Reading from serial port fails

Question

I have the following C program:

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

int main()
{
    int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);
    if(fd < 0)
    {
        perror("Could not open device");
    }
    printf("Device opened\n");

    struct termios options;
    tcgetattr(fd, &options);
    cfmakeraw(&options);
    cfsetispeed(&options, B19200);
    cfsetospeed(&options, B19200);
    tcsetattr(fd, TCSANOW, &options);

    char txpacket[] = {0x23, 0x06, 0x00, 0x00, 0xdd, 0xf9};
    ssize_t written = write(fd, txpacket, sizeof(txpacket));
    printf("Written %d bytes\n", written);

    printf("Starting to wait for target to respond\n");
    while(1)
    {
        fd_set readset;
        FD_ZERO(&readset);
        FD_SET(fd, &readset);
        int nCount = select(fd + 1, &readset, NULL, NULL, NULL);
        if(nCount > 0)
        {
            if(FD_ISSET(fd, &readset))
            {
                int i;
                char buffer[128];
                ssize_t bytesread = read(fd, buffer, sizeof(buffer));
                printf("Received %d bytes\n", bytesread);
                for(i = 0; i < bytesread; i++)
                {
                    printf("  %02x", buffer[i]);
                }
            }
        }
    }
}

This program opens the serial device /dev/ttyS0, writes a sequence of data to it and starts listening for a response. I get the following output:

Device opened
Written 6 bytes
Starting to wait for target to respond
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
Received 0 bytes
...

And the application consumes 100% CPU. I'm not able to receive any data, even though the target hardware actually transmits it.

What is wrong?

Answer 1

read() returning 0 indicates the end-of-file condition. You should check for that and break out of the loop if it occurs.

As to what's causing that - end-of-file on a serial port indicates it has detected a hangup, meaning that the DCD line has been dropped.

You can set the CLOCAL flag in options.c_cflag to ignore the modem control lines, if your device doesn't set them properly.

Answer 2

You should try without the O_NONBLOCK flag. in raw mode, if the settings of c_cc[VMIN] and c_cc[VTIME] is 0, the serial port behave like this (according to man cfmakeraw) :

If data is available, read returns immediately, with the lesser of the number of bytes available, or the number of bytes requested. If no data is available, read returns 0

So what you should try is :

options->c_cc[VMIN]=1;