libmodbus sample master, always gets errno == 9 for all operations after it's connected (but still works just fine)

modbus_master.c

#include <stdio.h>
#include <errno.h>

#include <modbus.h>

int main(int argc, char **argv) {

    modbus_t *mb;
    uint16_t tab_reg[32];
    int rc;

    printf("THIS IS THE MASTER!!!\n");
    if (argc > 1) {
        printf("Using serial port from command line: %s\n", argv[1]);
        mb = modbus_new_rtu(argv[1], 115200, 'N', 8, 1);
    } else {
        mb = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1);
    }
    modbus_set_debug(mb, TRUE);
    modbus_rtu_set_serial_mode(mb, MODBUS_RTU_RS232);
    //rc = modbus_set_slave(mb, 0x69);
    rc = modbus_set_slave(mb, 1);
    if (rc != 0) {
        fprintf(stderr, "set slave failed: %s\n", modbus_strerror(errno));
        modbus_free(mb);
        return -1;
    }
    rc = modbus_connect(mb);
    if (rc != 0) {
        fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
        modbus_free(mb);
        return -1;
    }

    rc = modbus_write_register(mb, 1, 250);
    if (rc != 0) {
        fprintf(stderr, "write failed: %d %s\n", errno, modbus_strerror(errno));
//        modbus_free(mb);
//        return -1;
    }

    rc = modbus_write_register(mb, 0, 55);
    if (rc != 0) {
        fprintf(stderr, "write failed: %d %s\n", errno, modbus_strerror(errno));
/*
        modbus_free(mb);
        return -1;
*/
    }
    rc = modbus_read_registers(mb, 2, 1, tab_reg);
    if (rc != 0) {
        fprintf(stderr, "write failed: %d %s\n", errno, modbus_strerror(errno));
/*
        modbus_free(mb);
        return -1;
*/
    }
    printf("result was :%d\n", tab_reg[0]);
    
    



    /* Read 5 registers from the address 987 */
    /*
        rc = modbus_read_registers(mb, 987, 5, tab_reg);
        if (rc != 0 ) {
                   fprintf(stderr, "read failed: %s\n", modbus_strerror(errno));
                   modbus_free(mb);
                   return -1;
        }
        int i;
        for (i = 0; i < 5; i++) {
            printf("Register %d is %d (%c)\n", i, tab_reg[i], tab_reg[i]);
        }
     */

    modbus_close(mb);
    modbus_free(mb);
}