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CONDVAR(9) Kernel Developer's Manual 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_wmesgkernel condition variable


#include <sys/param.h>
#include <sys/proc.h>
#include <sys/condvar.h>

cv_init(struct cv *cvp, const char *desc);

cv_destroy(struct cv *cvp);

cv_wait(struct cv *cvp, lock);

cv_wait_sig(struct cv *cvp, lock);

cv_wait_unlock(struct cv *cvp, lock);

cv_timedwait(struct cv *cvp, lock, int timo);

cv_timedwait_sbt(struct cv *cvp, lock, sbintime_t sbt, sbintime_t pr, int flags);

cv_timedwait_sig(struct cv *cvp, lock, int timo);

cv_timedwait_sig_sbt(struct cv *cvp, lock, sbintime_t sbt, sbintime_t pr, int flags);

cv_signal(struct cv *cvp);

cv_broadcast(struct cv *cvp);

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 (), where cvp is a pointer to space for a struct cv, and desc is a pointer to a null-terminated character string that describes the condition variable. Condition variables are destroyed with (). 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 () to unblock one waiter, or () or () 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. () 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 (), (), (), (), or (). 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 (), (), (), (), and () 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().

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

February 19, 2013 Debian