Hi,
I am on CentOS 6.5 with kernel version kernel-2.6.32-431.el6.
I was trying to find the implementation of recvmmsg(), and hopefully, I am not
looking at the wrong source
~/rpmbuild/BUILD/kernel-2.6.32-431.el6/linux-2.6.32-431.el6.x86_64/net/socket.c.
If I am, please kindly point me to the right source.
int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
unsigned int flags, struct timespec *timeout)
{
int fput_needed, err, datagrams;
struct socket *sock;
struct mmsghdr __user *entry;
struct compat_mmsghdr __user *compat_entry;
struct msghdr msg_sys;
struct timespec end_time;
if (timeout
poll_select_set_timeout(end_time, timeout-tv_sec,
timeout-tv_nsec))
return -EINVAL;
datagrams = 0;
sock = sockfd_lookup_light(fd, err, fput_needed);
if (!sock)
return err;
err = sock_error(sock-sk);
if (err)
goto out_put;
entry = mmsg;
compat_entry = (struct compat_mmsghdr __user *)mmsg;
while (datagrams vlen) {
/*
* No need to ask LSM for more than the first datagram.
*/
if (MSG_CMSG_COMPAT flags) {
err = __sys_recvmsg(sock, (struct msghdr __user
*)compat_entry,
msg_sys, flags, datagrams);
if (err 0)
break;
err = __put_user(err, compat_entry-msg_len);
++compat_entry;
} else {
err = __sys_recvmsg(sock, (struct msghdr __user *)entry,
msg_sys, flags, datagrams);
if (err 0)
break;
err = put_user(err, entry-msg_len);
++entry;
}
if (err)
break;
++datagrams;
if (timeout) {
ktime_get_ts(timeout);
*timeout = timespec_sub(end_time, *timeout);
if (timeout-tv_sec 0) {
timeout-tv_sec = timeout-tv_nsec = 0;
break;
}
/* Timeout, return less than vlen datagrams */
if (timeout-tv_nsec == 0 timeout-tv_sec == 0)
break;
}
/* Out of band data, return right away */
if (msg_sys.msg_flags MSG_OOB)
break;
}
out_put:
fput_light(sock-file, fput_needed);
if (err == 0)
return datagrams;
if (datagrams != 0) {
/*
* We may return less entries than requested (vlen) if the
* sock is non block and there aren't enough datagrams...
*/
if (err != -EAGAIN) {
/*
* ... or if recvmsg returns an error after we
* received some datagrams, where we record the
* error to return on the next call or if the
* app asks about it using getsockopt(SO_ERROR).
*/
sock-sk-sk_err = -err;
}
return datagrams;
}
return err;
}
Let's say there are 2 packets at the socket, and I try:
timespec t = {0, 0};
recvmmsg(fd, mmsg, vlen, 0, t);
Then, because of these lines:
if (timeout-tv_nsec == 0 timeout-tv_sec == 0)
break;
recvmmsg() will just return 1 packet instead of 2. Am I right?
It seems defecting the one of the main purposes of recvmmsg() -- trying to
reduce # of system calls when there are packet queued.
Maybe I miss something?
I created a small test case that it waits for epoll callback, sleeps for 2-5
seconds and then calls recvmmsg() with timeout=0 (not null; just like above).
Then, I found that recvmmsg() doesn't return all the packets that queued in 1
single call.
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