Fix modbus_reply for TCP when unit id == 0 (fixes #376)

According to the Modbus standard, the unit identifier is choosen by the
TCP client and can be any value:

"This field is used for intra-system routing purpose.  It is typically
used to communicate to a MODBUS+ or a MODBUS serial line slave through a
gateway between an Ethernet TCP-IP network and a MODBUS serial line.
This field is set by the MODBUS Client in the request and must be returned
with the same value in the response by the server."

So the current check in modbus_reply must be extended to check whether
we are in a RTU or TCP context.

Also the unit-test has to be adjusted.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

src/modbus.c

@@ -996,7 +996,8 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
     }
 
     /* Suppress any responses when the request was a broadcast */
-    return (slave == MODBUS_BROADCAST_ADDRESS) ? 0 : send_msg(ctx, rsp, rsp_length);
+    return (ctx->backend->backend_type == _MODBUS_BACKEND_TYPE_RTU &&
+            slave == MODBUS_BROADCAST_ADDRESS) ? 0 : send_msg(ctx, rsp, rsp_length);
 }
 
 int modbus_reply_exception(modbus_t *ctx, const uint8_t *req,

tests/unit-test-client.c

@@ -488,19 +488,28 @@ int main(int argc, char *argv[])
         printf("1-C/3 No response from slave %d with invalid request: ",
                INVALID_SERVER_ID);
         ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
+
+
+        rc = modbus_set_slave(ctx, MODBUS_BROADCAST_ADDRESS);
+        ASSERT_TRUE(rc != -1, "Invalid broacast address");
+
+        rc = modbus_read_registers(ctx, UT_REGISTERS_ADDRESS,
+                                   UT_REGISTERS_NB, tab_rp_registers);
+        printf("2/3 No reply after a broadcast query: ");
+        ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
     } else {
         /* Response in TCP mode */
         printf("1/3 Response from slave %d: ", INVALID_SERVER_ID);
         ASSERT_TRUE(rc == UT_REGISTERS_NB, "");
-    }
 
-    rc = modbus_set_slave(ctx, MODBUS_BROADCAST_ADDRESS);
-    ASSERT_TRUE(rc != -1, "Invalid broacast address");
+        rc = modbus_set_slave(ctx, MODBUS_BROADCAST_ADDRESS);
+        ASSERT_TRUE(rc != -1, "Invalid broacast address");
 
-    rc = modbus_read_registers(ctx, UT_REGISTERS_ADDRESS,
-                               UT_REGISTERS_NB, tab_rp_registers);
-    printf("2/3 No reply after a broadcast query: ");
-    ASSERT_TRUE(rc == -1 && errno == ETIMEDOUT, "");
+        rc = modbus_read_registers(ctx, UT_REGISTERS_ADDRESS,
+                                   UT_REGISTERS_NB, tab_rp_registers);
+        printf("2/3 Reply after a query with unit id == 0: ");
+        ASSERT_TRUE(rc == UT_REGISTERS_NB, "");
+    }
 
     /* Restore slave */
     modbus_set_slave(ctx, old_slave);