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  47 .TH CONDITION 5 "Jul 20, 1998"
  48 .SH NAME
  49 condition \- concepts related to condition variables
  51 .sp
  52 .LP
  53 Occasionally, a thread running within a mutex needs to wait for an event,  in
  54 which case it blocks or sleeps. When a thread is waiting for another thread to
  55 communicate its disposition, it uses a condition variable in conjunction with a
  56 mutex. Although a mutex is exclusive and the code it protects is sharable (at
  57 certain moments), condition variables enable the synchronization of differing
  58 events that share a mutex, but not necessarily data. Several condition
  59 variables may be used by threads to signal each other  when a task is complete,
  60 which then allows the next waiting thread to take  ownership of the mutex.
  61 .sp
  62 .LP
  63 A condition variable enables threads to atomically block and test the condition
  64 under the protection of a  mutual exclusion lock (mutex) until the condition is
  65 satisfied. If the condition is false, a thread blocks on a condition variable
  66 and atomically releases the mutex that is waiting for the condition to change.
  67 If another thread changes the condition, it may wake up waiting threads by
  68 signaling the associated condition variable. The waiting threads, upon
  69 awakening, reacquire the mutex and re-evaluate the condition.
  70 .SS "Initialize"
  71 .sp
  72 .LP
  73 Condition variables and mutexes should be global. Condition variables that are
  74 allocated in writable memory can synchronize threads among processes if they
  75 are shared by the cooperating processes (see \fBmmap\fR(2)) and are initialized
  76 for this purpose.
  77 .sp
  78 .LP
  79 The scope of a condition variable is either intra-process or inter-process.
  80 This is dependent upon whether the argument is passed implicitly or explicitly
  81 to the initialization  of that condition variable. A condition variable does
  82 not need to be explicitly initialized. A condition variable is initialized with
  83 all zeros, by default, and its scope is set  to within the calling process. For
  84 inter-process synchronization, a condition variable must be initialized once,
  85 and only once, before use.
  86 .sp
  87 .LP
  88 A condition variable must not be simultaneously initialized by multiple threads
  89 or re-initialized while in use by other threads.
  90 .sp
  91 .LP
  92 Condition variables attributes may be set to the default or customized at
  93 initialization.  POSIX threads even allow the default values to be customized.
  94 Establishing these attributes varies depending upon whether POSIX or Solaris
  95 threads are used. Similar to the distinctions between POSIX and Solaris thread
  96 creation, POSIX condition variables implement the default, intra-process,
  97 unless an attribute object is modified for inter-process prior to the
  98 initialization of the condition variable. Solaris condition variables also
  99 implement as the default,  intra-process; however, they set this attribute
 100 according to the argument, \fItype\fR, passed to their initialization function.
 101 .SS "Condition Wait"
 102 .sp
 103 .LP
 104 The condition wait interface allows a thread to wait for a condition and
 105 atomically release the associated mutex that it needs to hold to check the
 106 condition. The thread waits for another thread to make the condition true and
 107 that thread's resulting call to signal and wakeup the waiting thread.
 108 .SS "Condition Signaling"
 109 .sp
 110 .LP
 111 A condition signal allows a thread to unblock the next thread waiting on the
 112 condition variable, whereas, a condition broadcast allows a thread to unblock
 113 all threads waiting on the  condition variable.
 114 .SS "Destroy"
 115 .sp
 116 .LP
 117 The condition destroy functions destroy any state, but not the space,
 118 associated with the condition variable.
 120 .sp
 121 .LP
 122 See \fBattributes\fR(5) for descriptions of the following attributes:
 123 .sp
 125 .sp
 126 .TS
 127 box;
 128 c | c
 129 l | l .
 131 _
 132 MT-Level        MT-Safe
 133 .TE
 136 .sp
 137 .LP
 138 \fBfork\fR(2), \fBmmap\fR(2), \fBsetitimer\fR(2), \fBshmop\fR(2),
 139 \fBcond_broadcast\fR(3C), \fBcond_destroy\fR(3C), \fBcond_init\fR(3C),
 140 \fBcond_signal\fR(3C), \fBcond_timedwait\fR(3C), \fBcond_wait\fR(3C),
 141 \fBpthread_cond_broadcast\fR(3C), \fBpthread_cond_destroy\fR(3C),
 142 \fBpthread_cond_init\fR(3C), \fBpthread_cond_signal\fR(3C),
 143 \fBpthread_cond_timedwait\fR(3C), \fBpthread_cond_wait\fR(3C),
 144 \fBpthread_condattr_init\fR(3C), \fBsignal\fR(3C), \fBattributes\fR(5),
 145 \fBmutex\fR(5), \fBstandards\fR(5)
 146 .SH NOTES
 147 .sp
 148 .LP
 149 If more than one thread is blocked on a condition variable, the order in which
 150 threads are unblocked is determined by the scheduling policy.
 151 .sp
 152 .LP
 153 \fBUSYNC_THREAD\fR does not support multiple mapplings to the same logical
 154 synch object. If you need to \fBmmap()\fR a synch object to different locations
 155 within the same address space, then the synch object should be initialized as a
 156 shared object \fBUSYNC_PROCESS\fR for Solaris, and