June 4, 2008 CONDVAR 9

### NAME

cv, condvar, cv_init, cv_destroy, cv_wait, cv_wait_sig, cv_timedwait, cv_timedwait_sig, cv_signal, cv_broadcast, cv_has_waiters - condition variables

### SYNOPSIS

```void` cv_init(```kcondvar_t *cv``, const char *wmesg`)
```void` cv_destroy(```kcondvar_t *cv`)
```void` cv_wait(```kcondvar_t *cv``, kmutex_t *mtx`)
```int` cv_wait_sig(```kcondvar_t *cv``, kmutex_t *mtx`)
```int` cv_timedwait(```kcondvar_t *cv``, kmutex_t *mtx``, int ticks`)
```int` cv_timedwait_sig(```kcondvar_t *cv``, kmutex_t *mtx``, int ticks`)
```void` cv_signal(```kcondvar_t *cv`)
```void` cv_broadcast(```kcondvar_t *cv`)
```bool` cv_has_waiters(```kcondvar_t *cv`)

options DIAGNOSTIC options LOCKDEBUG

### DESCRIPTION

Condition variables (CVs) are used in the kernel to synchronize access to resources that are limited (for example, memory) and to wait for pending I/O operations to complete.

The type provides storage for the CV object. This should be treated as an opaque object and not examined directly by consumers.

### OPTIONS

options DIAGNOSTIC

  Kernels compiled with the ```DIAGNOSTIC` option perform basic sanity checks on CV operations.

options LOCKDEBUG

  Kernels compiled with the ```LOCKDEBUG` option perform potentially CPU intensive sanity checks on CV operations.

### FUNCTIONS

cv_init(```cv``, wmesg`)

  Initialize a CV for use. No other operations can be performed on the CV until it has been initialized.

  The ```wmesg` argument specifies a string of no more than 8 characters that describes the resource or condition associated with the CV. The kernel does not use this argument directly but makes it available for utilities such as ps(1) to display.

cv_destroy(```cv`)

  Release resources used by a CV. The CV must not be in use when it is destroyed, and must not be used afterwards.

cv_wait(```cv``, mtx`)

  Cause the current LWP to wait non-interruptably for access to a resource, or for an I/O operation to complete. The LWP will resume execution when awoken by another thread using cv_signal(``) or cv_broadcast(``).

  ```mtx` specifies a kernel mutex to be used as an interlock, and must be held by the calling LWP on entry to cv_wait(``). It will be released once the LWP has prepared to sleep, and will be reacquired before cv_wait(``) returns.

  A small window exists between testing for availability of a resource and waiting for the resource with cv_wait(``), in which the resource may become available again. The interlock is used to guarantee that the resource will not be signalled as available until the calling LWP has begun to wait for it.

  Non-interruptable waits have the potential to deadlock the system, and so must be kept short (typically, under one second).

cv_wait_sig(```cv``, mtx`)

  As per cv_wait(``), but causes the current LWP to wait interruptably. If the LWP receives a signal, or is interrupted by another condition such as its containing process exiting, the wait is ended early and an error code returned.

  If cv_wait_sig(``) returns as a result of a signal, the return value is ```ERESTART` if the signal has the ```SA_RESTART` property. If awoken normally, the value is zero, and ```EINTR` under all other conditions.

cv_timedwait(```cv``, mtx``, ticks`)

  As per cv_wait(``), but will return early if a timeout specified by the ```ticks` argument expires.

  ```ticks` is an architecture and system dependent value related to the number of clock interrupts per second. See hz(9) for details. The mstohz(9) macro can be used to convert a timeout expressed in milliseconds to one suitable for cv_timedwait(``). If the ```ticks` argument is zero, cv_timedwait(``) behaves exactly like cv_wait(``).

  If the timeout expires before the LWP is awoken, the return value is ```EWOULDBLOCK`. If awoken normally, the return value is zero.

cv_timedwait_sig(```cv``, mtx``, ticks`)

  As per cv_wait_sig(``), but also accepts a timeout value and will return ```EWOULDBLOCK` if the timeout expires.

cv_signal(```cv`)

  Awaken one LWP (potentially among many) that is waiting on the specified condition variable. The mutex passed to the wait function ```mtx(.blm Pp`) must also be held when calling cv_signal(``).

  (Note that cv_signal(``) is erroneously named in that it does not send a signal in the traditional sense to LWPs waiting on a CV.)

cv_broadcast(```cv`)

  Awaken all LWPs waiting on the specified condition variable. The mutex passed to the wait function ```mtx(.blm Pp`) must also be held when calling cv_broadcast(``).

cv_has_waiters(```cv`)

  Return ```true` if one or more LWPs are waiting on the specified condition variable.

  cv_has_waiters(``) cannot test reliably for interruptable waits. It should only be used to test for non-interruptable waits made using cv_wait(``).

  cv_has_waiters(``) should only be used when making diagnostic assertions, and must be called while holding the interlocking mutex passed to cv_wait(``).

### EXAMPLES

`

    Consuming a resource:
            /*
             * Lock the resource.  Its mutex will also serve as the
             * interlock.
             */
            mutex_enter(&res->mutex);
            /*
             * Wait for the resource to become available.
             */
            while (res->state == BUSY)
                    cv_wait(&res->condvar, &res->mutex);
            /*
             * It's now available to us.  Take ownership of the
             * resource, and consume it.
             */
            res->state = BUSY;
            mutex_exit(&res->mutex);
            consume(res);
    Releasing a resource for the next consumer to use:
            mutex_enter(&res->mutex);
            res->state = IDLE;
            cv_signal(&res->condvar);
            mutex_exit(&res->mutex);

`

### CODE REFERENCES

This section describes places within the NetBSD source tree where code implementing condition variables can be found. All pathnames are relative to ```/usr/src`.

The core of the CV implementation is in ```sys/kern/kern_condvar.c`.

The header file ```sys/sys/condvar.h` describes the public interface.

### SEE ALSO

sigaction(2), errno(9), mb(9), mstohz(9), mutex(9), rwlock(9)

### HISTORY

The CV primitives first appeared in NetBSD5.0.