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man epoll
其中关于EAGAIN 的部分我难以理解，谁能指点一下？
[code]
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 scenario 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 indefi-
nitely. The epoll interface, when used with the EPOLLET flag ( Edge Triggered ) should use non-block-
ing file descriptors to avoid having a blocking read or write starve the task that is handling multi-
ple 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 EPOLLONESHOT 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 speci-
fied, 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 clarifiction to avoid stalls in the application
event loop. In this example, 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=%d0,
client);
return -1;
}
}
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 ) ?
A9 No you donâ |
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楼主: wyezl
发表于 2006-08-25 09:39
