1.\" $FreeBSD: src/share/man/man3/pthread_testcancel.3,v 1.15 2007/10/22 10:08:01 ru Exp $ 2.Dd September 14, 2021 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.Ss Cancelability States 73The cancelability state of a thread determines the action taken upon 74receipt of a cancellation request. 75The thread may control cancellation in 76a number of ways. 77.Pp 78Each thread maintains its own 79.Dq cancelability state 80which may be encoded in two bits: 81.Bl -hang 82.It Em Cancelability Enable 83When cancelability is 84.Dv PTHREAD_CANCEL_DISABLE , 85cancellation requests against the target thread are held pending. 86.It Em Cancelability Type 87When cancelability is enabled and the cancelability type is 88.Dv PTHREAD_CANCEL_ASYNCHRONOUS , 89new or pending cancellation requests may be acted upon at any time. 90When cancelability is enabled and the cancelability type is 91.Dv PTHREAD_CANCEL_DEFERRED , 92cancellation requests are held pending until a cancellation point (see 93below) is reached. 94If cancelability is disabled, the setting of the 95cancelability type has no immediate effect as all cancellation requests 96are held pending; however, once cancelability is enabled again the new 97type will be in effect. 98.El 99.Ss Cancellation Points 100Cancellation points will occur when a thread is executing the following 101functions: 102.Fn clock_nanosleep , 103.Fn close , 104.Fn creat , 105.Fn fcntl , 106.Fn fdatasync , 107.Fn fsync , 108.Fn msync , 109.Fn nanosleep , 110.Fn open , 111.Fn pause , 112.Fn pthread_cond_timedwait , 113.Fn pthread_cond_wait , 114.Fn pthread_join , 115.Fn pthread_testcancel , 116.Fn read , 117.Fn sigwaitinfo , 118.Fn sigsuspend , 119.Fn sigwait , 120.Fn sleep , 121.Fn system , 122.Fn tcdrain , 123.Fn wait , 124.Fn waitpid , 125.Fn write . 126.Sh RETURN VALUES 127If successful, the 128.Fn pthread_setcancelstate 129and 130.Fn pthread_setcanceltype 131functions will return zero. 132Otherwise, an error number shall be returned to 133indicate the error. 134.Pp 135The 136.Fn pthread_setcancelstate 137and 138.Fn pthread_setcanceltype 139functions are used to control the points at which a thread may be 140asynchronously canceled. 141For cancellation control to be usable in modular 142fashion, some rules must be followed. 143.Pp 144For purposes of this discussion, consider an object to be a generalization 145of a procedure. 146It is a set of procedures and global variables written as 147a unit and called by clients not known by the object. 148Objects may depend 149on other objects. 150.Pp 151First, cancelability should only be disabled on entry to an object, never 152explicitly enabled. 153On exit from an object, the cancelability state should 154always be restored to its value on entry to the object. 155.Pp 156This follows from a modularity argument: if the client of an object (or the 157client of an object that uses that object) has disabled cancelability, it is 158because the client does not want to have to worry about how to clean up if the 159thread is canceled while executing some sequence of actions. 160If an object 161is called in such a state and it enables cancelability and a cancellation 162request is pending for that thread, then the thread will be canceled, 163contrary to the wish of the client that disabled. 164.Pp 165Second, the cancelability type may be explicitly set to either 166.Em deferred 167or 168.Em asynchronous 169upon entry to an object. 170But as with the cancelability state, on exit from 171an object that cancelability type should always be restored to its value on 172entry to the object. 173.Pp 174Finally, only functions that are cancel-safe may be called from a thread that 175is asynchronously cancelable. 176.Sh ERRORS 177The function 178.Fn pthread_setcancelstate 179may fail with: 180.Bl -tag -width Er 181.It Bq Er EINVAL 182The specified state is not 183.Dv PTHREAD_CANCEL_ENABLE 184or 185.Dv PTHREAD_CANCEL_DISABLE . 186.El 187.Pp 188The function 189.Fn pthread_setcanceltype 190may fail with: 191.Bl -tag -width Er 192.It Bq Er EINVAL 193The specified state is not 194.Dv PTHREAD_CANCEL_DEFERRED 195or 196.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 197.El 198.Sh SEE ALSO 199.Xr pthread_cancel 3 200.Sh STANDARDS 201The 202.Fn pthread_testcancel 203function conforms to 204.St -p1003.1-96 . 205.Sh AUTHORS 206This manual page was written by 207.An David Leonard Aq Mt d@openbsd.org 208for the 209.Ox 210implementation of 211.Xr pthread_cancel 3 . 212