 29, 2015 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.Bl -tag -width "Fn pthread_cond_timedwait" -compact 104.It Fn accept 105.It Fn accept4 106.It Fn aio_suspend 107.It Fn connect 108.It Fn close 109.It Fn creat 110.It Fn fcntl 111The 112.Fn fcntl 113function is a cancellation point if 114.Fa cmd 115is 116.Dv F_SETLKW . 117.It Fn fsync 118.It Fn kevent 119The 120.Fn kevent 121function is a cancellation point if it is potentially blocking, 122i.e. when the 123.Fa nevents 124argument is non-zero. 125.It Fn mq_receive 126.It Fn mq_send 127.It Fn mq_timedreceive 128.It Fn mq_timedsend 129.It Fn msync 130.It Fn nanosleep 131.It Fn open 132.It Fn openat 133.It Fn pause 134.It Fn poll 135.It Fn pselect 136.It Fn pthread_cond_timedwait 137.It Fn pthread_cond_wait 138.It Fn pthread_join 139.It Fn pthread_testcancel 140.It Fn read 141.It Fn readv 142.It Fn recv 143.It Fn recvfrom 144.It Fn recvmsg 145.It Fn select 146.It Fn sem_timedwait 147.It Fn sem_wait 148.It Fn send 149.It Fn sendmsg 150.It Fn sendto 151.It Fn sigsuspend 152.It Fn sigtimedwait 153.It Fn sigwaitinfo 154.It Fn sigwait 155.It Fn sleep 156.It Fn system 157.It Fn tcdrain 158.It Fn usleep 159.It Fn wait 160.It Fn wait3 161.It Fn wait4 162.It Fn waitpid 163.It Fn write 164.It Fn writev 165.El 166.Sh NOTES 167The 168.Fn pthread_setcancelstate 169and 170.Fn pthread_setcanceltype 171functions are used to control the points at which a thread may be 172asynchronously canceled. 173For cancellation control to be usable in modular 174fashion, some rules must be followed. 175.Pp 176For purposes of this discussion, consider an object to be a generalization 177of a procedure. 178It is a set of procedures and global variables written as 179a unit and called by clients not known by the object. 180Objects may depend 181on other objects. 182.Pp 183First, cancelability should only be disabled on entry to an object, never 184explicitly enabled. 185On exit from an object, the cancelability state should 186always be restored to its value on entry to the object. 187.Pp 188This follows from a modularity argument: if the client of an object (or the 189client of an object that uses that object) has disabled cancelability, it is 190because the client does not want to have to worry about how to clean up if the 191thread is canceled while executing some sequence of actions. 192If an object 193is called in such a state and it enables cancelability and a cancellation 194request is pending for that thread, then the thread will be canceled, 195contrary to the wish of the client that disabled. 196.Pp 197Second, the cancelability type may be explicitly set to either 198.Em deferred 199or 200.Em asynchronous 201upon entry to an object. 202But as with the cancelability state, on exit from 203an object that cancelability type should always be restored to its value on 204entry to the object. 205.Pp 206Finally, only functions that are cancel-safe may be called from a thread that 207is asynchronously cancelable. 208.Sh RETURN VALUES 209If successful, the 210.Fn pthread_setcancelstate 211and 212.Fn pthread_setcanceltype 213functions will return zero. 214Otherwise, an error number shall be returned to 215indicate the error. 216.Sh ERRORS 217The function 218.Fn pthread_setcancelstate 219may fail with: 220.Bl -tag -width Er 221.It Bq Er EINVAL 222The specified state is not 223.Dv PTHREAD_CANCEL_ENABLE 224or 225.Dv PTHREAD_CANCEL_DISABLE . 226.El 227.Pp 228The function 229.Fn pthread_setcanceltype 230may fail with: 231.Bl -tag -width Er 232.It Bq Er EINVAL 233The specified state is not 234.Dv PTHREAD_CANCEL_DEFERRED 235or 236.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 237.El 238.Sh SEE ALSO 239.Xr pthread_cancel 3 240.Sh STANDARDS 241The 242.Fn pthread_testcancel 243function conforms to 244.St -p1003.1-96 . 245The standard allows implementations to make many more functions 246cancellation points. 247.Sh AUTHORS 248This manual page was written by 249.An David Leonard Aq Mt d@openbsd.org 250for the 251.Ox 252implementation of 253.Xr pthread_cancel 3 . 254