1.\" $FreeBSD: src/share/man/man3/pthread_testcancel.3,v 1.15 2007/10/22 10:08:01 ru Exp $ 2.Dd July 10, 2009 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 close , 103.Fn creat , 104.Fn fcntl , 105.Fn fsync , 106.Fn msync , 107.Fn nanosleep , 108.Fn open , 109.Fn pause , 110.Fn pthread_cond_timedwait , 111.Fn pthread_cond_wait , 112.Fn pthread_join , 113.Fn pthread_testcancel , 114.Fn read , 115.Fn sigwaitinfo , 116.Fn sigsuspend , 117.Fn sigwait , 118.Fn sleep , 119.Fn system , 120.Fn tcdrain , 121.Fn wait , 122.Fn waitpid , 123.Fn write . 124.Sh RETURN VALUES 125If successful, the 126.Fn pthread_setcancelstate 127and 128.Fn pthread_setcanceltype 129functions will return zero. 130Otherwise, an error number shall be returned to 131indicate the error. 132.Pp 133The 134.Fn pthread_setcancelstate 135and 136.Fn pthread_setcanceltype 137functions are used to control the points at which a thread may be 138asynchronously canceled. 139For cancellation control to be usable in modular 140fashion, some rules must be followed. 141.Pp 142For purposes of this discussion, consider an object to be a generalization 143of a procedure. 144It is a set of procedures and global variables written as 145a unit and called by clients not known by the object. 146Objects may depend 147on other objects. 148.Pp 149First, cancelability should only be disabled on entry to an object, never 150explicitly enabled. 151On exit from an object, the cancelability state should 152always be restored to its value on entry to the object. 153.Pp 154This follows from a modularity argument: if the client of an object (or the 155client of an object that uses that object) has disabled cancelability, it is 156because the client does not want to have to worry about how to clean up if the 157thread is canceled while executing some sequence of actions. 158If an object 159is called in such a state and it enables cancelability and a cancellation 160request is pending for that thread, then the thread will be canceled, 161contrary to the wish of the client that disabled. 162.Pp 163Second, the cancelability type may be explicitly set to either 164.Em deferred 165or 166.Em asynchronous 167upon entry to an object. 168But as with the cancelability state, on exit from 169an object that cancelability type should always be restored to its value on 170entry to the object. 171.Pp 172Finally, only functions that are cancel-safe may be called from a thread that 173is asynchronously cancelable. 174.Sh ERRORS 175The function 176.Fn pthread_setcancelstate 177may fail with: 178.Bl -tag -width Er 179.It Bq Er EINVAL 180The specified state is not 181.Dv PTHREAD_CANCEL_ENABLE 182or 183.Dv PTHREAD_CANCEL_DISABLE . 184.El 185.Pp 186The function 187.Fn pthread_setcanceltype 188may fail with: 189.Bl -tag -width Er 190.It Bq Er EINVAL 191The specified state is not 192.Dv PTHREAD_CANCEL_DEFERRED 193or 194.Dv PTHREAD_CANCEL_ASYNCHRONOUS . 195.El 196.Sh SEE ALSO 197.Xr pthread_cancel 3 198.Sh STANDARDS 199The 200.Fn pthread_testcancel 201function conforms to 202.St -p1003.1-96 . 203.Sh AUTHORS 204This manual page was written by 205.An David Leonard Aq Mt d@openbsd.org 206for the 207.Ox 208implementation of 209.Xr pthread_cancel 3 . 210