1.\" $FreeBSD: src/share/man/man3/pthread_testcancel.3,v 1.15 2007/10/22 10:08:01 ru Exp $ 2.Dd January 17, 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 fsync , 107.Fn msync , 108.Fn nanosleep , 109.Fn open , 110.Fn pause , 111.Fn pthread_cond_timedwait , 112.Fn pthread_cond_wait , 113.Fn pthread_join , 114.Fn pthread_testcancel , 115.Fn read , 116.Fn sigwaitinfo , 117.Fn sigsuspend , 118.Fn sigwait , 119.Fn sleep , 120.Fn system , 121.Fn tcdrain , 122.Fn wait , 123.Fn waitpid , 124.Fn write . 125.Sh RETURN VALUES 126If successful, the 127.Fn pthread_setcancelstate 128and 129.Fn pthread_setcanceltype 130functions will return zero. 131Otherwise, an error number shall be returned to 132indicate the error. 133.Pp 134The 135.Fn pthread_setcancelstate 136and 137.Fn pthread_setcanceltype 138functions are used to control the points at which a thread may be 139asynchronously canceled. 140For cancellation control to be usable in modular 141fashion, some rules must be followed. 142.Pp 143For purposes of this discussion, consider an object to be a generalization 144of a procedure. 145It is a set of procedures and global variables written as 146a unit and called by clients not known by the object. 147Objects may depend 148on other objects. 149.Pp 150First, cancelability should only be disabled on entry to an object, never 151explicitly enabled. 152On exit from an object, the cancelability state should 153always be restored to its value on entry to the object. 154.Pp 155This follows from a modularity argument: if the client of an object (or the 156client of an object that uses that object) has disabled cancelability, it is 157because the client does not want to have to worry about how to clean up if the 158thread is canceled while executing some sequence of actions. 159If an object 160is called in such a state and it enables cancelability and a cancellation 161request is pending for that thread, then the thread will be canceled, 162contrary to the wish of the client that disabled. 163.Pp 164Second, the cancelability type may be explicitly set to either 165.Em deferred 166or 167.Em asynchronous 168upon entry to an object. 169But as with the cancelability state, on exit from 170an object that cancelability type should always be restored to its value on 171entry to the object. 172.Pp 173Finally, only functions that are cancel-safe may be called from a thread that 174is asynchronously cancelable. 175.Sh ERRORS 176The function 177.Fn pthread_setcancelstate 178may fail with: 179.Bl -tag -width Er 180.It Bq Er EINVAL 181The specified state is not 182.Dv PTHREAD_CANCEL_ENABLE 183or 184.Dv PTHREAD_CANCEL_DISABLE . 185.El 186.Pp 187The function 188.Fn pthread_setcanceltype 189may fail with: 190.Bl -tag -width Er 191.It Bq Er EINVAL 192The specified state is not 193.Dv PTHREAD_CANCEL_DEFERRED 194or 195.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 196.El 197.Sh SEE ALSO 198.Xr pthread_cancel 3 199.Sh STANDARDS 200The 201.Fn pthread_testcancel 202function conforms to 203.St -p1003.1-96 . 204.Sh AUTHORS 205This manual page was written by 206.An David Leonard Aq Mt d@openbsd.org 207for the 208.Ox 209implementation of 210.Xr pthread_cancel 3 . 211