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modbus_proxy() modbus_proxy() Name modbus_proxy - forward a request to a backend device and relay the response Synopsis int modbus_proxy(modbus_t *frontend_ctx, modbus_t *backend_ctx, const uint8_t *req, int req_length); Description The modbus_proxy() function shall forward a Modbus request received on frontend_ctx to a device connected through backend_ctx, and relay the re- sponse back to the original requester on frontend_ctx. This function is intended for building Modbus gateways or proxies that bridge two different backends. A typical use case is a TCP-to-RTU gateway: a client sends a TCP request which is forwarded to an RTU device on a ser- ial bus, and the RTU response is relayed back as a TCP response. The request req of length req_length should be a raw request as returned by modbus_receive <modbus_receive.md>. The slave address is extracted from req and set on backend_ctx before forwarding. The function handles protocol translation between backends automatically: * the PDU (Protocol Data Unit) is extracted from the frontend framing * it is re-framed for the backend protocol * the backend response PDU is extracted and re-framed for the frontend pro- tocol * the transaction identifier from the original request is preserved On communication errors, an appropriate Modbus gateway exception response is sent back to the frontend client: * MODBUS_EXCEPTION_GATEWAY_PATH (0x0A) if the request cannot be forwarded or if a non-timeout communication error occurs * MODBUS_EXCEPTION_GATEWAY_TARGET (0x0B) if the backend device does not re- spond (timeout) or sends an invalid response Return value The function shall return the length of the response sent on frontend_ctx if successful. Otherwise it shall return -1 and set errno. Errors * EINVAL, frontend_ctx or backend_ctx is NULL, or req is NULL, or req_length is less than 1. * EMBBADDATA, the PDU extracted from the request is invalid (too short or too long). Example modbus_t *tcp_ctx; modbus_t *rtu_ctx; uint8_t query[MODBUS_TCP_MAX_ADU_LENGTH]; int rc; tcp_ctx = modbus_new_tcp("0.0.0.0", 1502); rtu_ctx = modbus_new_rtu("/dev/ttyUSB0", 38400, 'E', 8, 1); int s = modbus_tcp_listen(tcp_ctx, 1); modbus_connect(rtu_ctx); for (;;) { modbus_tcp_accept(tcp_ctx, &s); for (;;) { rc = modbus_receive(tcp_ctx, query); if (rc < 0) break; if (rc == 0) continue; /* Forward the TCP request to the RTU device and relay the response */ modbus_proxy(tcp_ctx, rtu_ctx, query, rc); } modbus_close(tcp_ctx); } modbus_close(rtu_ctx); modbus_free(tcp_ctx); modbus_free(rtu_ctx); See also * modbus_receive <modbus_receive.md> * modbus_reply <modbus_reply.md> * modbus_reply_exception <modbus_reply_exception.md> modbus_proxy()
Name | Synopsis | Description | Return value | Errors | Example | See also
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