Linux Man Pages

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 */
 };
 
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:
 
EPOLLIN 
The associated file is available for
 read (2) operations.
 
EPOLLOUT 
The associated file is available for
 write (2) operations.
 
EPOLLRDHUP  (since kernel 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.)
 
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 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 kernel 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 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).
 

RETURN VALUE

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

ERRORS

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 () is Linux-specific, and was introduced in kernel 2.5.44.

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) epoll_wait (2) poll (2) epoll (7)