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KQUEUE(2) System Calls Manual KQUEUE(2) NAME kqueue, kevent -- kernel event notification mechanism LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/event.h> 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 or- der 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 trig- gered 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 de- scriptor 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). However, if rfork(2) 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) 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) 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). 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 struc- ture. The same array may be used for the changelist and eventlist. The EV_SET() macro is provided for ease of initializing a kevent struc- ture. 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 un- changed. 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 al- ways passed through the kernel unchanged, providing addi- tional 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 au- tomatically enables it, unless overridden by the EV_DIS- ABLE 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 at- tached 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 in- put, 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 re- maining 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 pre- viously 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 be- havior of the filter is slightly different depend- ing on the descriptor type. Sockets Sockets which have previously been passed to listen(2) return when there is an incoming con- nection 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 over- ridden 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 shut- down, 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), where read events are triggered for regular files un- conditionally. This event can be triggered un- conditionally 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 de- tails on this filter see sigevent(3) and aio(4). 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 de- scriptor. The events to monitor are: NOTE_ATTRIB The file referenced by the de- scriptor had its attributes changed. NOTE_CLOSE A file descriptor referencing the monitored file, was closed. The closed file de- scriptor did not have write access. NOTE_CLOSE_WRITE A file descriptor referencing the monitored file, was closed. The closed file de- scriptor had write access. This note, as well as NOTE_CLOSE, are not activated when files are closed forcibly by unmount(2) or revoke(2). 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 re- name operation. The NOTE_EXTEND event is not re- ported 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 de- scriptor was opened. NOTE_READ A read occurred on the file referenced by the descriptor. NOTE_RENAME The file referenced by the de- scriptor was renamed. NOTE_REVOKE Access to the file was revoked via revoke(2) or the underly- ing file system was unmounted. NOTE_WRITE A write occurred on the file referenced by the descriptor. On return, fflags contains the events which trig- gered 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 re- quested events. If a process can normally see an- other 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 re- turned by wait(2). NOTE_FORK The process has called fork(). NOTE_EXEC The process has executed a new process via execve(2) or a simi- lar call. NOTE_TRACK Follow a process across fork() calls. The parent process regis- ters 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 trig- gered the filter. EVFILT_PROCDESC Takes the process descriptor created by pdfork(2) to monitor as the identifier and the events to watch for in fflags, and returns when the associ- ated 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. On return, fflags contains the events which trig- gered the filter. EVFILT_SIGNAL Takes the signal number to monitor as the identi- fier and returns when the given signal is delivered to the process. This coexists with the signal() and sigaction() facilities, and has a lower prece- dence. The filter will record all attempts to de- liver a signal to a process, even if the signal has been marked as SIG_IGN, except for the SIGCHLD sig- nal, which, if ignored, will not be recorded by the filter. Event notification happens after normal signal delivery processing. data returns the num- ber 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). NOTE_JAIL_ATTACH A process has attached to the jail via jail_attach(2) or a similar call. The process ID will be stored in data. If more than one process has at- tached 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 trig- gered 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) or jail_get(2) as the identifier and the events to watch for in fflags, and returns when the jail per- forms 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 re- turn, 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 trig- gered 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 cur- rent 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 expira- tion) and re-started using the new parameters con- tained 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 fol- lowing: NOTE_TRIGGER Cause the event to be trig- gered. 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 re- turned, and errno will be set to indicate the error condition. If the time limit expires, then kevent() returns 0. EXAMPLES #include <sys/event.h> #include <err.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <string.h> 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)) 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 config- urable 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_read(2), aio_return(2), poll(2), read(2), select(2), sigaction(2), write(2), pthread_setcancelstate(3), signal(3) 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 kevent1() function first appeared in FreeBSD 14.0. AUTHORS The kqueue() system 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. FreeBSD 15.0 September 12, 2025 KQUEUE(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | CANCELLATION BEHAVIOUR | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO | HISTORY | AUTHORS | BUGS
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