man3/pthread_cancel.3

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  26 .TH PTHREAD_CANCEL 3 2019-03-06 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 pthread_cancel \- send a cancellation request to a thread
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <pthread.h>
  32 .PP
  33 .BI "int pthread_cancel(pthread_t " thread );
  34 .PP
  35 Compile and link with \fI\-pthread\fP.
  36 .fi
  37 .SH DESCRIPTION
  38 The
  39 .BR pthread_cancel ()
  40 function sends a cancellation request to the thread
  41 .IR thread .
  42 Whether and when the target thread
  43 reacts to the cancellation request depends on
  44 two attributes that are under the control of that thread:
  45 its cancelability
  46 .I state
  47 and
  48 .IR type .
  49 .PP
  50 A thread's cancelability state, determined by
  51 .BR pthread_setcancelstate (3),
  52 can be
  53 .I enabled
  54 (the default for new threads) or
  55 .IR disabled .
  56 If a thread has disabled cancellation,
  57 then a cancellation request remains queued until the thread
  58 enables cancellation.
  59 If a thread has enabled cancellation,
  60 then its cancelability type determines when cancellation occurs.
  61 .PP
  62 A thread's cancellation type, determined by
  63 .BR pthread_setcanceltype (3),
  64 may be either
  65 .IR asynchronous
  66 or
  67 .IR deferred
  68 (the default for new threads).
  69 Asynchronous cancelability
  70 means that the thread can be canceled at any time
  71 (usually immediately, but the system does not guarantee this).
  72 Deferred cancelability means that cancellation will be delayed until
  73 the thread next calls a function that is a
  74 .IR "cancellation point" .
  75 A list of functions that are or may be cancellation points is provided in
  76 .BR pthreads (7).
  77 .PP
  78 When a cancellation requested is acted on, the following steps occur for
  79 .IR thread
  80 (in this order):
  81 .IP 1. 3
  82 Cancellation clean-up handlers are popped
  83 (in the reverse of the order in which they were pushed) and called.
  84 (See
  85 .BR pthread_cleanup_push (3).)
  86 .IP 2.
  87 Thread-specific data destructors are called,
  88 in an unspecified order.
  89 (See
  90 .BR pthread_key_create (3).)
  91 .IP 3.
  92 The thread is terminated.
  93 (See
  94 .BR pthread_exit (3).)
  95 .PP
  96 The above steps happen asynchronously with respect to the
  97 .BR pthread_cancel ()
  98 call;
  99 the return status of
 100 .BR pthread_cancel ()
 101 merely informs the caller whether the cancellation request
 102 was successfully queued.
 103 .PP
 104 After a canceled thread has terminated,
 105 a join with that thread using
 106 .BR pthread_join (3)
 107 obtains
 108 .B PTHREAD_CANCELED
 109 as the thread's exit status.
 110 (Joining with a thread is the only way to know that cancellation
 111 has completed.)
 112 .SH RETURN VALUE
 113 On success,
 114 .BR pthread_cancel ()
 115 returns 0;
 116 on error, it returns a nonzero error number.
 117 .SH ERRORS
 118 .TP
 119 .B ESRCH
 120 No thread with the ID
 121 .I thread
 122 could be found.
 123 .\" .SH VERSIONS
 124 .\" Available since glibc 2.0
 125 .SH ATTRIBUTES
 126 For an explanation of the terms used in this section, see
 127 .BR attributes (7).
 128 .TS
 129 allbox;
 130 lb lb lb
 131 l l l.
 132 Interface       Attribute       Value
 133 T{
 134 .BR pthread_cancel ()
 135 T}      Thread safety   MT-Safe
 136 .TE
 137 .sp 1
 138 .SH CONFORMING TO
 139 POSIX.1-2001, POSIX.1-2008.
 140 .SH NOTES
 141 On Linux, cancellation is implemented using signals.
 142 Under the NPTL threading implementation,
 143 the first real-time signal (i.e., signal 32) is used for this purpose.
 144 On LinuxThreads, the second real-time signal is used,
 145 if real-time signals are available, otherwise
 146 .B SIGUSR2
 147 is used.
 148 .SH EXAMPLE
 149 The program below creates a thread and then cancels it.
 150 The main thread joins with the canceled thread to check
 151 that its exit status was
 152 .BR PTHREAD_CANCELED .
 153 The following shell session shows what happens when we run the program:
 154 .PP
 155 .in +4n
 156 .EX
 157 $ ./a.out
 158 thread_func(): started; cancellation disabled
 159 main(): sending cancellation request
 160 thread_func(): about to enable cancellation
 161 main(): thread was canceled
 162 .EE
 163 .in
 164 .SS Program source
 165 \&
 166 .EX
 167 #include <pthread.h>
 168 #include <stdio.h>
 169 #include <errno.h>
 170 #include <stdlib.h>
 171 #include <unistd.h>
 172
 173 #define handle_error_en(en, msg) \e
 174         do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
 175
 176 static void *
 177 thread_func(void *ignored_argument)
 178 {
 179     int s;
 180
 181     /* Disable cancellation for a while, so that we don\(aqt
 182        immediately react to a cancellation request */
 183
 184     s = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
 185     if (s != 0)
 186         handle_error_en(s, "pthread_setcancelstate");
 187
 188     printf("thread_func(): started; cancellation disabled\en");
 189     sleep(5);
 190     printf("thread_func(): about to enable cancellation\en");
 191
 192     s = pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
 193     if (s != 0)
 194         handle_error_en(s, "pthread_setcancelstate");
 195
 196     /* sleep() is a cancellation point */
 197
 198     sleep(1000);        /* Should get canceled while we sleep */
 199
 200     /* Should never get here */
 201
 202     printf("thread_func(): not canceled!\en");
 203     return NULL;
 204 }
 205
 206 int
 207 main(void)
 208 {
 209     pthread_t thr;
 210     void *res;
 211     int s;
 212
 213     /* Start a thread and then send it a cancellation request */
 214
 215     s = pthread_create(&thr, NULL, &thread_func, NULL);
 216     if (s != 0)
 217         handle_error_en(s, "pthread_create");
 218
 219     sleep(2);           /* Give thread a chance to get started */
 220
 221     printf("main(): sending cancellation request\en");
 222     s = pthread_cancel(thr);
 223     if (s != 0)
 224         handle_error_en(s, "pthread_cancel");
 225
 226     /* Join with thread to see what its exit status was */
 227
 228     s = pthread_join(thr, &res);
 229     if (s != 0)
 230         handle_error_en(s, "pthread_join");
 231
 232     if (res == PTHREAD_CANCELED)
 233         printf("main(): thread was canceled\en");
 234     else
 235         printf("main(): thread wasn\(aqt canceled (shouldn\(aqt happen!)\en");
 236     exit(EXIT_SUCCESS);
 237 }
 238 .EE
 239 .SH SEE ALSO
 240 .ad l
 241 .nh
 242 .BR pthread_cleanup_push (3),
 243 .BR pthread_create (3),
 244 .BR pthread_exit (3),
 245 .BR pthread_join (3),
 246 .BR pthread_key_create (3),
 247 .BR pthread_setcancelstate (3),
 248 .BR pthread_setcanceltype (3),
 249 .BR pthread_testcancel (3),
 250 .BR pthreads (7)