.\" Copyright (c) 2000 Jonathan Lemon .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: head/lib/libc/sys/kqueue.2 298984 2016-05-03 15:58:35Z kib $ .\" .Dd May 3, 2016 .Dt KQUEUE 2 .Os .Sh NAME .Nm kqueue , .Nm kevent .Nd kernel event notification mechanism .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/types.h .In sys/event.h .In sys/time.h .Ft int .Fn kqueue "void" .Ft int .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout" .Fn EV_SET "kev" ident filter flags fflags data udata .Sh DESCRIPTION The .Fn 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. .Pp 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. .Pp 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. .Pp 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 .Fn close on a file descriptor will remove any kevents that reference the descriptor. .Pp The .Fn kqueue system call creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with .Xr fork 2 . However, if .Xr rfork 2 is called without the .Dv RFFDG flag, then the descriptor table is shared, which will allow sharing of the kqueue between two processes. .Pp The .Fn kevent system call is used to register events with the queue, and return any pending events to the user. The .Fa changelist argument is a pointer to an array of .Va kevent structures, as defined in .In sys/event.h . All changes contained in the .Fa changelist are applied before any pending events are read from the queue. The .Fa nchanges argument gives the size of .Fa changelist . The .Fa eventlist argument is a pointer to an array of kevent structures. The .Fa nevents argument determines the size of .Fa eventlist . When .Fa nevents is zero, .Fn kevent will return immediately even if there is a .Fa timeout specified unlike .Xr select 2 . If .Fa 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 .Fa timeout is a NULL pointer, .Fn kevent waits indefinitely. To effect a poll, the .Fa timeout argument should be non-NULL, pointing to a zero-valued .Va timespec structure. The same array may be used for the .Fa changelist and .Fa eventlist . .Pp The .Fn EV_SET macro is provided for ease of initializing a kevent structure. .Pp The .Va kevent structure is defined as: .Bd -literal 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 */ }; .Ed .Pp The fields of .Fa struct kevent are: .Bl -tag -width "Fa filter" .It Fa ident Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor. .It Fa filter Identifies the kernel filter used to process this event. The pre-defined system filters are described below. .It Fa flags Actions to perform on the event. .It Fa fflags Filter-specific flags. .It Fa data Filter-specific data value. .It Fa udata Opaque user-defined value passed through the kernel unchanged. .El .Pp The .Va flags field can contain the following values: .Bl -tag -width EV_DISPATCH .It Dv 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. .It Dv EV_ENABLE Permit .Fn kevent to return the event if it is triggered. .It Dv EV_DISABLE Disable the event so .Fn kevent will not return it. The filter itself is not disabled. .It Dv EV_DISPATCH Disable the event source immediately after delivery of an event. See .Dv EV_DISABLE above. .It Dv 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. .It Dv EV_RECEIPT This flag is useful for making bulk changes to a kqueue without draining any pending events. When passed as input, it forces .Dv EV_ERROR to always be returned. When a filter is successfully added the .Va data field will be zero. .It Dv 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. .It Dv 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. .It Dv EV_EOF Filters may set this flag to indicate filter-specific EOF condition. .It Dv EV_ERROR See .Sx RETURN VALUES below. .El .Pp The predefined system filters are listed below. Arguments may be passed to and from the filter via the .Va fflags and .Va data fields in the kevent structure. .Bl -tag -width "Dv EVFILT_PROCDESC" .It Dv 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. .Bl -tag -width 2n .It Sockets Sockets which have previously been passed to .Fn listen return when there is an incoming connection pending. .Va data contains the size of the listen backlog. .Pp Other socket descriptors return when there is data to be read, subject to the .Dv 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 .Dv NOTE_LOWAT flag in .Va fflags , and specifying the new low water mark in .Va data . On return, .Va data contains the number of bytes of protocol data available to read. .Pp If the read direction of the socket has shutdown, then the filter also sets .Dv EV_EOF in .Va flags , and returns the socket error (if any) in .Va fflags . It is possible for EOF to be returned (indicating the connection is gone) while there is still data pending in the socket buffer. .It Vnodes Returns when the file pointer is not at the end of file. .Va data contains the offset from current position to end of file, and may be negative. .Pp This behavior is different from .Xr poll 2 , where read events are triggered for regular files unconditionally. This event can be triggered unconditionally by setting the .Dv NOTE_FILE_POLL flag in .Va fflags . .It "Fifos, Pipes" Returns when the there is data to read; .Va data contains the number of bytes available. .Pp When the last writer disconnects, the filter will set .Dv EV_EOF in .Va flags . This may be cleared by passing in .Dv EV_CLEAR , at which point the filter will resume waiting for data to become available before returning. .It "BPF devices" Returns when the BPF buffer is full, the BPF timeout has expired, or when the BPF has .Dq immediate mode enabled and there is any data to read; .Va data contains the number of bytes available. .El .It Dv EVFILT_WRITE Takes a descriptor as the identifier, and returns whenever it is possible to write to the descriptor. For sockets, pipes and fifos, .Va 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 .Dv EV_CLEAR . Note that this filter is not supported for vnodes or BPF devices. .Pp For sockets, the low water mark and socket error handling is identical to the .Dv EVFILT_READ case. .It Dv EVFILT_AIO The sigevent portion of the AIO request is filled in, with .Va sigev_notify_kqueue containing the descriptor of the kqueue that the event should be attached to, .Va sigev_notify_kevent_flags containing the kevent flags which should be .Dv EV_ONESHOT , .Dv EV_CLEAR or .Dv EV_DISPATCH , .Va sigev_value containing the udata value, and .Va sigev_notify set to .Dv SIGEV_KEVENT . When the .Fn aio_* system call is made, the event will be registered with the specified kqueue, and the .Va ident argument set to the .Fa struct aiocb returned by the .Fn aio_* system call. The filter returns under the same conditions as .Fn aio_error . .It Dv EVFILT_VNODE Takes a file descriptor as the identifier and the events to watch for in .Va fflags , and returns when one or more of the requested events occurs on the descriptor. The events to monitor are: .Bl -tag -width "Dv NOTE_CLOSE_WRITE" .It Dv NOTE_ATTRIB The file referenced by the descriptor had its attributes changed. .It Dv NOTE_CLOSE A file descriptor referencing the monitored file, was closed. The closed file descriptor did not have write access. .It Dv NOTE_CLOSE_WRITE A file descriptor referencing the monitored file, was closed. The closed file descriptor has write access. .Pp This note, as well as .Dv NOTE_CLOSE , are not activated when files are closed forcibly by .Xr unmount 2 or .Xr revoke 2 . Instead, .Dv NOTE_REVOKE is sent for such events. .It Dv NOTE_DELETE The .Fn unlink system call was called on the file referenced by the descriptor. .It Dv NOTE_EXTEND For regular file, the file referenced by the descriptor was extended. .Pp For directory, reports that a directory entry was added or removed, as the result of rename operation. The .Dv NOTE_EXTEND event is not reported when a name is changed inside the directory. .It Dv NOTE_LINK The link count on the file changed. In particular, the .Dv NOTE_LINK event is reported if a subdirectory was created or deleted inside the directory referenced by the descriptor. .It Dv NOTE_OPEN The file referenced by the descriptor was opened. .It Dv NOTE_READ A read occurred on the file referenced by the descriptor. .It Dv NOTE_RENAME The file referenced by the descriptor was renamed. .It Dv NOTE_REVOKE Access to the file was revoked via .Xr revoke 2 or the underlying file system was unmounted. .It Dv NOTE_WRITE A write occurred on the file referenced by the descriptor. .El .Pp On return, .Va fflags contains the events which triggered the filter. .It Dv EVFILT_PROC Takes the process ID to monitor as the identifier and the events to watch for in .Va 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: .Bl -tag -width "Dv NOTE_TRACKERR" .It Dv NOTE_EXIT The process has exited. The exit status will be stored in .Va data . .It Dv NOTE_FORK The process has called .Fn fork . .It Dv NOTE_EXEC The process has executed a new process via .Xr execve 2 or a similar call. .It Dv NOTE_TRACK Follow a process across .Fn fork calls. The parent process registers a new kevent to monitor the child process using the same .Va fflags as the original event. The child process will signal an event with .Dv NOTE_CHILD set in .Va fflags and the parent PID in .Va data . .Pp If the parent process fails to register a new kevent .Pq usually due to resource limitations , it will signal an event with .Dv NOTE_TRACKERR set in .Va fflags , and the child process will not signal a .Dv NOTE_CHILD event. .El .Pp On return, .Va fflags contains the events which triggered the filter. .It Dv EVFILT_PROCDESC Takes the process descriptor created by .Xr pdfork 2 to monitor as the identifier and the events to watch for in .Va fflags , and returns when the associated process performs one or more of the requested events. The events to monitor are: .Bl -tag -width "Dv NOTE_EXIT" .It Dv NOTE_EXIT The process has exited. The exit status will be stored in .Va data . .El .Pp On return, .Va fflags contains the events which triggered the filter. .It Dv 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 .Fn signal and .Fn 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 .Dv SIG_IGN , except for the .Dv SIGCHLD signal, which, if ignored, won't be recorded by the filter. Event notification happens after normal signal delivery processing. .Va data returns the number of times the signal has occurred since the last call to .Fn kevent . This filter automatically sets the .Dv EV_CLEAR flag internally. .It Dv EVFILT_TIMER Establishes an arbitrary timer identified by .Va ident . When adding a timer, .Va data specifies the timeout period. The timer will be periodic unless .Dv EV_ONESHOT is specified. On return, .Va data contains the number of times the timeout has expired since the last call to .Fn 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 .Va kern.kq_calloutmax sysctl. .Bl -tag -width "Dv NOTE_USECONDS" .It Dv NOTE_SECONDS .Va data is in seconds. .It Dv NOTE_MSECONDS .Va data is in milliseconds. .It Dv NOTE_USECONDS .Va data is in microseconds. .It Dv NOTE_NSECONDS .Va data is in nanoseconds. .El .Pp If .Va fflags is not set, the default is milliseconds. On return, .Va fflags contains the events which triggered the filter. .It Dv EVFILT_USER Establishes a user event identified by .Va ident which is not associated with any kernel mechanism but is triggered by user level code. The lower 24 bits of the .Va fflags may be used for user defined flags and manipulated using the following: .Bl -tag -width "Dv NOTE_FFLAGSMASK" .It Dv NOTE_FFNOP Ignore the input .Va fflags . .It Dv NOTE_FFAND Bitwise AND .Va fflags . .It Dv NOTE_FFOR Bitwise OR .Va fflags . .It Dv NOTE_FFCOPY Copy .Va fflags . .It Dv NOTE_FFCTRLMASK Control mask for .Va fflags . .It Dv NOTE_FFLAGSMASK User defined flag mask for .Va fflags . .El .Pp A user event is triggered for output with the following: .Bl -tag -width "Dv NOTE_FFLAGSMASK" .It Dv NOTE_TRIGGER Cause the event to be triggered. .El .Pp On return, .Va fflags contains the users defined flags in the lower 24 bits. .El .Sh CANCELLATION BEHAVIOUR If .Fa nevents is non-zero, i.e. the function is potentially blocking, the call is a cancellation point. Otherwise, i.e. if .Fa 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. .Sh RETURN VALUES The .Fn 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. .Pp The .Fn kevent system call returns the number of events placed in the .Fa eventlist , up to the value given by .Fa nevents . If an error occurs while processing an element of the .Fa changelist and there is enough room in the .Fa eventlist , then the event will be placed in the .Fa eventlist with .Dv EV_ERROR set in .Va flags and the system error in .Va data . Otherwise, .Dv -1 will be returned, and .Dv errno will be set to indicate the error condition. If the time limit expires, then .Fn kevent returns 0. .Sh EXAMPLES .Bd -literal -compact #include #include #include #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\en", 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"); if (event.flags & EV_ERROR) errx(EXIT_FAILURE, "Event error: %s", strerror(event.data)); 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) { printf("Something was written in '%s'\en", argv[1]); } } } .Ed .Sh ERRORS The .Fn kqueue system call fails if: .Bl -tag -width Er .It Bq Er ENOMEM The kernel failed to allocate enough memory for the kernel queue. .It Bq Er ENOMEM The .Dv RLIMIT_KQUEUES rlimit (see .Xr getrlimit 2 ) for the current user would be exceeded. .It Bq Er EMFILE The per-process descriptor table is full. .It Bq Er ENFILE The system file table is full. .El .Pp The .Fn kevent system call fails if: .Bl -tag -width Er .It Bq Er EACCES The process does not have permission to register a filter. .It Bq Er EFAULT There was an error reading or writing the .Va kevent structure. .It Bq Er EBADF The specified descriptor is invalid. .It Bq Er EINTR A signal was delivered before the timeout expired and before any events were placed on the kqueue for return. .It Bq Er EINTR A cancellation request was delivered to the thread, but not yet handled. .It Bq Er EINVAL The specified time limit or filter is invalid. .It Bq Er ENOENT The event could not be found to be modified or deleted. .It Bq Er 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 .Va kern.kq_calloutmax sysctl. .It Bq Er ESRCH The specified process to attach to does not exist. .El .Pp When .Fn kevent call fails with .Er EINTR error, all changes in the .Fa changelist have been applied. .Sh SEE ALSO .Xr aio_error 2 , .Xr aio_read 2 , .Xr aio_return 2 , .Xr poll 2 , .Xr read 2 , .Xr select 2 , .Xr sigaction 2 , .Xr write 2 , .Xr pthread_setcancelstate 3 , .Xr signal 3 .Sh HISTORY The .Fn kqueue and .Fn kevent system calls first appeared in .Fx 4.1 . .Sh AUTHORS The .Fn kqueue system and this manual page were written by .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org . .Sh BUGS The .Fa timeout value is limited to 24 hours; longer timeouts will be silently reinterpreted as 24 hours.