I am working with a Beagle Bone serial port. I made a C program to read some data sent by another board; However, something that I cannot understand is happening.

When I run the program it does not work if I don't run the "screen" command first (screen /dev/ttyOX). I would like to know from the experts, what is happening and why I need to run "screen" first. Can you help me with this?

My code looks like:

#include <stdio.h>
#include <fcntl.h>  
#include <termios.h>
#include <unistd.h> 
#include <errno.h>
struct termios SerialPortSettings;
static int configureUART_Port(const char* pDevice);
void main()
          int lSerialPort;
          lSerialPort = configureUART_Port("/dev/ttyO4");
          char buf[2]="";
          int bytesRead = 0;
          printf("Attempt to read\r\n");
          bytesRead = read(lSerialPort, buf, 2);
          printf("BytesRead : %d\r\n",bytesRead);
static int configureUART_Port(const char *pDevice){
    static int lSerialPort = 0;
    lSerialPort = open(pDevice, O_RDWR | O_NOCTTY);
    if(lSerialPort == -1)
        printf("\n  Error! in Opening ttyO4\n");
        printf("Port Successfully oppened\r\n");
        tcgetattr(lSerialPort, &SerialPortSettings);
        /*Setea La velocidad Tx/Rx*/
        /*Parity Bit 0 HC-05*/
        SerialPortSettings.c_cflag &= ~PARENB;
        SerialPortSettings.c_cflag &= ~CSTOPB;
        /*Configure 8 bits por byte*/
        SerialPortSettings.c_cflag &= CS8;
        SerialPortSettings.c_cflag &= ~CSIZE; /* Clears the Mask       */
        SerialPortSettings.c_cflag |=  CS8;
        SerialPortSettings.c_cflag &= ~CRTSCTS;
        SerialPortSettings.c_cflag |= CREAD | CLOCAL;
        SerialPortSettings.c_iflag &= ~(IXON | IXOFF | IXANY);
        SerialPortSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);
        SerialPortSettings.c_cc[VMIN]  = 2;
        /*Wait for data*/
        SerialPortSettings.c_cc[VTIME] = 0; 
        /*Update configuration*/
    return lSerialPort;    

  • Welcome to Stack Exchange! Could you please elaborate a little on what is happening when "it doesn't work" and what you expect to happen? – Martin Konrad May 13 '20 at 1:28
  • I have never heard of ttyOx - shouldn't it say ttySx if you want to open the serial port? – Martin Konrad May 13 '20 at 1:28
  • Sorry I will change the questions title, I am not pretty sure I am new in working with linux, I think that ttyO4 its okay since when I run 'screen /dev/ttyO4' I can see the data send from the other board, I read that screen keeps running in BG, and after this if I run my programa I can see the data. What I would Like to know is why this happens? – nicolas espla May 13 '20 at 1:32

I found the solution: when running screen, it enables the raw mode. I changed the code of my program to enable it and it works.

Btw, ttyOx is a symbolic link to ttySx.

Thank you

  • Welcome to the site, and congratulations on solving the problem yourself. For the sake of others facing a similar problem, would you mind editing your answer to include an explanation how you had to change your code to accomplish this? – AdminBee May 13 '20 at 7:16

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