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  47 .TH CONDITION 5 "May 16, 2020"
  48 .SH NAME
  49 condition \- concepts related to condition variables
  51 Occasionally, a thread running within a mutex needs to wait for an event,  in
  52 which case it blocks or sleeps. When a thread is waiting for another thread to
  53 communicate its disposition, it uses a condition variable in conjunction with a
  54 mutex. Although a mutex is exclusive and the code it protects is sharable (at
  55 certain moments), condition variables enable the synchronization of differing
  56 events that share a mutex, but not necessarily data. Several condition
  57 variables may be used by threads to signal each other  when a task is complete,
  58 which then allows the next waiting thread to take  ownership of the mutex.
  59 .sp
  60 .LP
  61 A condition variable enables threads to atomically block and test the condition
  62 under the protection of a  mutual exclusion lock (mutex) until the condition is
  63 satisfied. If the condition is false, a thread blocks on a condition variable
  64 and atomically releases the mutex that is waiting for the condition to change.
  65 If another thread changes the condition, it may wake up waiting threads by
  66 signaling the associated condition variable. The waiting threads, upon
  67 awakening, reacquire the mutex and re-evaluate the condition.
  68 .SS "Initialize"
  69 Condition variables and mutexes should be global. Condition variables that are
  70 allocated in writable memory can synchronize threads among processes if they
  71 are shared by the cooperating processes (see \fBmmap\fR(2)) and are initialized
  72 for this purpose.
  73 .sp
  74 .LP
  75 The scope of a condition variable is either intra-process or inter-process.
  76 This is dependent upon whether the argument is passed implicitly or explicitly
  77 to the initialization  of that condition variable. A condition variable does
  78 not need to be explicitly initialized. A condition variable is initialized with
  79 all zeros, by default, and its scope is set  to within the calling process. For
  80 inter-process synchronization, a condition variable must be initialized once,
  81 and only once, before use.
  82 .sp
  83 .LP
  84 A condition variable must not be simultaneously initialized by multiple threads
  85 or re-initialized while in use by other threads.
  86 .sp
  87 .LP
  88 Condition variables attributes may be set to the default or customized at
  89 initialization.  POSIX threads even allow the default values to be customized.
  90 Establishing these attributes varies depending upon whether POSIX or Solaris
  91 threads are used. Similar to the distinctions between POSIX and Solaris thread
  92 creation, POSIX condition variables implement the default, intra-process,
  93 unless an attribute object is modified for inter-process prior to the
  94 initialization of the condition variable. Solaris condition variables also
  95 implement as the default,  intra-process; however, they set this attribute
  96 according to the argument, \fItype\fR, passed to their initialization function.
  97 .SS "Condition Wait"
  98 The condition wait interface allows a thread to wait for a condition and
  99 atomically release the associated mutex that it needs to hold to check the
 100 condition. The thread waits for another thread to make the condition true and
 101 that thread's resulting call to signal and wakeup the waiting thread.
 102 .SS "Condition Signaling"
 103 A condition signal allows a thread to unblock the next thread waiting on the
 104 condition variable, whereas, a condition broadcast allows a thread to unblock
 105 all threads waiting on the  condition variable.
 106 .SS "Destroy"
 107 The condition destroy functions destroy any state, but not the space,
 108 associated with the condition variable.
 110 See \fBattributes\fR(5) for descriptions of the following attributes:
 111 .sp
 113 .sp
 114 .TS
 115 box;
 116 c | c
 117 l | l .
 119 _
 120 MT-Level        MT-Safe
 121 .TE
 124 \fBfork\fR(2), \fBmmap\fR(2), \fBsetitimer\fR(2), \fBshmop\fR(2),
 125 \fBcond_broadcast\fR(3C), \fBcond_destroy\fR(3C), \fBcond_init\fR(3C),
 126 \fBcond_signal\fR(3C), \fBcond_timedwait\fR(3C), \fBcond_wait\fR(3C),
 127 \fBpthread_cond_broadcast\fR(3C), \fBpthread_cond_destroy\fR(3C),
 128 \fBpthread_cond_init\fR(3C), \fBpthread_cond_signal\fR(3C),
 129 \fBpthread_cond_timedwait\fR(3C), \fBpthread_cond_wait\fR(3C),
 130 \fBpthread_condattr_init\fR(3C), \fBsignal\fR(3C), \fBattributes\fR(5),
 131 \fBmutex\fR(5), \fBstandards\fR(5)
 132 .SH NOTES
 133 If more than one thread is blocked on a condition variable, the order in which
 134 threads are unblocked is determined by the scheduling policy.
 135 .sp
 136 .LP
 137 \fBUSYNC_THREAD\fR does not support multiple mappings to the same logical
 138 synch object. If you need to \fBmmap()\fR a synch object to different locations
 139 within the same address space, then the synch object should be initialized as a
 140 shared object \fBUSYNC_PROCESS\fR for Solaris, and