Manpage of EPOLL_CTL


Section: Linux Programmer's Manual (2)
Updated: 2016-10-08


epoll_ctl - control interface for an epoll file descriptor  


#include <sys/epoll.h>

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


This system call performs control operations on the epoll(7) instance referred to by the file descriptor epfd. It requests that the operation opbe performed for the target file descriptor, fd.

Valid values for the opargument are:

Register the target file descriptor fdon the epollinstance referred to by the file descriptor epfdand associate the event eventwith the internal file linked to fd.
Change the event eventassociated with the target file descriptor fd.
Remove (deregister) the target file descriptor fdfrom the epollinstance referred to by epfd. The eventis ignored and can be NULL (but see BUGS below).

The eventargument describes the object linked to the file descriptor fd. The struct epoll_eventis 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 eventsmember is a bit mask composed using the following available event types:

The associated file is available for read(2) operations.
The associated file is available for write(2) operations.
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.)
There is urgent data available for read(2) operations.
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.
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.
Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for epollis 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 epollinterface. The user must call epoll_ctl() with EPOLL_CTL_MODto rearm the file descriptor with a new event mask.
EPOLLWAKEUP (since Linux 3.5)
If EPOLLONESHOTand EPOLLETare clear and the process has the CAP_BLOCK_SUSPENDcapability, ensure that the system does not enter "suspend" or "hibernate" 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 EPOLLWAKEUPfor 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 EPOLLEXCLUSIVEis not set) is for all epoll file descriptors to receive an event. EPOLLEXCLUSIVEis thus useful for avoiding thundering herd problems in certain scenarios.

If the same file descriptor is in multiple epoll instances, some with the EPOLLEXCLUSIVEflag, 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. EPOLLHUPand EPOLLERRcan 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 eventsyield an error. EPOLLEXCLUSIVEmay be used only in an EPOLL_CTL_ADDoperation; attempts to employ it with EPOLL_CTL_MODyield an error. If EPOLLEXCLUSIVEhas been set using epoll_ctl(), then a subsequent EPOLL_CTL_MODon the same epfdfdpair yields an error. A call to epoll_ctl() that specifies EPOLLEXCLUSIVEin eventsand specifies the target file descriptor fdas an epoll instance will likewise fail. The error in all of these cases is EINVAL.



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


epfdor fdis not a valid file descriptor.
opwas EPOLL_CTL_ADD, and the supplied file descriptor fdis already registered with this epoll instance.
epfdis not an epollfile descriptor, or fdis the same as epfd, or the requested operation opis not supported by this interface.
An invalid event type was specified along with EPOLLEXCLUSIVEin events.
opwas EPOLL_CTL_MODand eventsincluded EPOLLEXCLUSIVE.
opwas EPOLL_CTL_MODand the EPOLLEXCLUSIVEflag has previously been applied to this epfdfdpair.
EPOLLEXCLUSIVEwas specified in eventand fdrefers to an epoll instance.
fdrefers to an epoll instance and this EPOLL_CTL_ADDoperation would result in a circular loop of epoll instances monitoring one another.
opwas EPOLL_CTL_MODor EPOLL_CTL_DEL, and fdis not registered with this epoll instance.
There was insufficient memory to handle the requested opcontrol operation.
The limit imposed by /proc/sys/fs/epoll/max_user_watcheswas encountered while trying to register (EPOLL_CTL_ADD) a new file descriptor on an epoll instance. See epoll(7) for further details.
The target file fddoes not support epoll. This error can occur if fdrefers to, for example, a regular file or a directory.


epoll_ctl() was added to the kernel in version 2.6.  


epoll_ctl() is Linux-specific. Library support is provided in glibc starting with version 2.3.2.  


The epollinterface supports all file descriptors that support poll(2).  


In kernel versions before 2.6.9, the EPOLL_CTL_DELoperation required a non-null pointer in event, even though this argument is ignored. Since Linux 2.6.9, eventcan be specified as NULL when using EPOLL_CTL_DEL. Applications that need to be portable to kernels before 2.6.9 should specify a non-null pointer in event.

If EPOLLWAKEUPis specified in flags, but the caller does not have the CAP_BLOCK_SUSPENDcapability, then the EPOLLWAKEUPflag is silently ignored. This unfortunate behavior is necessary because no validity checks were performed on the flagsargument in the original implementation, and the addition of the EPOLLWAKEUPwith a check that caused the call to fail if the caller did not have the CAP_BLOCK_SUSPENDcapability caused a breakage in at least one existing user-space application that happened to randomly (and uselessly) specify this bit. A robust application should therefore double check that it has the CAP_BLOCK_SUSPENDcapability if attempting to use the EPOLLWAKEUPflag.  


epoll_create(2), epoll_wait(2), poll(2), epoll(7)




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