1.\" $FreeBSD$ 2.Dd August 16, 2016 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 fdatasync 118.It Fn fsync 119.It Fn kevent 120The 121.Fn kevent 122function is a cancellation point if it is potentially blocking, 123i.e. when the 124.Fa nevents 125argument is non-zero. 126.It Fn mq_receive 127.It Fn mq_send 128.It Fn mq_timedreceive 129.It Fn mq_timedsend 130.It Fn msync 131.It Fn nanosleep 132.It Fn open 133.It Fn openat 134.It Fn pause 135.It Fn poll 136.It Fn ppoll 137.It Fn pselect 138.It Fn pthread_cond_timedwait 139.It Fn pthread_cond_wait 140.It Fn pthread_join 141.It Fn pthread_testcancel 142.It Fn read 143.It Fn readv 144.It Fn recv 145.It Fn recvfrom 146.It Fn recvmsg 147.It Fn select 148.It Fn sem_timedwait 149.It Fn sem_wait 150.It Fn send 151.It Fn sendmsg 152.It Fn sendto 153.It Fn sigsuspend 154.It Fn sigtimedwait 155.It Fn sigwaitinfo 156.It Fn sigwait 157.It Fn sleep 158.It Fn system 159.It Fn tcdrain 160.It Fn usleep 161.It Fn wait 162.It Fn wait3 163.It Fn wait4 164.It Fn wait6 165.It Fn waitid 166.It Fn waitpid 167.It Fn write 168.It Fn writev 169.El 170.Sh NOTES 171The 172.Fn pthread_setcancelstate 173and 174.Fn pthread_setcanceltype 175functions are used to control the points at which a thread may be 176asynchronously canceled. 177For cancellation control to be usable in modular 178fashion, some rules must be followed. 179.Pp 180For purposes of this discussion, consider an object to be a generalization 181of a procedure. 182It is a set of procedures and global variables written as 183a unit and called by clients not known by the object. 184Objects may depend 185on other objects. 186.Pp 187First, cancelability should only be disabled on entry to an object, never 188explicitly enabled. 189On exit from an object, the cancelability state should 190always be restored to its value on entry to the object. 191.Pp 192This follows from a modularity argument: if the client of an object (or the 193client of an object that uses that object) has disabled cancelability, it is 194because the client does not want to have to worry about how to clean up if the 195thread is canceled while executing some sequence of actions. 196If an object 197is called in such a state and it enables cancelability and a cancellation 198request is pending for that thread, then the thread will be canceled, 199contrary to the wish of the client that disabled. 200.Pp 201Second, the cancelability type may be explicitly set to either 202.Em deferred 203or 204.Em asynchronous 205upon entry to an object. 206But as with the cancelability state, on exit from 207an object that cancelability type should always be restored to its value on 208entry to the object. 209.Pp 210Finally, only functions that are cancel-safe may be called from a thread that 211is asynchronously cancelable. 212.Sh RETURN VALUES 213If successful, the 214.Fn pthread_setcancelstate 215and 216.Fn pthread_setcanceltype 217functions will return zero. 218Otherwise, an error number shall be returned to 219indicate the error. 220.Sh ERRORS 221The function 222.Fn pthread_setcancelstate 223may fail with: 224.Bl -tag -width Er 225.It Bq Er EINVAL 226The specified state is not 227.Dv PTHREAD_CANCEL_ENABLE 228or 229.Dv PTHREAD_CANCEL_DISABLE . 230.El 231.Pp 232The function 233.Fn pthread_setcanceltype 234may fail with: 235.Bl -tag -width Er 236.It Bq Er EINVAL 237The specified state is not 238.Dv PTHREAD_CANCEL_DEFERRED 239or 240.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 241.El 242.Sh SEE ALSO 243.Xr pthread_cancel 3 244.Sh STANDARDS 245The 246.Fn pthread_testcancel 247function conforms to 248.St -p1003.1-96 . 249The standard allows implementations to make many more functions 250cancellation points. 251.Sh AUTHORS 252This manual page was written by 253.An David Leonard Aq Mt d@openbsd.org 254for the 255.Ox 256implementation of 257.Xr pthread_cancel 3 . 258