--- name: "kevent" section: "2" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/kevent" --- # kevent(2) kernel event notification mechanism ## NAME **kqueue**, **kevent** – kernel event notification mechanism ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *int*\ **kqueue**(*void*); *int*\ **kqueuex**(*u\_int flags*); *int*\ **kqueue1**(*int flags*); *int*\ **kevent**(*int kq*, *const struct kevent \*changelist*, *int nchanges*, *struct kevent \*eventlist*, *int nevents*, *const struct timespec \*timeout*); **EV\_SET**(*kev*, *ident*, *filter*, *flags*, *fflags*, *data*, *udata*); ## DESCRIPTION The **kqueue**() system call provides a generic method of notifying the user when an event happens or a condition holds, based on the results of small pieces of kernel code termed filters. A kevent is identified by the (ident, filter) pair; there may only be one unique kevent per kqueue. The filter is executed upon the initial registration of a kevent in order to detect whether a preexisting condition is present, and is also executed whenever an event is passed to the filter for evaluation. If the filter determines that the condition should be reported, then the kevent is placed on the kqueue for the user to retrieve. The filter is also run when the user attempts to retrieve the kevent from the kqueue. If the filter indicates that the condition that triggered the event no longer holds, the kevent is removed from the kqueue and is not returned. Multiple events which trigger the filter do not result in multiple kevents being placed on the kqueue; instead, the filter will aggregate the events into a single struct kevent. Calling **close**() on a file descriptor will remove any kevents that reference the descriptor. The **kqueue**() system call creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with [fork(2)](/fork/2.md). However, if [rfork(2)](/rfork/2.md) is called without the RFFDG flag, then the descriptor table is shared, which will allow sharing of the kqueue between two processes. The **kqueuex**() system call also creates a new kernel event queue, and additionally takes a *flags* argument, which is a bitwise-inclusive OR of the following flags: - KQUEUE\_CLOEXEC The returned file descriptor is automatically closed on [execve(2)](/execve/2.md) - KQUEUE\_CPONFORK When this flag is set, the created kqueue is copied into the child process on [fork(2)](/fork/2.md) calls. The kqueue descriptor index of the new kqueue will be inherited by the child, that is, the numeric value of the descriptor will remain the same. Copying is deep, that is, each registered event in the original kqueue is copied (and not shared) into the new kqueue. This is contrary to how other descriptor types are handled upon [fork(2)](/fork/2.md), where the copied file descriptor references the same file object as the source descriptor (shallow copy). By default, in other words, when the flag is not set, kqueues from the parent are not copied on fork to the child process. The corresponding file descriptor indeces are unused in the child. Registered events that reference file descriptors which are not duplicated on fork, are not copied into the new kqueue. For instance, if the event references a file descriptor opened with the O\_CLOEXEC flag set, it is not copied. Similarly, if event references a kqueue opened without the KQUEUE\_CPONFORK flag, the event is not copied. The **kqueue**() system call is equivalent to calling **kqueuex**() with *flags* set to 0. The **kqueue1**() function exists for compatibility with NetBSD. The *flags* argument accepts zero or more of the following values: - O\_CLOEXEC The returned file descriptor is automatically closed on [execve(2)](/execve/2.md) The **kevent**() system call is used to register events with the queue, and return any pending events to the user. The *changelist* argument is a pointer to an array of *kevent* structures, as defined in ⟨sys/event.h⟩. All changes contained in the *changelist* are applied before any pending events are read from the queue. The *nchanges* argument gives the size of *changelist*. The *eventlist* argument is a pointer to an array of kevent structures. The *nevents* argument determines the size of *eventlist*. When *nevents* is zero, **kevent**() will return immediately even if there is a *timeout* specified unlike [select(2)](/select/2.md). If *timeout* is a non-NULL pointer, it specifies a maximum interval to wait for an event, which will be interpreted as a struct timespec. If *timeout* is a NULL pointer, **kevent**() waits indefinitely. To effect a poll, the *timeout* argument should be non-NULL, pointing to a zero-valued *timespec* structure. The same array may be used for the *changelist* and *eventlist*. The **EV\_SET**() macro is provided for ease of initializing a kevent structure. The *kevent* structure is defined as: ``` struct kevent { uintptr_t ident; /* identifier for this event */ short filter; /* filter for event */ u_short flags; /* action flags for kqueue */ u_int fflags; /* filter flag value */ int64_t data; /* filter data value */ void *udata; /* opaque user data identifier */ uint64_t ext[4]; /* extensions */ }; ``` The fields of *struct*, *kevent* are: - *ident* Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor. - *filter* Identifies the kernel filter used to process this event. The pre-defined system filters are described below. - *flags* Actions to perform on the event. - *fflags* Filter-specific flags. - *data* Filter-specific data value. - *udata* Opaque user-defined value passed through the kernel unchanged. - *ext* Extended data passed to and from kernel. The meaning of the *ext\[0\]* and *ext\[1\]* members is defined by the filter. If a filter does not use them, these members are passed through the kernel unchanged. The *ext\[2\]* and *ext\[3\]* members are always passed through the kernel unchanged, providing additional user-defined values. The *flags* field can contain the following values: - EV\_ADD Adds the event to the kqueue. Re-adding an existing event will modify the parameters of the original event, and not result in a duplicate entry. Adding an event automatically enables it, unless overridden by the EV\_DISABLE flag. - EV\_ENABLE Permit **kevent**() to return the event if it is triggered. - EV\_DISABLE Disable the event so **kevent**() will not return it. The filter itself is not disabled. - EV\_DISPATCH Disable the event source immediately after delivery of an event. See EV\_DISABLE above. - EV\_DELETE Removes the event from the kqueue. Events which are attached to file descriptors are automatically deleted on the last close of the descriptor. - EV\_RECEIPT This flag is useful for making bulk changes to a kqueue without draining any pending events. When passed as input, it forces EV\_ERROR to always be returned. When a filter is successfully added the *data* field will be zero. Note that if this flag is encountered and there is no remaining space in *eventlist* to hold the EV\_ERROR event, then subsequent changes will not get processed. - EV\_ONESHOT Return only the first occurrence of the filter being triggered. After the user retrieves the event from the kqueue, it is deleted. - EV\_CLEAR Reset the state of the event after it is retrieved by the user. This is useful for filters which report state transitions instead of the current state. Note that some filters may automatically set this flag internally. - EV\_EOF Filters may set this flag to indicate filter-specific EOF condition. - EV\_ERROR See RETURN VALUES below. - EV\_KEEPUDATA Preserve the *udata* associated with an existing event. This allows other aspects of the event to be modified without requiring the caller to know the *udata* value previously registered with the event. This is especially useful with NOTE\_TRIGGER or EV\_ENABLE. This flag may not be used with EV\_ADD. The predefined system filters are listed below. Arguments may be passed to and from the filter via the *fflags* and *data* fields in the kevent structure. - EVFILT\_READ Takes a descriptor as the identifier, and returns whenever there is data available to read. The behavior of the filter is slightly different depending on the descriptor type. - Sockets Sockets which have previously been passed to [listen(2)](/listen/2.md) return when there is an incoming connection pending. *data* contains the size of the listen backlog. Other socket descriptors return when there is data to be read, subject to the SO\_RCVLOWAT value of the socket buffer. This may be overridden with a per-filter low water mark at the time the filter is added by setting the NOTE\_LOWAT flag in *fflags*, and specifying the new low water mark in *data*. On return, *data* contains the number of bytes of protocol data available to read. If the read direction of the socket has shutdown, then the filter also sets EV\_EOF in *flags*, and returns the socket error (if any) in *fflags*. It is possible for EOF to be returned (indicating the connection is gone) while there is still data pending in the socket buffer. - Vnodes Returns when the file pointer is not at the end of file. *data* contains the offset from current position to end of file, and may be negative. This behavior is different from [poll(2)](/poll/2.md), where read events are triggered for regular files unconditionally. This event can be triggered unconditionally by setting the NOTE\_FILE\_POLL flag in *fflags*. - Fifos, Pipes Returns when the there is data to read; *data* contains the number of bytes available. When the last writer disconnects, the filter will set EV\_EOF in *flags*. This will be cleared by the filter when a new writer connects, at which point the filter will resume waiting for data to become available before returning. - BPF devices Returns when the BPF buffer is full, the BPF timeout has expired, or when the BPF has “immediate mode” enabled and there is any data to read; *data* contains the number of bytes available. - Eventfds Returns when the counter is greater than 0; *data* contains the counter value, which must be cast to *uint64\_t*. - Kqueues Returns when pending events are present on the queue; *data* contains the number of events available. - EVFILT\_WRITE Takes a descriptor as the identifier, and returns whenever it is possible to write to the descriptor. For sockets, pipes and fifos, *data* will contain the amount of space remaining in the write buffer. The filter will set EV\_EOF when the reader disconnects, and for the fifo case, this will be cleared when a new reader connects. Note that this filter is not supported for vnodes. For sockets, the low water mark and socket error handling is identical to the EVFILT\_READ case. For eventfds, *data* will contain the maximum value that can be added to the counter without blocking. For BPF devices, when the descriptor is attached to an interface the filter always indicates that it is possible to write and *data* will contain the MTU size of the underlying interface. - EVFILT\_EMPTY Takes a descriptor as the identifier, and returns whenever there is no remaining data in the write buffer. - EVFILT\_AIO Events for this filter are not registered with **kevent**() directly but are registered via the *aio\_sigevent* member of an asynchronous I/O request when it is scheduled via an asynchronous I/O system call such as **aio\_read**(). The filter returns under the same conditions as **aio\_error**(). For more details on this filter see [sigevent(3)](/sigevent/3.md) and [aio(4)](/aio/4.md). - EVFILT\_VNODE Takes a file descriptor as the identifier and the events to watch for in *fflags*, and returns when one or more of the requested events occurs on the descriptor. The events to monitor are: - NOTE\_ATTRIB The file referenced by the descriptor had its attributes changed. - NOTE\_CLOSE A file descriptor referencing the monitored file, was closed. The closed file descriptor did not have write access. - NOTE\_CLOSE\_WRITE A file descriptor referencing the monitored file, was closed. The closed file descriptor had write access. This note, as well as NOTE\_CLOSE, are not activated when files are closed forcibly by [unmount(2)](/unmount/2.md) or [revoke(2)](/revoke/2.md). Instead, NOTE\_REVOKE is sent for such events. - NOTE\_DELETE The **unlink**() system call was called on the file referenced by the descriptor. - NOTE\_EXTEND For regular file, the file referenced by the descriptor was extended. For directory, reports that a directory entry was added or removed, as the result of rename operation. The NOTE\_EXTEND event is not reported when a name is changed inside the directory. - NOTE\_LINK The link count on the file changed. In particular, the NOTE\_LINK event is reported if a subdirectory was created or deleted inside the directory referenced by the descriptor. - NOTE\_OPEN The file referenced by the descriptor was opened. - NOTE\_READ A read occurred on the file referenced by the descriptor. - NOTE\_RENAME The file referenced by the descriptor was renamed. - NOTE\_REVOKE Access to the file was revoked via [revoke(2)](/revoke/2.md) or the underlying file system was unmounted. - NOTE\_WRITE A write occurred on the file referenced by the descriptor. On return, *fflags* contains the events which triggered the filter. - EVFILT\_PROC Takes the process ID to monitor as the identifier and the events to watch for in *fflags*, and returns when the process performs one or more of the requested events. If a process can normally see another process, it can attach an event to it. The events to monitor are: - NOTE\_EXIT The process has exited. The exit status will be stored in *data* in the same format as the status returned by [wait(2)](/wait/2.md). - NOTE\_REAP The process has been reaped by a call to the [wait(2)](/wait/2.md) family of functions. - NOTE\_FORK The process has called [fork(2)](/fork/2.md) or [vfork(2)](/vfork/2.md). - NOTE\_EXEC The process has executed a new process via [execve(2)](/execve/2.md) or a similar call. - NOTE\_TRACK Follow a process across **fork**() calls. The parent process registers a new kevent to monitor the child process using the same *fflags* as the original event. The child process will signal an event with NOTE\_CHILD set in *fflags* and the parent PID in *data*. If the parent process fails to register a new kevent (usually due to resource limitations), it will signal an event with NOTE\_TRACKERR set in *fflags*, and the child process will not signal a NOTE\_CHILD event. On return, *fflags* contains the events which triggered the filter. - EVFILT\_PROCDESC Takes the process descriptor created by [pdfork(2)](/pdfork/2.md) to monitor as the identifier and the events to watch for in *fflags*, and returns when the associated process performs one or more of the requested events. The events to monitor are: - NOTE\_EXIT The process has exited. The exit status will be stored in *data*. - NOTE\_FORK The process has forked. The process identifier (PID) of the most recently forked child is returned in the *data* field. The identifier is advisory; if several children were spawned before the event is returned, only one of the PIDs is returned. Other mechanisms provide reliable reporting of fork events. For instance, debugging the process with a subscription for relevant events would serialize forks. See [ptrace(2)](/ptrace/2.md) and the description of the PT\_FOLLOW\_FORK request. Unlike the EVFILT\_PROC filter, the NOTE\_TRACK pseudo-event is not supported. If both NOTE\_EXIT and NOTE\_FORK events are reported, then the *data* field contains the exit status of the process, and the last child's PID is not returned. - NOTE\_PDSIGCHLD Activates on events that are reported through [pdwait(2)](/pdwait/2.md) on the process descriptor. After the event is reported, the [pdwait(2)](/pdwait/2.md) can be called to obtain the information about the process status change. On return, *fflags* contains the events which triggered the filter. - EVFILT\_SIGNAL Takes the signal number to monitor as the identifier and returns when the given signal is delivered to the process. This coexists with the **signal**() and **sigaction**() facilities, and has a lower precedence. The filter will record all attempts to deliver a signal to a process, even if the signal has been marked as SIG\_IGN, except for the SIGCHLD signal, which, if ignored, will not be recorded by the filter. Event notification happens before normal signal delivery processing. *data* returns the number of times the signal has occurred since the last call to **kevent**(). This filter automatically sets the EV\_CLEAR flag internally. - EVFILT\_JAIL Takes the jail ID to monitor as the identifier and the events to watch for in *fflags*, and returns when the jail performs one or more of the requested events. If a process can normally see a jail, it can attach an event to it. An identifier of zero will watch the process's own jail. The events to monitor are: - NOTE\_JAIL\_SET The jail has been changed via [jail\_set(2)](/jail_set/2.md). - NOTE\_JAIL\_ATTACH A process has attached to the jail via [jail\_attach(2)](/jail_attach/2.md) or a similar call. The process ID will be stored in *data*. If more than one process has attached since the last call to **kevent**(), *data* will be zero. - NOTE\_JAIL\_REMOVE The jail has been removed. - NOTE\_JAIL\_CHILD A child of the watched jail has been created. Its jail ID will be stored in *data*. If more than one jail has been created since the last call to **kevent**(), *data* will be zero. On return, *fflags* contains the events which triggered the filter. It will also contain NOTE\_JAIL\_MULTI if more than one NOTE\_JAIL\_ATTACH or NOTE\_JAIL\_CHILD event has been received since the last call to **kevent**(). - EVFILT\_JAILDESC Takes a jail descriptor returned by [jail\_set(2)](/jail_set/2.md) or [jail\_get(2)](/jail_get/2.md) as the identifier and the events to watch for in *fflags*, and returns when the jail performs one or more of the requested events. The events to monitor and the resulting *fflags* are the same as those listed in EVFILT\_JAIL, above. - EVFILT\_TIMER Establishes an arbitrary timer identified by *ident*. When adding a timer, *data* specifies the moment to fire the timer (for NOTE\_ABSTIME) or the timeout period. The timer will be periodic unless EV\_ONESHOT or NOTE\_ABSTIME is specified. On return, *data* contains the number of times the timeout has expired since the last call to **kevent**(). For non-monotonic timers, this filter automatically sets the EV\_CLEAR flag internally. The filter accepts the following flags in the *fflags* argument: - NOTE\_SECONDS *data* is in seconds. - NOTE\_MSECONDS *data* is in milliseconds. - NOTE\_USECONDS *data* is in microseconds. - NOTE\_NSECONDS *data* is in nanoseconds. - NOTE\_ABSTIME The specified expiration time is absolute. If *fflags* is not set, the default is milliseconds. On return, *fflags* contains the events which triggered the filter. Periodic timers with a specified timeout of 0 will be silently adjusted to timeout after 1 of the time units specified by the requested precision in *fflags*. If an absolute time is specified that has already passed, then it is treated as if the current time were specified and the event will fire as soon as possible. If an existing timer is re-added, the existing timer will be effectively canceled (throwing away any undelivered record of previous timer expiration) and re-started using the new parameters contained in *data* and *fflags*. There is a system wide limit on the number of timers which is controlled by the *kern.kq\_calloutmax* sysctl. - EVFILT\_USER Establishes a user event identified by *ident* which is not associated with any kernel mechanism but is triggered by user level code. The lower 24 bits of the *fflags* may be used for user defined flags and manipulated using the following: - NOTE\_FFNOP Ignore the input *fflags*. - NOTE\_FFAND Bitwise AND *fflags*. - NOTE\_FFOR Bitwise OR *fflags*. - NOTE\_FFCOPY Copy *fflags*. - NOTE\_FFCTRLMASK Control mask for *fflags*. - NOTE\_FFLAGSMASK User defined flag mask for *fflags*. A user event is triggered for output with the following: - NOTE\_TRIGGER Cause the event to be triggered. On return, *fflags* contains the users defined flags in the lower 24 bits. ## CANCELLATION BEHAVIOUR If *nevents* is non-zero, i.e., the function is potentially blocking, the call is a cancellation point. Otherwise, i.e., if *nevents* is zero, the call is not cancellable. Cancellation can only occur before any changes are made to the kqueue, or when the call was blocked and no changes to the queue were requested. ## RETURN VALUES The **kqueue**() system call creates a new kernel event queue and returns a file descriptor. If there was an error creating the kernel event queue, a value of -1 is returned and errno set. The **kevent**() system call returns the number of events placed in the *eventlist*, up to the value given by *nevents*. If an error occurs while processing an element of the *changelist* and there is enough room in the *eventlist*, then the event will be placed in the *eventlist* with EV\_ERROR set in *flags* and the system error in *data*. Otherwise, -1 will be returned, and errno will be set to indicate the error condition. If the time limit expires, then **kevent**() returns 0. ## EXAMPLES ``` #include #include #include #include #include #include int main(int argc, char **argv) { struct kevent event; /* Event we want to monitor */ struct kevent tevent; /* Event triggered */ int kq, fd, ret; if (argc != 2) err(EXIT_FAILURE, "Usage: %s path\n", argv[0]); fd = open(argv[1], O_RDONLY); if (fd == -1) err(EXIT_FAILURE, "Failed to open '%s'", argv[1]); /* Create kqueue. */ kq = kqueue(); if (kq == -1) err(EXIT_FAILURE, "kqueue() failed"); /* Initialize kevent structure. */ EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE, 0, NULL); /* Attach event to the kqueue. */ ret = kevent(kq, &event, 1, NULL, 0, NULL); if (ret == -1) err(EXIT_FAILURE, "kevent register"); for (;;) { /* Sleep until something happens. */ ret = kevent(kq, NULL, 0, &tevent, 1, NULL); if (ret == -1) { err(EXIT_FAILURE, "kevent wait"); } else if (ret > 0) { if (tevent.flags & EV_ERROR) errx(EXIT_FAILURE, "Event error: %s", strerror(event.data)); else printf("Something was written in '%s'\n", argv[1]); } } /* kqueues are destroyed upon close() */ (void)close(kq); (void)close(fd); } ``` ## ERRORS The **kqueue**() system call fails if: - \[ENOMEM\] The kernel failed to allocate enough memory for the kernel queue. - \[ENOMEM\] The RLIMIT\_KQUEUES rlimit (see [getrlimit(2)](/getrlimit/2.md)) for the current user would be exceeded. - \[EMFILE\] The per-process descriptor table is full. - \[ENFILE\] The system file table is full. The **kevent**() system call fails if: - \[EACCES\] The process does not have permission to register a filter. - \[EFAULT\] There was an error reading or writing the *kevent* structure. - \[EBADF\] The specified descriptor is invalid. - \[EINTR\] A signal was delivered before the timeout expired and before any events were placed on the kqueue for return. - \[EINTR\] A cancellation request was delivered to the thread, but not yet handled. - \[EINVAL\] The specified time limit or filter is invalid. - \[EINVAL\] The specified length of the event or change lists is negative. - \[ENOENT\] The event could not be found to be modified or deleted. - \[ENOMEM\] No memory was available to register the event or, in the special case of a timer, the maximum number of timers has been exceeded. This maximum is configurable via the *kern.kq\_calloutmax* sysctl. - \[ESRCH\] The specified process to attach to does not exist. When **kevent**() call fails with EINTR error, all changes in the *changelist* have been applied. ## SEE ALSO [aio\_error(2)](/aio_error/2.md), [aio\_read(2)](/aio_read/2.md), [aio\_return(2)](/aio_return/2.md), [poll(2)](/poll/2.md), [read(2)](/read/2.md), [select(2)](/select/2.md), [sigaction(2)](/sigaction/2.md), [write(2)](/write/2.md), [pthread\_setcancelstate(3)](/pthread_setcancelstate/3.md), [signal(3)](/signal/3.md) Jonathan Lemon, “Kqueue: A Generic and Scalable Event Notification Facility”, *Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference*, USENIX Association, June 25-30, 2001. ## HISTORY The **kqueue**() and **kevent**() system calls first appeared in FreeBSD 4.1. The **kqueuex**() system call and **kqueue1**() function first appeared in FreeBSD 14.0. ## AUTHORS The *kqueue* subsystem and this manual page were written by Jonathan Lemon ⟨*jlemon@FreeBSD.org*⟩. ## BUGS In versions older than FreeBSD 12.0, ⟨sys/event.h⟩ failed to parse without including ⟨sys/types.h⟩ manually.