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.