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epoll_ctl() - Unix, Linux System Call

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epoll_ctl - control interface for an epoll descriptor


#include <sys/epoll.h> 

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


Control an epoll descriptor, epfd, by requesting that the operation op be performed on the target file descriptor, fd. The event 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 */ };

The events member is a bit set composed using the following available event types :

Error CodeDescription
EPOLLINThe associated file is available forread(2) operations.
EPOLLOUT The associated file is available for write(2) operations.
EPOLLRDHUP 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.)
EPOLLPRI There is urgent data available for read(2) operations.
EPOLLERR Error condition happened on the associated file descriptor. epoll_wait(2) will always wait 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.
EPOLLET Sets the Edge Triggered behaviour for the associated file descriptor. The default behaviour for epoll is Level Triggered. See epoll(7) for more detailed information about Edge and Level Triggered event distribution architectures.
EPOLLONESHOT (since kernel 2.6.2) Sets the one-shot behaviour 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(2) with EPOLL_CTL_MOD to re-enable the file descriptor with a new event mask.
The epoll interface supports all file descriptors that support poll(2). Valid values for the op parameter are :
EPOLL_CTL_ADD Add the target file descriptor fd to the epoll descriptor epfd and associate the event event with the internal file linked to fd.
EPOLL_CTL_MOD Change the event event associated with the target file descriptor fd.
EPOLL_CTL_DEL Remove the target file descriptor fd from the epoll file descriptor, epfd. The event is ignored and can be NULL (but see BUGS below).


When successful, epoll_ctl(2) returns zero. When an error occurs, epoll_ctl(2) returns -1 and errno is set appropriately.


Error CodeDescription
EBADF epfd or fd is not a valid file descriptor.
EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is already in epfd.
EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd, or the requested operation op is not supported by this interface.
ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not in epfd.
ENOMEM There was insufficient memory to handle the requested op control operation.
EPERM The target file fd does not support epoll.


epoll_ctl(2) is a new API introduced in Linux kernel 2.5.44. The interface should be finalized by Linux kernel 2.5.66.


In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a non-NULL pointer in event, even though this argument is ignored. Since kernel 2.6.9, event can be specified as NULL when using EPOLL_CTL_DEL.


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