CONDVAR(9) MidnightBSD Kernel Developer’s Manual CONDVAR(9)


condvar, cv_init, cv_destroy, cv_wait, cv_wait_sig, cv_wait_unlock, cv_timedwait, cv_timedwait_sig, cv_signal, cv_broadcast, cv_broadcastpri, cv_wmesg — kernel 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_sig(struct cv *cvp, lock, int timo);


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 cv_init(), 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 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. The cv_wait_unlock() function does not reacquire the lock before returning. 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().


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)

MidnightBSD 0.3 June 5, 2007 MidnightBSD 0.3