Tribblix: 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)