SITL: make it possible to connect two SITL instances together

this adds --client to set uartC as a tcp client rather than a TCP
server

libraries/AP_HAL_SITL/SITL_State.h

@@ -55,6 +55,9 @@ public:
     uint16_t voltage_pin_value;  // pin 13
     uint16_t current_pin_value;  // pin 12
 
+    // return TCP client address for uartC
+    const char *get_client_address(void) const { return _client_address; }
+
 private:
     void _parse_command_line(int argc, char * const argv[]);
     void _set_param_default(const char *parm);
@@ -187,6 +190,9 @@ private:
 
     // internal SITL model
     Aircraft *sitl_model;
+
+    // TCP address to connect uartC to
+    const char *_client_address;
 };
 
 #endif // CONFIG_HAL_BOARD == HAL_BOARD_SITL

libraries/AP_HAL_SITL/SITL_cmdline.cpp

@@ -75,6 +75,11 @@ void SITL_State::_parse_command_line(int argc, char * const argv[])
     _rcout_port = 5502;
     _simin_port = 5501;
     _fdm_address = "127.0.0.1";
+    _client_address = NULL;
+
+    enum long_options {
+        CMDLINE_CLIENT=0
+    };
 
     const struct GetOptLong::option options[] = {
         {"help",            false,  0, 'h'},
@@ -89,6 +94,7 @@ void SITL_State::_parse_command_line(int argc, char * const argv[])
         {"home",            true,   0, 'O'},
         {"model",           true,   0, 'M'},
         {"frame",           true,   0, 'F'},
+        {"client",          true,   0, CMDLINE_CLIENT},
         {0, false, 0, 0}
     };
 
@@ -135,6 +141,9 @@ void SITL_State::_parse_command_line(int argc, char * const argv[])
         case 'F':
             _fdm_address = gopt.optarg;
             break;
+        case CMDLINE_CLIENT:
+            _client_address = gopt.optarg;
+            break;
         default:
             _usage();
             exit(1);

libraries/AP_HAL_SITL/UARTDriver.cpp

@@ -64,6 +64,14 @@ void SITLUARTDriver::begin(uint32_t baud, uint16_t rxSpace, uint16_t txSpace)
         _fd = _sitlState->gps_pipe();
         break;
 
+    case 2:
+        if (_sitlState->get_client_address() != NULL) {
+            _tcp_start_client(_sitlState->get_client_address());
+        } else {
+            _tcp_start_connection(false);
+        }
+        break;
+
     case 4:
         /* gps2 */
         _connected = true;
@@ -228,7 +236,7 @@ void SITLUARTDriver::_tcp_start_connection(bool wait_for_connection)
 
         fprintf(stderr, "bind port %u for %u\n",
                 (unsigned)ntohs(sockaddr.sin_port),
-                (unsigned)_portNumber),
+                (unsigned)_portNumber);
 
         ret = bind(_listen_fd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
         if (ret == -1) {
@@ -263,6 +271,66 @@ void SITLUARTDriver::_tcp_start_connection(bool wait_for_connection)
     }
 }
 
+
+/*
+  start a TCP client connection for the serial port. 
+ */
+void SITLUARTDriver::_tcp_start_client(const char *address)
+{
+    int one=1;
+    struct sockaddr_in sockaddr;
+    int ret;
+
+    if (_connected) {
+        return;
+    }
+
+    if (_fd != -1) {
+        close(_fd);
+    }
+
+    char *addr2 = strdup(address);
+    char *p = strchr(addr2, ':');
+    if (p == NULL) {
+        fprintf(stderr, "need IP:port\n");
+        exit(1);
+    }
+    *p = 0;
+    uint16_t port = htons(atoi(p+1));
+
+    memset(&sockaddr,0,sizeof(sockaddr));
+
+#ifdef HAVE_SOCK_SIN_LEN
+    sockaddr.sin_len = sizeof(sockaddr);
+#endif
+    sockaddr.sin_port = port;
+    sockaddr.sin_family = AF_INET;
+    sockaddr.sin_addr.s_addr = inet_addr(addr2);
+
+    free(addr2);
+
+    _fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (_fd == -1) {
+        fprintf(stderr, "socket failed - %s\n", strerror(errno));
+        exit(1);
+    }
+
+    /* we want to be able to re-use ports quickly */
+    setsockopt(_fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
+
+    ret = connect(_fd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
+    if (ret == -1) {
+        fprintf(stderr, "connect failed on port %u - %s\n",
+                (unsigned)ntohs(sockaddr.sin_port),
+                strerror(errno));
+        exit(1);
+    }
+
+    setsockopt(_fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
+    setsockopt(_fd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));
+    _connected = true;
+}
+
 /*
   see if a new connection is coming in
  */

libraries/AP_HAL_SITL/UARTDriver.h

@@ -65,7 +65,11 @@ private:
     uint16_t _rxSpace;
     uint16_t _txSpace;
 
+    // IPv4 address of target for uartC
+    const char *_tcp_client_addr;
+
     void _tcp_start_connection(bool wait_for_connection);
+    void _tcp_start_client(const char *address);
     void _check_connection(void);
     static bool _select_check(int );
     static void _set_nonblocking(int );