1.\" $NetBSD: pthread_cond.3,v 1.6 2012/11/12 23:11:05 uwe Exp $ 2.\" 3.\" Copyright (c) 2002, 2008 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 16.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 17.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 18.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 19.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 20.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 21.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 22.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 23.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 24.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 25.\" POSSIBILITY OF SUCH DAMAGE. 26.\" 27.\" Copyright (c) 1997 Brian Cully <shmit@kublai.com> 28.\" All rights reserved. 29.\" 30.\" Redistribution and use in source and binary forms, with or without 31.\" modification, are permitted provided that the following conditions 32.\" are met: 33.\" 1. Redistributions of source code must retain the above copyright 34.\" notice, this list of conditions and the following disclaimer. 35.\" 2. Redistributions in binary form must reproduce the above copyright 36.\" notice, this list of conditions and the following disclaimer in the 37.\" documentation and/or other materials provided with the distribution. 38.\" 3. Neither the name of the author nor the names of any co-contributors 39.\" may be used to endorse or promote products derived from this software 40.\" without specific prior written permission. 41.\" 42.\" THIS SOFTWARE IS PROVIDED BY JOHN BIRRELL AND CONTRIBUTORS ``AS IS'' AND 43.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 44.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 45.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 46.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 47.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 48.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 49.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 50.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 51.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 52.\" SUCH DAMAGE. 53.\" 54.\" ---------------------------------------------------------------------------- 55.Dd July 8, 2010 56.Dt PTHREAD_COND 3 57.Os 58.Sh NAME 59.Nm pthread_cond 60.Nd condition variable interface 61.Sh LIBRARY 62.Lb libpthread 63.\" ---------------------------------------------------------------------------- 64.Sh SYNOPSIS 65.In pthread.h 66.Ft int 67.Fn pthread_cond_init "pthread_cond_t * restrict cond" \ 68"const pthread_condattr_t * restrict attr" 69.Vt pthread_cond_t cond No = Dv PTHREAD_COND_INITIALIZER; 70.Ft int 71.Fn pthread_cond_destroy "pthread_cond_t *cond" 72.Ft int 73.Fn pthread_cond_broadcast "pthread_cond_t *cond" 74.Ft int 75.Fn pthread_cond_signal "pthread_cond_t *cond" 76.Ft int 77.Fn pthread_cond_wait "pthread_cond_t * restrict cond" \ 78"pthread_mutex_t * restrict mutex" 79.Ft int 80.Fn pthread_cond_timedwait "pthread_cond_t * restrict cond" \ 81"pthread_mutex_t * restrict mutex" "const struct timespec * restrict abstime" 82.\" ---------------------------------------------------------------------------- 83.Sh DESCRIPTION 84Condition variables are intended to be used to communicate changes in 85the state of data shared between threads. 86Condition variables are always associated with a mutex to provide 87synchronized access to the shared data. 88A single predicate should always be associated with a 89condition variable. 90The predicate should identify a state of the 91shared data that must be true before the thread proceeds. 92.Pp 93The 94.Fn pthread_cond_init 95function creates a new condition variable, with attributes specified with 96.Fa attr . 97If 98.Fa attr 99is 100.Dv NULL 101the default attributes are used. 102The 103.Fn pthread_cond_destroy 104function frees the resources allocated by the condition variable 105.Fa cond . 106.Pp 107The macro 108.Dv PTHREAD_COND_INITIALIZER 109can be used to initialize a condition variable when it can be statically 110allocated and the default attributes are appropriate. 111The effect is similar to calling 112.Fn pthread_cond_init 113with 114.Fa attr 115specified as 116.Dv NULL , 117except that no error checking is done. 118.Pp 119.\" ----- 120The difference between 121.Fn pthread_cond_broadcast 122and 123.Fn pthread_cond_signal 124is that the former unblocks all threads waiting for the condition variable, 125whereas the latter blocks only one waiting thread. 126If no threads are waiting on 127.Fa cond , 128neither function has any effect. 129If more than one thread is blocked on a condition variable, 130the used scheduling policy determines the order in which threads are unblocked. 131The same mutex used for waiting must be held while calling either function. 132Although neither function strictly enforces this requirement, 133undefined behavior may follow if the mutex is not held. 134.Pp 135.\" ----- 136The 137.Fn pthread_cond_wait 138function atomically blocks the current thread waiting on the condition 139variable specified by 140.Fa cond , 141and unlocks the mutex specified by 142.Fa mutex . 143The 144.Fn pthread_cond_timedwait 145function behaves similarly, but unblocks also 146if the system time reaches the time specified in 147.Fa abstime , 148represented as 149.Em struct timespec 150(see 151.Xr timespec 3 ) . 152With both functions the waiting thread unblocks after another thread calls 153.Fn pthread_cond_signal 154or 155.Fn pthread_cond_broadcast 156with the same condition variable and by holding the same 157.Fa mutex 158that was associated with 159.Fa cond 160by either one of the blocking functions. 161The current thread holds the lock on 162.Fa mutex 163upon return from either function. 164.\" ----- 165.Pp 166Note that a call to 167.Fn pthread_cond_wait 168or 169.Fn pthread_cond_timedwait 170may wake up spontaneously, without a call to 171.Fn pthread_cond_signal 172or 173.Fn pthread_cond_broadcast . 174The caller should prepare for this by invoking either function 175within a predicate loop that tests whether the thread should proceed. 176.Pp 177.\" ----- 178As noted, when calling either function that waits on a condition variable, 179a temporary binding is established between the condition variable 180.Fa cond 181and the mutex 182.Fa mutex . 183During this time, the effect of an attempt by any thread to wait on 184that condition variable using a different mutex is undefined. 185The same mutex must be held while broadcasting or signaling on 186.Fa cond . 187Additionally, the same mutex must be used for concurrent calls to 188.Fn pthread_cond_wait 189and 190.Fn pthread_cond_timedwait . 191Only when a condition variable is known to be quiescent may an application 192change the mutex associated with it. 193In this implementation, none of the functions enforce this requirement, but 194if the mutex is not held or independent mutexes are used the resulting 195behaviour is undefined. 196.\" ---------------------------------------------------------------------------- 197.Sh RETURN VALUES 198If successful, all functions return zero. 199Otherwise, an error number will be returned to indicate the error. 200.Sh ERRORS 201The 202.Fn pthread_cond_init 203function may fail if: 204.Bl -tag -width Er 205.It Bq Er EINVAL 206The value specified by 207.Fa attr 208is invalid. 209.El 210.Pp 211.\" ----- 212The 213.Fn pthread_cond_destroy 214function may fail if: 215.Bl -tag -width Er 216.It Bq Er EBUSY 217The variable 218.Fa cond 219is locked by another thread. 220.It Bq Er EINVAL 221The value specified by 222.Fa cond 223is invalid. 224.El 225.Pp 226.\" ----- 227Both 228.Fn pthread_cond_broadcast 229and 230.Fn pthread_cond_signal 231may fail if: 232.Bl -tag -width Er 233.It Bq Er EINVAL 234The value specified by 235.Fa cond 236is invalid. 237.El 238.Pp 239.\" ----- 240Both 241.Fn pthread_cond_wait 242and 243.Fn pthread_cond_timedwait 244may fail if: 245.Bl -tag -width Er 246.It Bq Er EINVAL 247The value specified by 248.Fa cond 249or the value specified by 250.Fa mutex 251is invalid. 252.It Bq Er EPERM 253The value specified by 254.Fa mutex 255was not locked in the condition wait. 256.El 257.Pp 258The 259.Fn pthread_cond_timedwait 260function may additionally fail if: 261.Bl -tag -width Er 262.It Bq Er ETIMEDOUT 263The system time has reached or exceeded the time specified in 264.Fa abstime . 265.El 266.Sh SEE ALSO 267.Xr pthread 3 , 268.Xr pthread_barrier 3 , 269.Xr pthread_condattr 3 , 270.Xr pthread_mutex 3 , 271.Xr pthread_rwlock 3 , 272.Xr pthread_spin 3 273.Sh STANDARDS 274These functions conform to 275.St -p1003.1-2001 . 276