Kqueue is a scalable event notification interface introduced in FreeBSD

FreeBSD

FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

 4.1, also supported in NetBSD

NetBSD

NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...

, OpenBSD

OpenBSD

OpenBSD is a Unix-like computer operating system descended from Berkeley Software Distribution , a Unix derivative developed at the University of California, Berkeley. It was forked from NetBSD by project leader Theo de Raadt in late 1995...

, DragonflyBSD, and Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

. It's the foundation of Apple's Grand Central Dispatch.

Kqueue provides efficient input and output event pipelines between the kernel

Kernel (computing)

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...

 and userland. Thus, it is possible to modify event filters as well as receive pending events while using only a single system call

System call

In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...

 to kevent(2) per main event loop

Event loop

In computer science, the event loop, message dispatcher, message loop, message pump, or run loop is a programming construct that waits for and dispatches events or messages in a program...

 iteration. This contrasts with older traditional polling

Polling (computer science)

Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output , and is also referred to as polled or software driven .Polling is sometimes used...

 system calls such as poll(2) and select(2)

Select (Unix)

select is a system call and application programming interface in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels...

 which are less efficient, especially when polling for events on a large number of file descriptors.

Kqueue does not only handle file descriptor

File descriptor

In computer programming, a file descriptor is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems...

 events but is also used for various other notifications such as file modification monitoring

File alteration monitor

In computing, the File Alteration Monitor, also known as FAM and sgi_fam, provides a subsystem developed by Silicon Graphics for Unix-like operating systems. The FAM subsystem allows applications to watch certain files and be notified when they are modified...

, signals

Signal (computing)

A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...

, asynchronous I/O

Asynchronous I/O

Asynchronous I/O, or non-blocking I/O, is a form of input/output processing that permits other processing to continue before the transmission has finished....

 events (AIO), child process

Child process

A child process in computing is a process created by another process .A child process inherits most of its attributes, such as open files, from its parent. In UNIX, a child process is in fact created as a copy of the parent...

 state change monitoring and timers which support nanosecond

Nanosecond

A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....

 resolution.

Some other operating systems

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 which traditionally only supported select(2)

Select (Unix)

select is a system call and application programming interface in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels...

 and poll(2) also currently provide more efficient polling alternatives, such as epoll

Epoll

epoll is a scalable I/O event notification mechanism for Linux, first introduced in Linux 2.5.44 . It is meant to replace the older POSIX select and poll system calls, to achieve better performance in more demanding applications, where the number of watched file descriptors is large...

 on Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 and I/O Completion Ports on Illumos

Illumos

Illumos is a derivative of OS/Net , which basically is a Solaris/OpenSolaris kernel with the bulk of the drivers, core libraries, and basic utilities. It is dependent on OS/Net, which Illumos will follow very closely while allowing to retain changes to code which might be unacceptable to upstream...

 and Solaris.

See also

Kqueue equivalent for other platforms:

• on Linux: epoll
  Epoll
  epoll is a scalable I/O event notification mechanism for Linux, first introduced in Linux 2.5.44 . It is meant to replace the older POSIX select and poll system calls, to achieve better performance in more demanding applications, where the number of watched file descriptors is large...
  
  
• on Solaris: I/O Completion Ports