modbus_new_rtu(3) create a libmodbus context for RTU


modbus_t *modbus_new_rtu(const char *device, int baud, char parity, int data_bit, int stop_bit);


The modbus_new_rtu() function shall allocate and initialize a modbus_t structure to communicate in RTU mode on a serial line.

The device argument specifies the name of the serial port handled by the OS, eg. /dev/ttyS0 or /dev/ttyUSB0. On Windows, it's necessary to prepend COM name with \\.\ for COM number greater than 9, eg. \\\\.\\COM10. See m[blue][] for details

The baud argument specifies the baud rate of the communication, eg. 9600, 19200, 57600, 115200, etc.

The parity argument can have one of the following values

N for none

E for even

O for odd

The data_bits argument specifies the number of bits of data, the allowed values are 5, 6, 7 and 8.

The stop_bits argument specifies the bits of stop, the allowed values are 1 and 2.


The modbus_new_rtu() function shall return a pointer to a modbus_t structure if successful. Otherwise it shall return NULL and set errno to one of the values defined below.



An invalid argument was given.


modbus_t *ctx;
ctx = modbus_new_rtu("/dev/ttyUSB0", 115200, 'N', 8, 1);
if (ctx == NULL) {
    fprintf(stderr, "Unable to create the libmodbus context\n");
    return -1;


The libmodbus documentation was written by Stéphane Raimbault <m[blue][email protected]m[][1]>