1PTHREAD_COND_BROADCAST(3P) POSIX Programmer's ManualPTHREAD_COND_BROADCAST(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 pthread_cond_broadcast, pthread_cond_signal — broadcast or signal a
13 condition
14
16 #include <pthread.h>
17 int pthread_cond_broadcast(pthread_cond_t *cond);
19 int pthread_cond_signal(pthread_cond_t *cond);
20
22 These functions shall unblock threads blocked on a condition variable.
23 The pthread_cond_broadcast() function shall unblock all threads cur‐
25 rently blocked on the specified condition variable cond.
26 The pthread_cond_signal() function shall unblock at least one of the
28 threads that are blocked on the specified condition variable cond (if
29 any threads are blocked on cond).
30 If more than one thread is blocked on a condition variable, the sched‐
32 uling policy shall determine the order in which threads are unblocked.
33 When each thread unblocked as a result of a pthread_cond_broadcast() or
34 pthread_cond_signal() returns from its call to pthread_cond_wait() or
35 pthread_cond_timedwait(), the thread shall own the mutex with which it
36 called pthread_cond_wait() or pthread_cond_timedwait(). The thread(s)
37 that are unblocked shall contend for the mutex according to the sched‐
38 uling policy (if applicable), and as if each had called
39 pthread_mutex_lock().
40 The pthread_cond_broadcast() or pthread_cond_signal() functions may be
42 called by a thread whether or not it currently owns the mutex that
43 threads calling pthread_cond_wait() or pthread_cond_timedwait() have
44 associated with the condition variable during their waits; however, if
45 predictable scheduling behavior is required, then that mutex shall be
46 locked by the thread calling pthread_cond_broadcast() or
47 pthread_cond_signal().
48 The pthread_cond_broadcast() and pthread_cond_signal() functions shall
50 have no effect if there are no threads currently blocked on cond.
51 The behavior is undefined if the value specified by the cond argument
53 to pthread_cond_broadcast() or pthread_cond_signal() does not refer to
54 an initialized condition variable.
55
57 If successful, the pthread_cond_broadcast() and pthread_cond_signal()
58 functions shall return zero; otherwise, an error number shall be
59 returned to indicate the error.
60
62 These functions shall not return an error code of [EINTR].
63 The following sections are informative.
65
67 None.
68
70 The pthread_cond_broadcast() function is used whenever the shared-vari‐
71 able state has been changed in a way that more than one thread can pro‐
72 ceed with its task. Consider a single producer/multiple consumer prob‐
73 lem, where the producer can insert multiple items on a list that is
74 accessed one item at a time by the consumers. By calling the
75 pthread_cond_broadcast() function, the producer would notify all con‐
76 sumers that might be waiting, and thereby the application would receive
77 more throughput on a multi-processor. In addition, pthread_cond_broad‐
78 cast() makes it easier to implement a read-write lock. The
79 pthread_cond_broadcast() function is needed in order to wake up all
80 waiting readers when a writer releases its lock. Finally, the two-phase
81 commit algorithm can use this broadcast function to notify all clients
82 of an impending transaction commit.
83 It is not safe to use the pthread_cond_signal() function in a signal
85 handler that is invoked asynchronously. Even if it were safe, there
86 would still be a race between the test of the Boolean
87 pthread_cond_wait() that could not be efficiently eliminated.
88 Mutexes and condition variables are thus not suitable for releasing a
90 waiting thread by signaling from code running in a signal handler.
91
93 If an implementation detects that the value specified by the cond argu‐
94 ment to pthread_cond_broadcast() or pthread_cond_signal() does not
95 refer to an initialized condition variable, it is recommended that the
96 function should fail and report an [EINVAL] error.
97 Multiple Awakenings by Condition Signal
99 On a multi-processor, it may be impossible for an implementation of
100 pthread_cond_signal() to avoid the unblocking of more than one thread
101 blocked on a condition variable. For example, consider the following
102 partial implementation of pthread_cond_wait() and pthread_cond_sig‐
103 nal(), executed by two threads in the order given. One thread is trying
104 to wait on the condition variable, another is concurrently executing
105 pthread_cond_signal(), while a third thread is already waiting.
106 pthread_cond_wait(mutex, cond):
109 value = cond->value; /* 1 */
110 pthread_mutex_unlock(mutex); /* 2 */
111 pthread_mutex_lock(cond->mutex); /* 10 */
112 if (value == cond->value) { /* 11 */
113 me->next_cond = cond->waiter;
114 cond->waiter = me;
115 pthread_mutex_unlock(cond->mutex);
116 unable_to_run(me);
117 } else
118 pthread_mutex_unlock(cond->mutex); /* 12 */
119 pthread_mutex_lock(mutex); /* 13 */
120 pthread_cond_signal(cond):
122 pthread_mutex_lock(cond->mutex); /* 3 */
123 cond->value++; /* 4 */
124 if (cond->waiter) { /* 5 */
125 sleeper = cond->waiter; /* 6 */
126 cond->waiter = sleeper->next_cond; /* 7 */
127 able_to_run(sleeper); /* 8 */
128 }
129 pthread_mutex_unlock(cond->mutex); /* 9 */
130 The effect is that more than one thread can return from its call to
132 pthread_cond_wait() or pthread_cond_timedwait() as a result of one call
133 to pthread_cond_signal(). This effect is called ``spurious wakeup''.
134 Note that the situation is self-correcting in that the number of
135 threads that are so awakened is finite; for example, the next thread to
136 call pthread_cond_wait() after the sequence of events above blocks.
137 While this problem could be resolved, the loss of efficiency for a
139 fringe condition that occurs only rarely is unacceptable, especially
140 given that one has to check the predicate associated with a condition
141 variable anyway. Correcting this problem would unnecessarily reduce the
142 degree of concurrency in this basic building block for all higher-level
143 synchronization operations.
144 An added benefit of allowing spurious wakeups is that applications are
146 forced to code a predicate-testing-loop around the condition wait.
147 This also makes the application tolerate superfluous condition broad‐
148 casts or signals on the same condition variable that may be coded in
149 some other part of the application. The resulting applications are thus
150 more robust. Therefore, POSIX.1‐2008 explicitly documents that spurious
151 wakeups may occur.
152
154 None.
155
157 pthread_cond_destroy(), pthread_cond_timedwait()
158 The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Syn‐
160 chronization, <pthread.h>
161
163 Portions of this text are reprinted and reproduced in electronic form
164 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
165 table Operating System Interface (POSIX), The Open Group Base Specifi‐
166 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
167 Electrical and Electronics Engineers, Inc and The Open Group. In the
168 event of any discrepancy between this version and the original IEEE and
169 The Open Group Standard, the original IEEE and The Open Group Standard
170 is the referee document. The original Standard can be obtained online
171 at http://www.opengroup.org/unix/online.html .
172 Any typographical or formatting errors that appear in this page are
174 most likely to have been introduced during the conversion of the source
175 files to man page format. To report such errors, see https://www.ker‐
176 nel.org/doc/man-pages/reporting_bugs.html .
177IEEE/The Open Group 2017 PTHREAD_COND_BROADCAST(3P)