Create send_vecio() function

This is a generic way of sending multiple packets with one system call.

It takes a pointer to vecio_buf (mmsghdr on Linux, TRANSMIT_PACKETS_ELEMENT
on Windows) and vecio_buf length and calls either sendmmsg(2) or TransmitPackets()

src/socket.cc

@@ -277,6 +277,23 @@ rtp_error_t kvz_rtp::socket::sendto(
     return __sendtov(addr, buffers, flags, bytes_sent);
 }
 
+rtp_error_t kvz_rtp::socket::send_vecio(vecio_buf *buffers, size_t nbuffers, int flags)
+{
+    if (buffers == nullptr || nbuffers == 0)
+        return RTP_INVALID_VALUE;
+
+#ifdef __linux__
+    if (sendmmsg(socket_, buffers, nbuffers, flags) < 0) {
+        LOG_ERROR("Failed to flush the message queue: %s", strerror(errno));
+        return RTP_SEND_ERROR;
+    }
+
+    return RTP_OK;
+#else
+    /* TODO:  */
+#endif
+}
+
 rtp_error_t kvz_rtp::socket::__recvfrom(uint8_t *buf, size_t buf_len, int flags, sockaddr_in *sender, int *bytes_read)
 {
     socklen_t *len_ptr = nullptr;

src/socket.hh

@@ -19,8 +19,10 @@ namespace kvz_rtp {
 #ifdef _WIN32
     typedef SOCKET socket_t;
     typedef unsigned socklen_t;
+    typedef TRANSMIT_PACKETS_ELEMENT vecio_buf;
 #else
-    typedef int    socket_t;
+    typedef int socket_t;
+    typedef mmsghdr vecio_buf;
 #endif
 
     const int MAX_BUFFER_COUNT = 256;
@@ -71,6 +73,14 @@ namespace kvz_rtp {
             rtp_error_t sendto(sockaddr_in& addr, std::vector<std::pair<size_t, uint8_t *>> buffers, int flags);
             rtp_error_t sendto(sockaddr_in& addr, std::vector<std::pair<size_t, uint8_t *>> buffers, int flags, int *bytes_sent);
 
+            /* Special sendto() functions, used to send multiple UDP packets with one system call
+             * Internally it uses sendmmsg(2) (Linux) or TransmitPackets (Windows)
+             *
+             * Return RTP_OK on success
+             * Return RTP_INVALID_VALUE if one of the parameters is invalid
+             * Return RTP_SEND_ERROR if sendmmsg/TransmitPackets failed */
+            rtp_error_t send_vecio(vecio_buf *buffers, size_t nbuffers, int flags);
+
             /* Same as recvfrom(2), receives a message from remote
              *
              * Write the sender address to "sender" if it's not NULL