illumos: manual page: condition.7

CONDITION(7) Standards, Environments, and Macros CONDITION(7)

NAME

condition - concepts related to condition variables

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.

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.

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 mmap(2))
and are initialized for this purpose.

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.

A condition variable must not be simultaneously initialized by multiple
threads or re-initialized while in use by other threads.

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,
type, passed to their initialization function.

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.

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.

Destroy

The condition destroy functions destroy any state, but not the space,
associated with the condition variable.

ATTRIBUTES

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.

USYNC_THREAD does not support multiple mappings to the same logical synch
object. If you need to mmap() a synch object to different locations
within the same address space, then the synch object should be
initialized as a shared object USYNC_PROCESS for Solaris, and
PTHREAD_PROCESS_PRIVATE for POSIX.

May 16, 2020 CONDITION(7)