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Manual Reference Pages  -  CALLOUT_DRAIN (9)


callout_active, callout_deactivate, callout_drain, callout_handle_init, callout_init, callout_init_mtx, callout_init_rm, callout_init_rw, callout_pending, callout_reset, callout_reset_curcpu, callout_reset_on, callout_reset_sbt, callout_reset_sbt_curcpu, callout_reset_sbt_on, callout_schedule, callout_schedule_curcpu, callout_schedule_on, callout_schedule_sbt, callout_schedule_sbt_curcpu, callout_schedule_sbt_on, callout_stop, timeout, untimeout - execute a function after a specified length of time


Legacy Api
Return Values


.In sys/types.h
.In sys/systm.h
typedef void timeout_t (void *);

int callout_active struct callout *c void callout_deactivate struct callout *c int callout_drain struct callout *c void callout_handle_init struct callout_handle *handle
struct callout_handle handle = CALLOUT_HANDLE_INITIALIZER(&handle);

void callout_init struct callout *c int mpsafe void callout_init_mtx struct callout *c struct mtx *mtx int flags void callout_init_rm struct callout *c struct rmlock *rm int flags void callout_init_rw struct callout *c struct rwlock *rw int flags int callout_pending struct callout *c int callout_reset struct callout *c int ticks timeout_t *func void *arg int callout_reset_curcpu struct callout *c int ticks timeout_t *func "void *arg" int callout_reset_on struct callout *c int ticks timeout_t *func "void *arg" "int cpu" int callout_reset_sbt struct callout *c sbintime_t sbt "sbintime_t pr" "timeout_t *func" "void *arg" "int flags" int callout_reset_sbt_curcpu struct callout *c sbintime_t sbt "sbintime_t pr" "timeout_t *func" "void *arg" "int flags" int callout_reset_sbt_on struct callout *c sbintime_t sbt "sbintime_t pr" "timeout_t *func" "void *arg" "int cpu" "int flags" int callout_schedule struct callout *c int ticks int callout_schedule_curcpu struct callout *c int ticks int callout_schedule_on struct callout *c int ticks int cpu int callout_schedule_sbt struct callout *c sbintime_t sbt "sbintime_t pr" "int flags" int callout_schedule_sbt_curcpu struct callout *c sbintime_t sbt "sbintime_t pr" "int flags" int callout_schedule_sbt_on struct callout *c sbintime_t sbt "sbintime_t pr" "int cpu" "int flags" int callout_stop struct callout *c struct callout_handle timeout timeout_t *func void *arg int ticks void untimeout timeout_t *func void *arg struct callout_handle handle


The callout API is used to schedule a call to an arbitrary function at a specific time in the future. Consumers of this API are required to allocate a callout structure (struct callout) for each pending function invocation. This structure stores state about the pending function invocation including the function to be called and the time at which the function should be invoked. Pending function calls can be cancelled or rescheduled to a different time. In addition, a callout structure may be reused to schedule a new function call after a scheduled call is completed.

Callouts only provide a single-shot mode. If a consumer requires a periodic timer, it must explicitly reschedule each function call. This is normally done by rescheduling the subsequent call within the called function.

Callout functions must not sleep. They may not acquire sleepable locks, wait on condition variables, perform blocking allocation requests, or invoke any other action that might sleep.

Each callout structure must be initialized by callout_init, callout_init_mtx, callout_init_rm, or callout_init_rw before it is passed to any of the other callout functions. The callout_init function initializes a callout structure in c that is not associated with a specific lock. If the mpsafe argument is zero, the callout structure is not considered to be "multi-processor safe"; and the Giant lock will be acquired before calling the callout function and released when the callout function returns.

The callout_init_mtx, callout_init_rm, and callout_init_rw functions initialize a callout structure in c that is associated with a specific lock. The lock is specified by the mtx, rm, or rw parameter. The associated lock must be held while stopping or rescheduling the callout. The callout subsystem acquires the associated lock before calling the callout function and releases it after the function returns. If the callout was cancelled while the callout subsystem waited for the associated lock, the callout function is not called, and the associated lock is released. This ensures that stopping or rescheduling the callout will abort any previously scheduled invocation.

Only regular mutexes may be used with callout_init_mtx; spin mutexes are not supported. A sleepable read-mostly lock ( one initialized with the RM_SLEEPABLE flag ) may not be used with callout_init_rm. Similarly, other sleepable lock types such as sx(9) and lockmgr(9) cannot be used with callouts because sleeping is not permitted in the callout subsystem.

These flags may be specified for callout_init_mtx, callout_init_rm, or callout_init_rw:
  The callout function will release the associated lock itself, so the callout subsystem should not attempt to unlock it after the callout function returns.
CALLOUT_SHAREDLOCK The lock is only acquired in read mode when running the callout handler. This flag is ignored by callout_init_mtx.

The function callout_stop cancels a callout c if it is currently pending. If the callout is pending, then callout_stop returns a non-zero value. If the callout is not set, has already been serviced, or is currently being serviced, then zero will be returned. If the callout has an associated lock, then that lock must be held when this function is called.

The function callout_drain is identical to callout_stop except that it will wait for the callout c to complete if it is already in progress. This function MUST NOT be called while holding any locks on which the callout might block, or deadlock will result. Note that if the callout subsystem has already begun processing this callout, then the callout function may be invoked before callout_drain returns. However, the callout subsystem does guarantee that the callout will be fully stopped before callout_drain returns.

The callout_reset and callout_schedule function families schedule a future function invocation for callout c. If c already has a pending callout, it is cancelled before the new invocation is scheduled. These functions return a non-zero value if a pending callout was cancelled and zero if there was no pending callout. If the callout has an associated lock, then that lock must be held when any of these functions are called.

The time at which the callout function will be invoked is determined by either the ticks argument or the sbt, pr, and flags arguments. When ticks is used, the callout is scheduled to execute after ticks /hz seconds. Non-positive values of ticks are silently converted to the value '1'.

The sbt, pr, and flags arguments provide more control over the scheduled time including support for higher resolution times, specifying the precision of the scheduled time, and setting an absolute deadline instead of a relative timeout. The callout is scheduled to execute in a time window which begins at the time specified in sbt and extends for the amount of time specified in pr. If sbt specifies a time in the past, the window is adjusted to start at the current time. A non-zero value for pr allows the callout subsystem to coalesce callouts scheduled close to each other into fewer timer interrupts, reducing processing overhead and power consumption. These flags may be specified to adjust the interpretation of sbt and pr:
C_ABSOLUTE Handle the sbt argument as an absolute time since boot. By default, sbt is treated as a relative amount of time, similar to ticks.
  Run the handler directly from hardware interrupt context instead of from the softclock thread. This reduces latency and overhead, but puts more constraints on the callout function. Callout functions run in this context may use only spin mutexes for locking and should be as small as possible because they run with absolute priority.
C_PREL Specifies relative event time precision as binary logarithm of time interval divided by acceptable time deviation: 1 -- 1/2, 2 -- 1/4, etc. Note that the larger of pr or this value is used as the length of the time window. Smaller values (which result in larger time intervals) allow the callout subsystem to aggregate more events in one timer interrupt.
C_HARDCLOCK Align the timeouts to hardclock calls if possible.

The callout_reset functions accept a func argument which identifies the function to be called when the time expires. It must be a pointer to a function that takes a single void * argument. Upon invocation, func will receive arg as its only argument. The callout_schedule functions reuse the func and arg arguments from the previous callout. Note that one of the callout_reset functions must always be called to initialize func and arg before one of the callout_schedule functions can be used.

The callout subsystem provides a softclock thread for each CPU in the system. Callouts are assigned to a single CPU and are executed by the softclock thread for that CPU. Initially, callouts are assigned to CPU 0. The callout_reset_on, callout_reset_sbt_on, callout_schedule_on and callout_schedule_sbt_on functions assign the callout to CPU cpu. The callout_reset_curcpu, callout_reset_sbt_curpu, callout_schedule_curcpu and callout_schedule_sbt_curcpu functions assign the callout to the current CPU. The callout_reset, callout_reset_sbt, callout_schedule and callout_schedule_sbt functions schedule the callout to execute in the softclock thread of the CPU to which it is currently assigned.

Softclock threads are not pinned to their respective CPUs by default. The softclock thread for CPU 0 can be pinned to CPU 0 by setting the kern.pin_default_swi loader tunable to a non-zero value. Softclock threads for CPUs other than zero can be pinned to their respective CPUs by setting the kern.pin_pcpu_swi loader tunable to a non-zero value.

The macros callout_pending, callout_active and callout_deactivate provide access to the current state of the callout. The callout_pending macro checks whether a callout is pending; a callout is considered pending when a timeout has been set but the time has not yet arrived. Note that once the timeout time arrives and the callout subsystem starts to process this callout, callout_pending will return FALSE even though the callout function may not have finished (or even begun) executing. The callout_active macro checks whether a callout is marked as active, and the callout_deactivate macro clears the callout’s active flag. The callout subsystem marks a callout as active when a timeout is set and it clears the active flag in callout_stop and callout_drain, but it does not clear it when a callout expires normally via the execution of the callout function.

    Avoiding Race Conditions

The callout subsystem invokes callout functions from its own thread context. Without some kind of synchronization, it is possible that a callout function will be invoked concurrently with an attempt to stop or reset the callout by another thread. In particular, since callout functions typically acquire a lock as their first action, the callout function may have already been invoked, but is blocked waiting for that lock at the time that another thread tries to reset or stop the callout.

There are three main techniques for addressing these synchronization concerns. The first approach is preferred as it is the simplest:

  1. Callouts can be associated with a specific lock when they are initialized by callout_init_mtx, callout_init_rm, or callout_init_rw. When a callout is associated with a lock, the callout subsystem acquires the lock before the callout function is invoked. This allows the callout subsystem to transparently handle races between callout cancellation, scheduling, and execution. Note that the associated lock must be acquired before calling callout_stop or one of the callout_reset or callout_schedule functions to provide this safety.

    A callout initialized via callout_init with mpsafe set to zero is implicitly associated with the Giant mutex. If Giant is held when cancelling or rescheduling the callout, then its use will prevent races with the callout function.

  2. The return value from callout_stop ( or the callout_reset and callout_schedule function families ) indicates whether or not the callout was removed. If it is known that the callout was set and the callout function has not yet executed, then a return value of FALSE indicates that the callout function is about to be called. For example:
    if (sc->sc_flags & SCFLG_CALLOUT_RUNNING) {
            if (callout_stop(&sc->sc_callout)) {
                    sc->sc_flags &= ~SCFLG_CALLOUT_RUNNING;
                    /* successfully stopped */
            } else {
                     * callout has expired and callout
                     * function is about to be executed
  3. The callout_pending, callout_active and callout_deactivate macros can be used together to work around the race conditions. When a callout’s timeout is set, the callout subsystem marks the callout as both active and pending. When the timeout time arrives, the callout subsystem begins processing the callout by first clearing the pending flag. It then invokes the callout function without changing the active flag, and does not clear the active flag even after the callout function returns. The mechanism described here requires the callout function itself to clear the active flag using the callout_deactivate macro. The callout_stop and callout_drain functions always clear both the active and pending flags before returning.

    The callout function should first check the pending flag and return without action if callout_pending returns TRUE. This indicates that the callout was rescheduled using callout_reset just before the callout function was invoked. If callout_active returns FALSE then the callout function should also return without action. This indicates that the callout has been stopped. Finally, the callout function should call callout_deactivate to clear the active flag. For example:

    if (callout_pending(&sc->sc_callout)) {
            /* callout was reset */
    if (!callout_active(&sc->sc_callout)) {
            /* callout was stopped */
    /* rest of callout function */

    Together with appropriate synchronization, such as the mutex used above, this approach permits the callout_stop and callout_reset functions to be used at any time without races. For example:

    /* The callout is effectively stopped now. */

    If the callout is still pending then these functions operate normally, but if processing of the callout has already begun then the tests in the callout function cause it to return without further action. Synchronization between the callout function and other code ensures that stopping or resetting the callout will never be attempted while the callout function is past the callout_deactivate call.

    The above technique additionally ensures that the active flag always reflects whether the callout is effectively enabled or disabled. If callout_active returns false, then the callout is effectively disabled, since even if the callout subsystem is actually just about to invoke the callout function, the callout function will return without action.

There is one final race condition that must be considered when a callout is being stopped for the last time. In this case it may not be safe to let the callout function itself detect that the callout was stopped, since it may need to access data objects that have already been destroyed or recycled. To ensure that the callout is completely finished, a call to callout_drain should be used. In particular, a callout should always be drained prior to destroying its associated lock or releasing the storage for the callout structure.


.Bf Sy The functions below are a legacy API that will be removed in a future release. New code should not use these routines.

The function timeout schedules a call to the function given by the argument func to take place after ticks /hz seconds. Non-positive values of ticks are silently converted to the value '1'. func should be a pointer to a function that takes a void * argument. Upon invocation, func will receive arg as its only argument. The return value from timeout is a struct callout_handle which can be used in conjunction with the untimeout function to request that a scheduled timeout be canceled.

The function callout_handle_init can be used to initialize a handle to a state which will cause any calls to untimeout with that handle to return with no side effects.

Assigning a callout handle the value of CALLOUT_HANDLE_INITIALIZER performs the same function as callout_handle_init and is provided for use on statically declared or global callout handles.

The function untimeout cancels the timeout associated with handle using the func and arg arguments to validate the handle. If the handle does not correspond to a timeout with the function func taking the argument arg no action is taken. handle must be initialized by a previous call to timeout, callout_handle_init, or assigned the value of CALLOUT_HANDLE_INITIALIZER &handle before being passed to untimeout. The behavior of calling untimeout with an uninitialized handle is undefined.

As handles are recycled by the system, it is possible (although unlikely) that a handle from one invocation of timeout may match the handle of another invocation of timeout if both calls used the same function pointer and argument, and the first timeout is expired or canceled before the second call. The timeout facility offers O(1) running time for timeout and untimeout. Timeouts are executed from softclock with the Giant lock held. Thus they are protected from re-entrancy.


The callout_active macro returns the state of a callout’s active flag.

The callout_pending macro returns the state of a callout’s pending flag.

The callout_reset and callout_schedule function families return non-zero if the callout was pending before the new function invocation was scheduled.

The callout_stop and callout_drain functions return non-zero if the callout was still pending when it was called or zero otherwise. The timeout function returns a struct callout_handle that can be passed to untimeout.


The current timeout and untimeout routines are based on the work of
.An Adam M. Costello and
.An George Varghese , published in a technical report entitled Redesigning the BSD Callout and Timer Facilities and modified slightly for inclusion in
.Fx by
.An Justin T. Gibbs . The original work on the data structures used in this implementation was published by
.An G. Varghese and
.An A. Lauck in the paper Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient Implementation of a Timer Facility in the The current implementation replaces the long standing BSD linked list callout mechanism which offered O(n) insertion and removal running time but did not generate or require handles for untimeout operations.
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