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# epoll_ctl() - Unix, Linux System Call

# NAME

epoll_ctl - control interface for an epoll descriptor

# SYNOPSIS

    #include <sys/epoll.h>
    int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)

# DESCRIPTION

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 Code | Description
--- | ---
EPOLLIN | The 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 :

Code | Description
--- | ---
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).

# RETURN VALUE

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

# ERRORS

Error Code | Description
--- | ---
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.

# CONFORMING TO

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.

# BUGS

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.

# SEE ALSO

- epoll_create (2)

- poll (2)

- epoll_wait (2)

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