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Man Pages

Manual Reference Pages  -  CONDVAR (9)


condvar, cv_init, cv_destroy, cv_wait, cv_wait_sig, cv_wait_unlock, cv_timedwait, cv_timedwait_sbt, cv_timedwait_sig, cv_timedwait_sig_sbt, cv_signal, cv_broadcast, cv_broadcastpri, cv_wmesg - kernel condition variable


Return Values
See Also


.In sys/param.h
.In sys/proc.h
.In sys/condvar.h void cv_init struct cv *cvp const char *desc void cv_destroy struct cv *cvp void cv_wait struct cv *cvp lock int cv_wait_sig struct cv *cvp lock void cv_wait_unlock struct cv *cvp lock int cv_timedwait struct cv *cvp lock int timo int cv_timedwait_sbt struct cv *cvp lock sbintime_t sbt "sbintime_t pr" "int flags" int cv_timedwait_sig struct cv *cvp lock int timo int cv_timedwait_sig_sbt struct cv *cvp lock sbintime_t sbt "sbintime_t pr" "int flags" void cv_signal struct cv *cvp void cv_broadcast struct cv *cvp void cv_broadcastpri struct cv *cvp int pri const char * cv_wmesg struct cv *cvp


Condition variables are used in conjunction with mutexes to wait for conditions to occur. Condition variables are created with cv_init, where cvp is a pointer to space for a
.Vt struct cv , and desc is a pointer to a null-terminated character string that describes the condition variable. Condition variables are destroyed with cv_destroy. Threads wait on condition variables by calling cv_wait, cv_wait_sig, cv_wait_unlock, cv_timedwait, or cv_timedwait_sig. Threads unblock waiters by calling cv_signal to unblock one waiter, or cv_broadcast or cv_broadcastpri to unblock all waiters. In addition to waking waiters, cv_broadcastpri ensures that all of the waiters have a priority of at least pri by raising the priority of any threads that do not. cv_wmesg returns the description string of cvp, as set by the initial call to cv_init.

The lock argument is a pointer to either a mutex(9), rwlock(9), or sx(9) lock. A mutex(9) argument must be initialized with MTX_DEF and not MTX_SPIN. A thread must hold lock before calling cv_wait, cv_wait_sig, cv_wait_unlock, cv_timedwait, or cv_timedwait_sig. When a thread waits on a condition, lock is atomically released before the thread is blocked, then reacquired before the function call returns. In addition, the thread will fully drop the Giant mutex (even if recursed) while the it is suspended and will reacquire the Giant mutex before the function returns. The cv_wait_unlock function does not reacquire the lock before returning. Note that the Giant mutex may be specified as lock. However, Giant may not be used as lock for the cv_wait_unlock function. All waiters must pass the same lock in conjunction with cvp.

When cv_wait, cv_wait_sig, cv_wait_unlock, cv_timedwait, and cv_timedwait_sig unblock, their calling threads are made runnable. cv_timedwait and cv_timedwait_sig wait for at most timo / HZ seconds before being unblocked and returning EWOULDBLOCK; otherwise, they return 0. cv_wait_sig and cv_timedwait_sig return prematurely with a value of EINTR or ERESTART if a signal is caught, or 0 if signaled via cv_signal or cv_broadcast.

cv_timedwait_sbt and cv_timedwait_sig_sbt functions take sbt argument instead of timo. It allows to specify relative or absolute unblock time with higher resolution in form of
.Vt sbintime_t . The parameter pr allows to specify wanted absolute event precision. The parameter flags allows to pass additional callout_reset_sbt flags.


If successful, cv_wait_sig, cv_timedwait, and cv_timedwait_sig return 0. Otherwise, a non-zero error code is returned.

cv_wmesg returns the description string that was passed to cv_init.


cv_wait_sig and cv_timedwait_sig will fail if:
  A signal was caught and the system call should be interrupted.
  A signal was caught and the system call should be restarted.

cv_timedwait and cv_timedwait_sig will fail if:
  Timeout expired.


locking(9), mtx_pool(9), mutex(9), rwlock(9), sema(9), sleep(9), sx(9), timeout(9)
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