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epoll_ctl(2) — Linux manual page

epoll_ctl - control interface for an epoll file descriptor

SYNOPSIS

#include <sys/epoll.h>

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

DESCRIPTION

This system call is used to add, modify, or remove entries in the interest list of the epoll(7) instance referred to by the file descriptor epfd. It requests that the operation op be performed for the target file descriptor, fd.

op argument

Valid values for the op argument are:

EPOLL_CTL_ADD

Add fd to the interest list and associate the settings specified in event with the internal file linked to fd.

EPOLL_CTL_MOD

Change the settings associated with fd in the interest list to the new settings specified in event.

EPOLL_CTL_DEL

Remove (deregister) the target file descriptor fd from the interest list. The event argument is ignored and can be NULL (but see BUGS below).

The event argument describes the object linked to the file descriptor fd. The struct epoll_event is defined as: 

typedef union epoll_data {
    void        *ptr;
    int          fd;
    uint32_t     u32;
    uint64_t     u64;
} epoll_data_t;

struct epoll_event {
    uint32_t     events;      /* Epoll events */
    epoll_data_t data;        /* User data variable */
};

events member

The events member is a bit mask composed by ORing together zero or more of the following available event types:

EPOLLIN

The associated file is available for read(2) operations.

EPOLLOUT

The associated file is available for write(2) operations.

NOTE:

一、EPOLLIN 即 可读，EPOLLOUT即可写

那到底何时 EPOLLIN、何时 EPOLLOUT？在下面文章中进行了介绍:

1、socket(7) Linux Programmer's Manual # SO_RCVLOWAT and SO_SNDLOWAT

2、I/O多路复用技术（multiplexing）是什么？ # A :

以select和tcp socket为例，

所谓可读事件，具体的说是指以下事件：

1 socket内核接收缓冲区中的可用字节数大于或等于其低水位SO_RCVLOWAT;

所谓可写事件，则是指：

1 socket的内核发送缓冲区的可用字节数大于或等于其低水位SO_SNDLOWAIT；

EPOLLRDHUP (since Linux 2.6.17)

Stream socket peer closed connection, or shut down writing half of connection. (This flag is especially useful for writing simple code to detect peer shutdown when using Edge Triggered monitoring.)

NOTE:

参见 csdn EPOLLRDHUP vs EPOLLHUP

EPOLLPRI

There is an exceptional condition on the file descriptor. See the discussion of POLLPRI in poll(2).

EPOLLERR

Error condition happened on the associated file descriptor. This event is also reported for the write end of a pipe when the read end has been closed. epoll_wait(2) will always report for this event; it is not necessary to set it in events.

EPOLLHUP

Hang up happened on the associated file descriptor. epoll_wait(2) will always wait for this event; it is not necessary to set it in events.

Note that when reading from a channel such as a pipe or a stream socket, this event merely indicates that the peer closed its end of the channel. Subsequent reads from the channel will return 0 (end of file) only after all outstanding data in the channel has been consumed.

NOTE:

参见TLPI中关于此的解释；

EPOLLET

Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for epoll is Level Triggered. See epoll(7) for more detailed information about Edge and Level Triggered event distribution architectures.

EPOLLONESHOT (since Linux 2.6.2)

Sets the one-shot behavior for the associated file descriptor. This means that after an event is pulled out with epoll_wait(2) the associated file descriptor is internally disabled and no other events will be reported by the epoll interface. The user must call epoll_ctl() with EPOLL_CTL_MOD to rearm the file descriptor with a new event mask.

EPOLLWAKEUP (since Linux 3.5)

If EPOLLONESHOT and EPOLLET are clear and the process has the CAP_BLOCK_SUSPEND capability, ensure that the system does not enter "suspend" or "hibernate"(冬眠、蛰伏) while this event is pending or being processed. The event is considered as being "processed" from the time when it is returned by a call to epoll_wait(2) until the next call to epoll_wait(2) on the same epoll(7) file descriptor, the closure of that file descriptor, the removal of the event file descriptor with EPOLL_CTL_DEL, or the clearing of EPOLLWAKEUP for the event file descriptor with EPOLL_CTL_MOD. See also BUGS.

EPOLLEXCLUSIVE (since Linux 4.5)

Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor, fd. When a wakeup event occurs and multiple epoll file descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or more of the epoll file descriptors will receive an event with epoll_wait(2). The default in this scenario (when EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to receive an event. EPOLLEXCLUSIVE is thus useful for avoiding thundering herd problems in certain scenarios.

If the same file descriptor is in multiple epoll instances, some with the EPOLLEXCLUSIVE flag, and others without, then events will be provided to all epoll instances that did not specify EPOLLEXCLUSIVE, and at least one of the epoll instances that did specify EPOLLEXCLUSIVE.

The following values may be specified in conjunction with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and EPOLLET. EPOLLHUP and EPOLLERR can also be specified, but this is not required: as usual, these events are always reported if they occur, regardless of whether they are specified in events. Attempts to specify other values in events yield the error EINVAL.

EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD operation; attempts to employ it with EPOLL_CTL_MOD yield an error. If EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent EPOLL_CTL_MOD on the same epfd, fd pair yields an error. A call to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and specifies the target file descriptor fd as an epoll instance will likewise fail. The error in all of these cases is EINVAL.