--- name: "kqueue" section: "2" platform: "linux" package: "libkqueue-dev" version: "2.0.3-1.1" release: "Debian 8 (Jessie)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/kqueue" --- # kqueue(2) kernel event notification mechanism ## NAME **kqueue**, **kevent** – kernel event notification mechanism ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **kqueue**(*void*); *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 **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 */ intptr_t data; /* filter data value */ void *udata; /* opaque user data identifier */ }; ``` 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. 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. - EV\_ONESHOT Causes the event to return only the first occurrence of the filter being triggered. After the user retrieves the event from the kqueue, it is deleted. - EV\_CLEAR After the event is retrieved by the user, its state is reset. 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. 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**() 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. - 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 may be cleared by passing in EV\_CLEAR, 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. - 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 may be cleared by use of EV\_CLEAR. Note that this filter is not supported for vnodes or BPF devices. For sockets, the low water mark and socket error handling is identical to the EVFILT\_READ case. - 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\_DELETE The **unlink**() system call was called on the file referenced by the descriptor. - NOTE\_WRITE A write occurred on the file referenced by the descriptor. - NOTE\_EXTEND The file referenced by the descriptor was extended. - NOTE\_ATTRIB The file referenced by the descriptor had its attributes changed. - NOTE\_LINK The link count on the file changed. - NOTE\_RENAME The file referenced by the descriptor was renamed. 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 overrides the **signal**() and **sigaction**() facilities, and has a higher precedence. The filter will record all attempts to deliver a signal to a process, even if the signal has been marked as SIG\_IGN. *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\_TIMER Establishes an arbitrary timer identified by *ident*. When adding a timer, *data* specifies the timeout period in milliseconds. The timer will be periodic unless EV\_ONESHOT is specified. On return, *data* contains the number of times the timeout has expired since the last call to **kevent**(). This filter automatically sets the EV\_CLEAR flag internally. 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 assosicated 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\_COPY 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. ## 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. ## ERRORS The **kqueue**() system call fails if: - \[ENOMEM\] The kernel failed to allocate enough memory for the kernel queue. - \[EMFILE\] 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. - \[EINVAL\] The specified time limit or filter is invalid. - \[ENOENT\] 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. ## 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), [signal(3)](/signal/3.md) ## HISTORY The **kqueue**() and **kevent**() system calls first appeared in FreeBSD 4.1. ## AUTHORS The **kqueue**() system and this manual page were written by Jonathan Lemon ⟨jlemon@FreeBSD.org⟩.