int modbus_report_slave_id(modbus_t *ctx, int max_dest, uint8_t *dest);


The modbus_report_slave_id() function shall send a request to the controller to obtain a description of the controller.

The response stored in dest contains:

  • the slave ID, this unique ID is in reality not unique at all so it’s not possible to depend on it to know how the information are packed in the response.

  • the run indicator status (0x00 = OFF, 0xFF = ON)

  • additional data specific to each controller. For example, libmodbus returns the version of the library as a string.

The function writes at most max_dest bytes from the response to dest so you must ensure that dest is large enough.


The function shall return the number of read data if successful.

If the output was truncated due to the max_dest limit then the return value is the number of bytes which would have been written to dest if enough space had been available. Thus, a return value greater than max_dest means that the response data was truncated.

Otherwise it shall return -1 and set errno.


uint8_t tab_bytes[MODBUS_MAX_PDU_LENGTH];


rc = modbus_report_slave_id(ctx, MODBUS_MAX_PDU_LENGTH, tab_bytes);
if (rc > 1) {
    printf("Run Status Indicator: %s\n", tab_bytes[1] ? "ON" : "OFF");


The libmodbus documentation was written by St├ęphane Raimbault <[email protected]>