I agree Pete -- its messy. Just by the names of errnos, it seems
appropriate to return what's been completed if we get EWOULDBLOCK,
while EAGAIN suggests we can just call recv again and get what we
want. But as you point out they're the same value. According to the
opengroup, impls _may_ assign the same value to both:
http://www.opengroup.org/pubs/online/7908799/xsh/errors.html
Strictly from a linux implementation perspective, epoll/poll tell us
that the bytes are on the socket, so even when EAGAIN is returned, we
can call recv again and get what we wanted. I've tested this a bunch,
and when EAGAIN is returned (which is infrequent), the next call
invariably returns successfully. There were two instances where the
code looped up-to around 200 times on EAGAIN under heavy load. But
looping does turn nbrecv into more of a brecv, although we avoid all
the fcntl calls to turn the socket into a blocking one just for the
recv call.
With the socket in non-blocking mode, the conditional:
if (ret == -1 && errno == EWOULDBLOCK)
{
return (len - comp); /* return amount completed */
}
Just doesn't work. It causes the caller to error and close the
socket. Not what we want.
I think we can get away with doing:
if (!ret) /* socket closed */
{
errno = EPIPE;
return (-1);
}
if (ret == -1 && (errno == EINTR || errno == EAGAIN || errno
== EWOULDBLOCK))
{
goto nbrecv_restart;
}
else if (ret == -1)
{
return (-1);
}
From a practical perspective, this seems to work, and an
implementation that has poll telling us that bytes are ready, but recv
returning EWOULDBLOCK because of anything other than small timing
issues in the kernel seems broken anyway.
The alternative is to return the bytes received with the errno, and on
EAGAIN, we would have to add the operation back onto the op queue with
a state variable of how much was received. The code is designed to
avoid doing this in the first place by polling until the bytes we need
are ready, so doing this would probably be messy.
-sam
On Dec 10, 2007, at 11:47 PM, Pete Wyckoff wrote:
[EMAIL PROTECTED] wrote on Mon, 10 Dec 2007 21:19 -0600:
while a loop will fix it, it would be really nice to understand how
we get
EAGAIN when we think that there are bytes there...
[..]
On Dec 7, 2007, at 4:55 PM, Sam Lang wrote:
I'm seeing recv on a socket in non-blocking mode returning EAGAIN
occasionally, even though epoll has just told us there's bytes
waiting. I
guess that's why the call was initially a blocking recv. I can
add a loop
around the non-blocking recv while it returns EAGAIN, unless
someone can
think of a better work around.
The function is getting a bit messy. I'm all for looping on E* and
thought Sam's original mail made sense. But on second glance:
int BMI_sockio_nbrecv(int s,
void *buf,
int len)
{
int ret, comp = len;
assert(fcntl(s, F_GETFL, 0) & O_NONBLOCK);
while (comp)
{
nbrecv_restart:
ret = recv(s, buf, comp, DEFAULT_MSG_FLAGS);
if (!ret) /* socket closed */
{
errno = EPIPE;
return (-1);
}
if (ret == -1 && errno == EWOULDBLOCK)
{
return (len - comp); /* return amount completed */
}
if (ret == -1 && (errno == EINTR || errno == EAGAIN))
{
goto nbrecv_restart;
}
else if (ret == -1)
{
return (-1);
}
comp -= ret;
buf = (char *)buf + ret;
}
return (len - comp);
}
Note that we get from standard headers:
/usr/include/asm-generic/errno.h:#define EWOULDBLOCK
EAGAIN /* Operation would block */
But maybe there are some systems where this is not true? Not ones
that use glibc, apparently.
Anyway, the first use of EWOULDBLOCK runs us back to the poll
loop, which is the right thing to do. The second use of EAGAIN
would lead to a busy loop on recv()->EAGAIN that isn't quite so
nice. But that code never gets hit.
I'm not sure that a poll readable result necessarily means we'll get
any bytes on the socket. There are numerous ways in which things
can get messy.
-- Pete
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