Serial port not reading

Question

I've got this strange issue with reading from a serial port on Linux. About 50% of the time, I won't get any data back from the serial port when I'm reading from it, yet if I use GTKTerm I get data back 100% of the time. I've double-checked my settings, and the terminal settings that I'm using are the exact same that GTKTerm uses, so I'm mystified as to why this would occur. The only thing that I could think of is that the port is set in canonical mode, not raw mode, but I set it properly - the device that I'm reading from does not send back newlines with all of the commands. However, everything is working properly when the device sends back commands with a newline character.

Here's the code I'm using:

int fd = open(serial_port, O_RDWR );
if( fd < 0 ){
    perror("open");
}

//Set the serial port settings
struct termios newio;
if( tcgetattr( fd, &newio ) < 0 )
    perror("tcgetattr");

if( cfsetospeed( &newio, B9600 ) < 0 )
    perror("cfsetospeed");

newio.c_cflag &= ~CSTOPB;
newio.c_cflag &= ~CSIZE;
newio.c_cflag |= CS8;

newio.c_cflag |= CREAD;
newio.c_iflag |= IGNPAR;
newio.c_iflag |= IGNBRK;
newio.c_iflag &= ~BRKINT;
newio.c_iflag &= ~ICRNL;
newio.c_iflag &= ~IXON;
newio.c_cflag |= CLOCAL;
newio.c_oflag = 0;
newio.c_lflag = 0;
newio.c_cc[VTIME] = 0;
newio.c_cc[VMIN] = 1;
if( tcsetattr( fd, TCSANOW, &newio ) < 0 )
    perror("tcsetattr");
tcflush( fd, TCOFLUSH );
tcflush( fd, TCIFLUSH );

Reading code (in separate thread)

void* thr(void* ign){
    char buffer[10];
    while( 1 ){
            int got = read(fd, buffer, 10);
            buffer[got] = 0;
            printf("got %s\n", buffer);
    }
}

Can you include an example of the code you're using to read/write from that fd? You may also want to strace GTKTerm and your code and pick out the TCSETS ioctl call from each to compare. — je4d
@rm5248: The code which configures the port seems okay. Maybe how the i/o is done is the problem. Also, be sure to carefully compare the port settings with stty -a -F /dev/ttyWhatever when your program has it open and when GTK has it open to see if anything is different. — wallyk
Well, after checking with stty, the only difference between what I have and what GTKTerm has is hupcl, which shouldn't matter because what I'm connecting to only uses the transmit/receive lines. I edited in the reading code, writing code is essentially the same. — rm5248

rm5248 rm5248 · Accepted Answer · 2012-04-03T15:33:53

Well, I found out the problem.

For some bizzare reason, the device that I'm sending to doesn't seem to have a big buffer or something; it turns out that when I send commands as a string all at once, I won't always get a response back. Sleeping between sending characters fixes the problem(though I admit, it's not a very good solution). What was probably happening was that I was getting interrupted at points, which would allow enough time for the device to process some of its input. GTKTerm will send out each character as it is received, and it's impossible to type fast enough for this error to occur.

Serial port not reading

1 Answers