Ubuntu C++ Serial Port communication with TM4C launchpad

I'm able to send and receive serial data from my Windows7 PC to the TM4C12GXL by using the Texasdisplay. It is quite a basic and easy program, and it works: The launchpad waits for an unsigned char input. If r button is pressed on keyboard, it displays red light, and similar for other buttons etc. etc.

Now I want to do a similar thing by using my Ubuntu 15.04 PC, by directly sending outputs of my C++ code in the PC. Connecting launchpad to the PC, but I can't even open the serial port.

first I check the ports:

dmesg | grep tty

gives the output:

[    0.000000] console [tty0] enabled
[10097.475939] cdc_acm 2-1.3:1.0: ttyACM0: USB ACM device
[10397.346325] cdc_acm 2-1.3:1.0: ttyACM0: USB ACM device

This is the C++ code in my Ubuntu PC:

#include <SerialStream.h>
#include <iostream>
#include <unistd.h>
#include <cstdlib>
#include <string>

using namespace LibSerial;
using namespace std;

int main(void)
{
    SerialStream my_serial_stream;
    my_serial_stream.Open("/dev/ttyACM0");  //opening the port
    if(!my_serial_stream.good())
    {
        std::cerr << "[" << __LINE__ << "]" << " , Port open error." << "\n";
    }

    my_serial_stream.SetBaudRate(SerialStreamBuf::BAUD_115200);
    if(!my_serial_stream.good())
    {
        std::cerr << "[" << __LINE__ << "]" << " , Baudrade error." << "\n";
    }
}

How I compile:

g++ uart.cpp -o uart -L/usr/lib -lserial

Output is:

[20] , Port open error.
[26] , Baudrade error.

It can't even open the serial port. What am I missing? Thank you.