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.TH CONDITION 5 "May 16, 2020"
.SH NAME
condition \- concepts related to condition variables
.SH DESCRIPTION
Occasionally, a thread running within a mutex needs to wait for an event, in
which case it blocks or sleeps. When a thread is waiting for another thread to
communicate its disposition, it uses a condition variable in conjunction with a
mutex. Although a mutex is exclusive and the code it protects is sharable (at
certain moments), condition variables enable the synchronization of differing
events that share a mutex, but not necessarily data. Several condition
variables may be used by threads to signal each other when a task is complete,
which then allows the next waiting thread to take ownership of the mutex.
.sp
.LP
A condition variable enables threads to atomically block and test the condition
under the protection of a mutual exclusion lock (mutex) until the condition is
satisfied. If the condition is false, a thread blocks on a condition variable
and atomically releases the mutex that is waiting for the condition to change.
If another thread changes the condition, it may wake up waiting threads by
signaling the associated condition variable. The waiting threads, upon
awakening, reacquire the mutex and re-evaluate the condition.
.SS "Initialize"
Condition variables and mutexes should be global. Condition variables that are
allocated in writable memory can synchronize threads among processes if they
are shared by the cooperating processes (see \fBmmap\fR(2)) and are initialized
for this purpose.
.sp
.LP
The scope of a condition variable is either intra-process or inter-process.
This is dependent upon whether the argument is passed implicitly or explicitly
to the initialization of that condition variable. A condition variable does
not need to be explicitly initialized. A condition variable is initialized with
all zeros, by default, and its scope is set to within the calling process. For
inter-process synchronization, a condition variable must be initialized once,
and only once, before use.
.sp
.LP
A condition variable must not be simultaneously initialized by multiple threads
or re-initialized while in use by other threads.
.sp
.LP
Condition variables attributes may be set to the default or customized at
initialization. POSIX threads even allow the default values to be customized.
Establishing these attributes varies depending upon whether POSIX or Solaris
threads are used. Similar to the distinctions between POSIX and Solaris thread
creation, POSIX condition variables implement the default, intra-process,
unless an attribute object is modified for inter-process prior to the
initialization of the condition variable. Solaris condition variables also
implement as the default, intra-process; however, they set this attribute
according to the argument, \fItype\fR, passed to their initialization function.
.SS "Condition Wait"
The condition wait interface allows a thread to wait for a condition and
atomically release the associated mutex that it needs to hold to check the
condition. The thread waits for another thread to make the condition true and
that thread's resulting call to signal and wakeup the waiting thread.
.SS "Condition Signaling"
A condition signal allows a thread to unblock the next thread waiting on the
condition variable, whereas, a condition broadcast allows a thread to unblock
all threads waiting on the condition variable.
.SS "Destroy"
The condition destroy functions destroy any state, but not the space,
associated with the condition variable.
.SH ATTRIBUTES
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp
.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE ATTRIBUTE VALUE
_
MT-Level MT-Safe
.TE
.SH SEE ALSO
\fBfork\fR(2), \fBmmap\fR(2), \fBsetitimer\fR(2), \fBshmop\fR(2),
\fBcond_broadcast\fR(3C), \fBcond_destroy\fR(3C), \fBcond_init\fR(3C),
\fBcond_signal\fR(3C), \fBcond_timedwait\fR(3C), \fBcond_wait\fR(3C),
\fBpthread_cond_broadcast\fR(3C), \fBpthread_cond_destroy\fR(3C),
\fBpthread_cond_init\fR(3C), \fBpthread_cond_signal\fR(3C),
\fBpthread_cond_timedwait\fR(3C), \fBpthread_cond_wait\fR(3C),
\fBpthread_condattr_init\fR(3C), \fBsignal\fR(3C), \fBattributes\fR(5),
\fBmutex\fR(5), \fBstandards\fR(5)
.SH NOTES
If more than one thread is blocked on a condition variable, the order in which
threads are unblocked is determined by the scheduling policy.
.sp
.LP
\fBUSYNC_THREAD\fR does not support multiple mappings to the same logical
synch object. If you need to \fBmmap()\fR a synch object to different locations
within the same address space, then the synch object should be initialized as a
shared object \fBUSYNC_PROCESS\fR for Solaris, and
\fBPTHREAD_PROCESS_PRIVATE\fR for POSIX.
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