posts - 297, comments - 15, trackbacks - 0

1.NAME
epoll - I/O event notification facility

SYNOPSIS
#include <sys/epoll.h>

DESCRIPTION
       epoll is a variant of poll(2) that can be used either as Edge or Level Triggered interface and scales well to
       large numbers of watched fds. Three system calls are provided to set up and control an epoll set: epoll_create(2),
       epoll_ctl(2), epoll_wait(2).

An epoll set is connected to a file descriptor created by epoll_create(2). Interest for certain file descriptors
is then registered via epoll_ctl(2). Finally, the actual wait is started by epoll_wait(2).

NOTES
       The epoll event distribution interface is able to behave both as Edge Triggered ( ET ) and Level Triggered ( LT ).
       The difference between ET and LT event distribution mechanism can be described as follows. Suppose that this sce-
       nario happens :

1 The file descriptor that represents the read side of a pipe ( RFD ) is added inside the epoll device.

2 Pipe writer writes 2Kb of data on the write side of the pipe.

3 A call to epoll_wait(2) is done that will return RFD as ready file descriptor.

4 The pipe reader reads 1Kb of data from RFD.

5 A call to epoll_wait(2) is done.

       If the RFD file descriptor has been added to the epoll interface using the EPOLLET flag, the call to epoll_wait(2)
       done in step 5 will probably hang because of the available data still present in the file input buffers and the
       remote peer might be expecting a response based on the data it already sent. The reason for this is that Edge
       Triggered event distribution delivers events only when events happens on the monitored file. So, in step 5 the
       caller might end up waiting for some data that is already present inside the input buffer. In the above example,
       an event on RFD will be generated because of the write done in 2 and the event is consumed in 3. Since the read
       operation done in 4 does not consume the whole buffer data, the call to epoll_wait(2) done in step 5 might lock
       indefinitely. The epoll interface, when used with the EPOLLET flag ( Edge Triggered ) should use non-blocking file
       descriptors to avoid having a blocking read or write starve the task that is handling multiple file descriptors.
       The suggested way to use epoll as an Edge Triggered (EPOLLET) interface is below, and possible pitfalls to avoid
       follow.
              i      with non-blocking file descriptors

ii by going to wait for an event only after read(2) or write(2) return EAGAIN

       On the contrary, when used as a Level Triggered interface, epoll is by all means a faster poll(2), and can be used
       wherever the latter is used since it shares the same semantics. Since even with the Edge Triggered epoll multiple
       events can be generated up on receival of multiple chunks of data, the caller has the option to specify the EPOL-
       LONESHOT flag, to tell epoll to disable the associated file descriptor after the receival of an event with
       epoll_wait(2). When the EPOLLONESHOT flag is specified, it is caller responsibility to rearm the file descriptor
       using epoll_ctl(2) with EPOLL_CTL_MOD.

EXAMPLE FOR SUGGESTED USAGE
       While the usage of epoll when employed like a Level Triggered interface does have the same semantics of poll(2),
       an Edge Triggered usage requires more clarification to avoid stalls in the application event loop. In this exam-
       ple, listener is a non-blocking socket on which listen(2) has been called. The function do_use_fd() uses the new
       ready file descriptor until EAGAIN is returned by either read(2) or write(2).   An event driven state machine
       application should, after having received EAGAIN, record its current state so that at the next call to do_use_fd()
       it will continue to read(2) or write(2) from where it stopped before.

struct epoll_event ev, *events;

for(;;) {
nfds = epoll_wait(kdpfd, events, maxevents, -1);

           for(n = 0; n < nfds; ++n) {
               if(events[n].data.fd == listener) {
                   client = accept(listener, (struct sockaddr *) &local,
                                   &addrlen);
                   if(client < 0){
                       perror("accept");
                       continue;
                   }
                   setnonblocking(client);
                   ev.events = EPOLLIN | EPOLLET;
                   ev.data.fd = client;
                   if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev) < 0) {
                       fprintf(stderr, "epoll set insertion error: fd=%d\n",
                               client);
                       return -1;
                   }
               }
               else
               else
                   do_use_fd(events[n].data.fd);
           }
       }

       When used as an Edge triggered interface, for performance reasons, it is possible to add the file descriptor
       inside the epoll interface ( EPOLL_CTL_ADD ) once by specifying ( EPOLLIN|EPOLLOUT ). This allows you to avoid
       continuously switching between EPOLLIN and EPOLLOUT calling epoll_ctl(2) with EPOLL_CTL_MOD.

QUESTIONS AND ANSWERS (from linux-kernel)
Q1 What happens if you add the same fd to an epoll_set twice?

A1 You will probably get EEXIST. However, it is possible that two threads may add the same fd twice. This is a
harmless condition.

Q2 Can two epoll sets wait for the same fd? If so, are events reported to both epoll sets fds?

A2 Yes. However, it is not recommended. Yes it would be reported to both.

Q3 Is the epoll fd itself poll/epoll/selectable?

A3 Yes.

Q4 What happens if the epoll fd is put into its own fd set?

A4 It will fail. However, you can add an epoll fd inside another epoll fd set.

Q5 Can I send the epoll fd over a unix-socket to another process?

A5 No.

Q6 Will the close of an fd cause it to be removed from all epoll sets automatically?

A6 Yes.

Q7 If more than one event comes in between epoll_wait(2) calls, are they combined or reported separately?

A7 They will be combined.

Q8 Does an operation on an fd affect the already collected but not yet reported events?

A8 You can do two operations on an existing fd. Remove would be meaningless for this case. Modify will re-read
available I/O.

       Q9     Do I need to continuously read/write an fd until EAGAIN when using the EPOLLET flag ( Edge Triggered
              behaviour ) ?
       Q9     Do I need to continuously read/write an fd until EAGAIN when using the EPOLLET flag ( Edge Triggered
              behaviour ) ?

       A9     No you don't. Receiving an event from epoll_wait(2) should suggest to you that such file descriptor is
              ready for the requested I/O operation. You have simply to consider it ready until you will receive the next
              EAGAIN. When and how you will use such file descriptor is entirely up to you. Also, the condition that the
              read/write I/O space is exhausted can be detected by checking the amount of data read/write from/to the
              target file descriptor. For example, if you call read(2) by asking to read a certain amount of data and
              read(2) returns a lower number of bytes, you can be sure to have exhausted the read I/O space for such file
              descriptor. Same is valid when writing using the write(2) function.

POSSIBLE PITFALLS AND WAYS TO AVOID THEM
o Starvation ( Edge Triggered )

If there is a large amount of I/O space, it is possible that by trying to drain it the other files will not get
processed causing starvation. This is not specific to epoll.

       The solution is to maintain a ready list and mark the file descriptor as ready in its associated data structure,
       thereby allowing the application to remember which files need to be processed but still round robin amongst all
       the ready files. This also supports ignoring subsequent events you receive for fd's that are already ready.

o If using an event cache...

       If you use an event cache or store all the fd's returned from epoll_wait(2), then make sure to provide a way to
       mark its closure dynamically (ie- caused by a previous event's processing). Suppose you receive 100 events from
       epoll_wait(2), and in event #47 a condition causes event #13 to be closed.   If you remove the structure and
       close() the fd for event #13, then your event cache might still say there are events waiting for that fd causing
       confusion.

       One solution for this is to call, during the processing of event 47, epoll_ctl(EPOLL_CTL_DEL) to delete fd 13 and
       close(), then mark its associated data structure as removed and link it to a cleanup list. If you find another
       event for fd 13 in your batch processing, you will discover the fd had been previously removed and there will be
       no confusion.

CONFORMING TO
       epoll(7) is a new API introduced in Linux kernel 2.5.44. Its interface should be finalized in Linux kernel
       2.5.66.

2.NAME
       epoll_wait - wait for an I/O event on an epoll file descriptor

SYNOPSIS
#include <sys/epoll.h>

int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout)

DESCRIPTION
       Wait for events on the epoll file descriptor epfd for a maximum time of timeout milliseconds. The memory area
       pointed to by events will contain the events that will be available for the caller. Up to maxevents are returned
       by epoll_wait(2).   The maxevents parameter must be greater than zero. Specifying a timeout of -1 makes
       epoll_wait(2) wait indefinitely, while specifying a timeout equal to zero makes epoll_wait(2) to return immedi-
       ately even if no events are available (return code equal to zero). 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 data of each returned structure will contain the same data the user set with a epoll_ctl(2)
(EPOLL_CTL_ADD,EPOLL_CTL_MOD) while the events member will contain the returned event bit field.

RETURN VALUE
       When successful, epoll_wait(2) returns the number of file descriptors ready for the requested I/O, or zero if no
       file descriptor became ready during the requested timeout milliseconds. When an error occurs, epoll_wait(2)
       returns -1 and errno is set appropriately.

ERRORS
EBADF epfd is not a valid file descriptor.
EFAULT The memory area pointed to by events is not accessible with write permissions.

EINTR The call was interrupted by a signal handler before any of the requested events occurred or the timeout
expired.

EINVAL epfd is not an epoll file descriptor, or maxevents is less than or equal to zero.

3.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 :

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

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

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 distribu-
              tion 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).

RETURN VALUE
When successful, epoll_ctl(2) returns zero. When an error occurs, epoll_ctl(2) returns -1 and errno is set appro-
priately.

ERRORS
EBADF epfd 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 sup-
ported 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.

4.NAME
epoll_wait - wait for an I/O event on an epoll file descriptor