Epoll

codice_1 is a Linux kernel system call for a scalable I/O event notification mechanism, first introduced in version 2.5.44 of the Linux kernel. Its function is to monitor multiple file descriptors to see whether I/O is possible on any of them. It is meant to replace the older POSIX codice_2 and codice_3 system calls, to achieve better performance in more demanding applications, where the number of watched file descriptors is large (unlike the older system calls, which operate in "O"("n") time, codice_1 operates in "O"(1) time).
codice_1 is similar to FreeBSD's codice_6, in that it consists of a set of user-space functions, each taking a file descriptor argument denoting the configurable kernel object, against which they cooperatively operate. codice_1 uses a red-black tree (RB-tree) data structure to keep track of all file descriptors that are currently being monitored.
API.
int epoll_create1(int flags);
Creates an codice_1 object and returns its file descriptor. The codice_9 parameter allows epoll behavior to be modified. It has only one valid value, codice_10. codice_11 is an older variant of codice_12 and is deprecated as of Linux kernel version 2.6.27 and glibc version 2.9.
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
Controls (configures) which file descriptors are watched by this object, and for which events. codice_13 can be ADD, MODIFY or DELETE.
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);
Waits for any of the events registered for with codice_14, until at least one occurs or the timeout elapses. Returns the occurred events in codice_15, up to codice_16 at once.
Triggering modes.
codice_1 provides both edge-triggered and level-triggered modes. In edge-triggered mode, a call to codice_18 will return only when a new event is enqueued with the codice_1 object, while in level-triggered mode, codice_18 will return as long as the condition holds.
For instance, if a pipe registered with codice_1 has received data, a call to codice_18 will return, signaling the presence of data to be read. Suppose, the reader only consumed part of data from the buffer. In level-triggered mode, further calls to codice_18 will return immediately, as long as the pipe's buffer contains data to be read. In edge-triggered mode, however, codice_18 will return only once new data is written to the pipe.
Criticism.
Bryan Cantrill pointed out that codice_1 had mistakes that could have been avoided, had it learned from its predecessors: input/output completion ports, event ports (Solaris) and kqueue. However, a large part of his criticism was addressed by codice_1's codice_27 and codice_28 options. codice_27 was added in version 2.6.2 of the Linux kernel mainline, released in February 2004. codice_28 was added in version 4.5, released in March 2016.