man3/pthread_cancel.3

   1 '\" t
   2 .\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
   3 .\"     <mtk.manpages@gmail.com>
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .TH pthread_cancel 3 (date) "Linux man-pages (unreleased)"
   8 .SH NAME
   9 pthread_cancel \- send a cancelation request to a thread
  10 .SH LIBRARY
  11 POSIX threads library
  12 .RI ( libpthread ", " \-lpthread )
  13 .SH SYNOPSIS
  14 .nf
  15 .B #include <pthread.h>
  16 .P
  17 .BI "int pthread_cancel(pthread_t " thread );
  18 .fi
  19 .SH DESCRIPTION
  20 The
  21 .BR pthread_cancel ()
  22 function sends a cancelation request to the thread
  23 .IR thread .
  24 Whether and when the target thread
  25 reacts to the cancelation request depends on
  26 two attributes that are under the control of that thread:
  27 its cancelability
  28 .I state
  29 and
  30 .IR type .
  31 .P
  32 A thread's cancelability state, determined by
  33 .BR pthread_setcancelstate (3),
  34 can be
  35 .I enabled
  36 (the default for new threads) or
  37 .IR disabled .
  38 If a thread has disabled cancelation,
  39 then a cancelation request remains queued until the thread
  40 enables cancelation.
  41 If a thread has enabled cancelation,
  42 then its cancelability type determines when cancelation occurs.
  43 .P
  44 A thread's cancelation type, determined by
  45 .BR pthread_setcanceltype (3),
  46 may be either
  47 .I asynchronous
  48 or
  49 .I deferred
  50 (the default for new threads).
  51 Asynchronous cancelability
  52 means that the thread can be canceled at any time
  53 (usually immediately, but the system does not guarantee this).
  54 Deferred cancelability means that cancelation will be delayed until
  55 the thread next calls a function that is a
  56 .IR "cancelation point" .
  57 A list of functions that are or may be cancelation points is provided in
  58 .BR pthreads (7).
  59 .P
  60 When a cancelation requested is acted on, the following steps occur for
  61 .I thread
  62 (in this order):
  63 .IP (1) 5
  64 Cancelation clean-up handlers are popped
  65 (in the reverse of the order in which they were pushed) and called.
  66 (See
  67 .BR pthread_cleanup_push (3).)
  68 .IP (2)
  69 Thread-specific data destructors are called,
  70 in an unspecified order.
  71 (See
  72 .BR pthread_key_create (3).)
  73 .IP (3)
  74 The thread is terminated.
  75 (See
  76 .BR pthread_exit (3).)
  77 .P
  78 The above steps happen asynchronously with respect to the
  79 .BR pthread_cancel ()
  80 call;
  81 the return status of
  82 .BR pthread_cancel ()
  83 merely informs the caller whether the cancelation request
  84 was successfully queued.
  85 .P
  86 After a canceled thread has terminated,
  87 a join with that thread using
  88 .BR pthread_join (3)
  89 obtains
  90 .B PTHREAD_CANCELED
  91 as the thread's exit status.
  92 (Joining with a thread is the only way to know that cancelation
  93 has completed.)
  94 .SH RETURN VALUE
  95 On success,
  96 .BR pthread_cancel ()
  97 returns 0;
  98 on error, it returns a nonzero error number.
  99 .SH ERRORS
 100 .TP
 101 .B ESRCH
 102 No thread with the ID
 103 .I thread
 104 could be found.
 105 .SH ATTRIBUTES
 106 For an explanation of the terms used in this section, see
 107 .BR attributes (7).
 108 .TS
 109 allbox;
 110 lbx lb lb
 111 l l l.
 112 Interface       Attribute       Value
 113 T{
 114 .na
 115 .nh
 116 .BR pthread_cancel ()
 117 T}      Thread safety   MT-Safe
 118 .TE
 119 .SH VERSIONS
 120 On Linux, cancelation is implemented using signals.
 121 Under the NPTL threading implementation,
 122 the first real-time signal (i.e., signal 32) is used for this purpose.
 123 On LinuxThreads, the second real-time signal is used,
 124 if real-time signals are available, otherwise
 125 .B SIGUSR2
 126 is used.
 127 .SH STANDARDS
 128 POSIX.1-2008.
 129 .SH HISTORY
 130 glibc 2.0
 131 POSIX.1-2001.
 132 .SH EXAMPLES
 133 The program below creates a thread and then cancels it.
 134 The main thread joins with the canceled thread to check
 135 that its exit status was
 136 .BR PTHREAD_CANCELED .
 137 The following shell session shows what happens when we run the program:
 138 .P
 139 .in +4n
 140 .EX
 141 $ ./a.out
 142 thread_func(): started; cancelation disabled
 143 main(): sending cancelation request
 144 thread_func(): about to enable cancelation
 145 main(): thread was canceled
 146 .EE
 147 .in
 148 .SS Program source
 149 \&
 150 .\" SRC BEGIN (pthread_cancel.c)
 151 .EX
 152 #include <errno.h>
 153 #include <pthread.h>
 154 #include <stdio.h>
 155 #include <stdlib.h>
 156 #include <unistd.h>
 157 \&
 158 #define handle_error_en(en, msg) \e
 159         do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
 160 \&
 161 static void *
 162 thread_func(void *ignored_argument)
 163 {
 164     int s;
 165 \&
 166     /* Disable cancelation for a while, so that we don\[aq]t
 167        immediately react to a cancelation request. */
 168 \&
 169     s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
 170     if (s != 0)
 171         handle_error_en(s, "pthread_setcancelstate");
 172 \&
 173     printf("%s(): started; cancelation disabled\en", __func__);
 174     sleep(5);
 175     printf("%s(): about to enable cancelation\en", __func__);
 176 \&
 177     s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
 178     if (s != 0)
 179         handle_error_en(s, "pthread_setcancelstate");
 180 \&
 181     /* sleep() is a cancelation point. */
 182 \&
 183     sleep(1000);        /* Should get canceled while we sleep */
 184 \&
 185     /* Should never get here. */
 186 \&
 187     printf("%s(): not canceled!\en", __func__);
 188     return NULL;
 189 }
 190 \&
 191 int
 192 main(void)
 193 {
 194     pthread_t thr;
 195     void *res;
 196     int s;
 197 \&
 198     /* Start a thread and then send it a cancelation request. */
 199 \&
 200     s = pthread_create(&thr, NULL, &thread_func, NULL);
 201     if (s != 0)
 202         handle_error_en(s, "pthread_create");
 203 \&
 204     sleep(2);           /* Give thread a chance to get started */
 205 \&
 206     printf("%s(): sending cancelation request\en", __func__);
 207     s = pthread_cancel(thr);
 208     if (s != 0)
 209         handle_error_en(s, "pthread_cancel");
 210 \&
 211     /* Join with thread to see what its exit status was. */
 212 \&
 213     s = pthread_join(thr, &res);
 214     if (s != 0)
 215         handle_error_en(s, "pthread_join");
 216 \&
 217     if (res == PTHREAD_CANCELED)
 218         printf("%s(): thread was canceled\en", __func__);
 219     else
 220         printf("%s(): thread wasn\[aq]t canceled (shouldn\[aq]t happen!)\en",
 221                __func__);
 222     exit(EXIT_SUCCESS);
 223 }
 224 .EE
 225 .\" SRC END
 226 .SH SEE ALSO
 227 .ad l
 228 .nh
 229 .BR pthread_cleanup_push (3),
 230 .BR pthread_create (3),
 231 .BR pthread_exit (3),
 232 .BR pthread_join (3),
 233 .BR pthread_key_create (3),
 234 .BR pthread_setcancelstate (3),
 235 .BR pthread_setcanceltype (3),
 236 .BR pthread_testcancel (3),
 237 .BR pthreads (7)