Linux C serial port reading

Question

I'm receiving data from device using serial port. The communication works fine, but there is a problem with reading data. I'm working on Linux (Ubuntu).

Here's my code:

int OpenPort(char *PortName, int *FileDesc) {
    *FileDesc = open (PortName, O_RDWR | O_NOCTTY | O_SYNC);
    if (fd < 0) {
            perror("fakap");
            return (P_OPEN_ERROR);
    }
    else return(P_OPEN_SUCCESS);

};

int SetPortAtributes (int fd, int speed, int parity) {
        struct termios tty;
        memset (&tty, 0, sizeof tty);
        if (tcgetattr (fd, &tty) != 0)
        {
                perror("error %d from tcgetattr");
                return -1;
        }

        cfsetospeed (&tty, speed);
        cfsetispeed (&tty, speed);

        tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
        // disable IGNBRK for mismatched speed tests; otherwise receive break
        // as \000 chars
        tty.c_iflag |= (IGNBRK | IGNPAR);,

        tty.c_oflag = 0;                // no remapping, no delays
        tty.c_cc[VMIN]  = 0;            // read doesn't block
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl
        tty.c_iflag &= ~(ISTRIP);
        tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
                                        // enable reading
        tty.c_cflag &= ~(PARENB | PARODD | HUPCL);      // shut off parity
        tty.c_cflag |= parity;
        tty.c_cflag &= ~CSTOPB;
        tty.c_cflag &= ~CRTSCTS;
        tty.c_lflag = 0;
        if (tcsetattr (fd, TCSANOW, &tty) != 0)
        {
                perror("error %d from tcsetattr");
                return -1;
        }
        return 0;
}

Read section:

void PortRead(int *FileDesc) {

    memset(&recBuff[0], 0, sizeof(recBuff)); //clear buff

    int n = read (*FileDesc, recBuff, sizeof(recBuff));  // read
    printf("n: %d \n", n);
    int i = 0;
    for(i = 0;i<n;i++) {
        recData.buf[i].b_int = recBuff[i]; // put rec buff to ANS_u type variable
    }
};

Everything works fine until I receive message larger then 8 bytes. read() doesn't read more than 8 bytes so I have to use read() second time to read to read all data. Everything works fine when I'm using GtkTerm but, there's a problem during C implementation.

Answer 1

It may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal.

Repeat the read for as long as it has data available.

Also, since the other end is a microcontroller which might be slower than your workstation by a large margin, perhaps the data simply isn't available yet when you do the read(). That's another reason to try again, of course.

So looks like you can try

int n = 0, offset = 0, bytes_expected = 40;
do 
{
    n = read (*FileDesc, recBuff+offset, sizeof(recBuff));
    offset += num;

} while (offset < bytes_expected);