Read return garbage data in serial port programming in linux

Question

I want to communicate with my PC using the RS232 port. I can open "/dev/ttyS0" and write data using write() function, But i can't read correct data from "dev/ttyS0" by using read(). read() function read the unnecessary data.Please tell me how to solve this problem?

My program code here :

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>

int main()
{
    int n = 0, fd = 0, bytes = 0;

    char buffer[10];

    struct termios term;

    fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);

    if (fd == -1)
    {
        perror("open");
        return;
    }
    else
    {
        fcntl(fd, F_SETFL, 0);
        perror("Port");
    }

    tcgetattr(fd, &term);

    cfsetispeed(&term, B115200);
    cfsetospeed(&term, B115200);

    term.c_cflag |= (CLOCAL | CREAD);
    term.c_cflag &= ~PARENB;
    term.c_cflag &= ~CSTOPB;
    term.c_cflag &= ~CSIZE;
    term.c_cflag |= CS8;
    term.c_cflag &= ~CRTSCTS;
    term.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    term.c_iflag &= ~(IXON | IXOFF | IXANY);
    term.c_oflag &= ~OPOST; 

    term.c_cc[VMIN] = 0;
    term.c_cc[VTIME] = 10;


    tcsetattr(fd, TCSANOW, &term); 

    printf("Enter the string...\n");
        scanf("%s", buffer);

        write(fd, buffer, sizeof(buffer));
        perror("write");

//  fcntl(fd, F_SETFL, FNDELAY);    

    sleep(1);

    bytes = read(fd, buffer, sizeof(buffer));
    perror("read");

    buffer[bytes] = '\0';
    printf("Bytes : %d\n", bytes);
    printf("%s\n", buffer);
    memset(buffer, '\0', 10);   
}

Please give an example of what data you are reading and what you were expecting — Manos Nikolaidis
– Manos Nikolaidis, Commented Jan 28, 2016 at 11:01
Why are you using nonblocking IO? Do you know what the effect of this is? — fuz
– fuz, Commented Jan 28, 2016 at 11:01
@FUZxxl: Good point. Perhaps fd isn't ready when tcgetattr is called (so maybe it just returns -EAGAIN. The code doesn't check errors on anything, so I'd suggest using strace to see what system calls are made, and what their return values are. Also, use a known-good terminal emulator like minicom to make sure things work properly. — Peter Cordes
– Peter Cordes, Commented Jan 29, 2016 at 17:06
@PeterCordes Library functions never clear errno. That's a POSIX rule. — fuz
– fuz, Commented Jan 29, 2016 at 18:34

Community · Accepted Answer · 2017-05-23 12:23:56Z

As I already answered HERE the following code works well. Did you try to change the serial line? Are you sure you are shortcutting pin 2 and 3 of serial DB9 connector?

int main()
{
    int n = 0, fd = 0, bytes = 0;
    char ch = 0;

    char buffer[128], *bufPtr;
    int nBytes = 0, tries = 0, x = 0;

    struct termios term;

    fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);

    if (fd == -1)
    {
        perror("open");
        return;
    }
    else
    {
        fcntl(fd, F_SETFL, 0);
        perror("Port");
    }

    if (n = tcgetattr(fd, &term) == -1)
    {
        perror("tcgetattr");
        return;
    }

    if (n = cfsetispeed(&term, B115200) == -1)
    {
        perror("cfsetispeed");
        return;
    }

    if (n = cfsetospeed(&term, B115200) == -1)
    {
        perror("cfsetospeed");
        return;
    }

    term.c_cflag |= (CLOCAL | CREAD);
    term.c_cflag &= ~PARENB;
    term.c_cflag &= ~CSTOPB;
    term.c_cflag &= ~CSIZE;
    term.c_cflag |= CS8;
    term.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    term.c_iflag &= ~(IXON | IXOFF | IXANY);
    term.c_cflag &= ~CRTSCTS;
    term.c_oflag &= ~OPOST;

    if (n = tcsetattr(fd, TCSANOW, &term) == -1)
    {
        perror("tcsetattr");
        return;
    }

    char stringToSend[128];

    printf("Enter the string...\n");
    scanf("%s", stringToSend);

    size_t len = strlen(stringToSend) +1 ;

    write(fd,stringToSend, len);
    perror("write");

    size_t receivedBytes = 0;
    bytes = 0;
    memset(buffer, 0x00, sizeof(buffer));
    while (receivedBytes<len)
    {
       bytes = read(fd, &buffer[receivedBytes], sizeof(buffer)-1);
       perror("read");

       if (bytes > 0)
           receivedBytes += bytes;
    }

    printf("Bytes : %d and data: %s\n", receivedBytes, buffer);
}

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