Add modbus-tcp support

src/modbus.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "net.h"
+
+// Functions
+#define MG_MODBUS_FUNC_READ_COILS 1
+#define MG_MODBUS_FUNC_READ_DISCRETE_INPUTS 2
+#define MG_MODBUS_FUNC_READ_HOLDING_REGISTERS 3
+#define MG_MODBUS_FUNC_READ_INPUT_REGISTERS 4
+#define MG_MODBUS_FUNC_WRITE_SINGLE_COIL 5
+#define MG_MODBUS_FUNC_WRITE_SINGLE_REGISTER 6
+#define MG_MODBUS_FUNC_WRITE_MULTIPLE_COILS 15
+#define MG_MODBUS_FUNC_WRITE_MULTIPLE_REGISTERS 16
+
+// Error codes
+#define MG_MODBUS_ERR_NONE 0
+#define MG_MODBUS_ERR_ILLEGAL_FUNCTION 1
+#define MG_MODBUS_ERR_ILLEGAL_ADDRESS 2
+#define MG_MODBUS_ERR_ILLEGAL_VALUE 3
+#define MG_MODBUS_ERR_DEVICE_FAILURE 4
+
+struct mg_modbus_req {
+  uint8_t func;      // Function code, one of MG_MODBUS_FUNC_*
+  uint8_t error;     // Error code user handler can set
+  uint16_t addr;     // Address
+  union {
+    bool *bits;
+    uint16_t *regs;
+  } u;
+  uint16_t len;      // Number of registers or bits
+};
+
+struct mg_connection *mg_modbus_listen(struct mg_mgr *mgr, const char *url,
+                                       mg_event_handler_t fn, void *fn_data);