my serial port data reading program isn't working

This is a discussion on my serial port data reading program isn't working within the C Programming forums, part of the General Programming Boards category; Hey all, I run linux, and my serial port is on /dev/ttyS0. I have a GPS receiver hooked up to ...

  1. #1
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    my serial port data reading program isn't working

    Hey all,

    I run linux, and my serial port is on /dev/ttyS0. I have a GPS receiver hooked up to it, which puts out lines like:
    $PSRF104,0,0,0,96000,237759,1946,12,4*1D

    I can't seem to read those lines. When i fireup cutecom and have the program connect to the serial port at 4800 8N1, it reads the lines perfectly from the device... so I know the device and serial port work.

    Here is the code I have:
    Code:
    #include <stdio.h>
    #include <string.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <termios.h>
    
    int main() {
      struct termios tp;
      int fd;
      char buffer[100];
      long baud = 4800;
    
      if ((fd = open("/dev/ttyS0", O_RDWR)) < 0) {
        perror("bad terminal device, try another");
        exit(-1);
      }
    
      tp.c_cflag = CS8|CLOCAL|baud;
      tp.c_oflag = 0;
      tp.c_iflag = IXON|IGNBRK|IGNCR|IGNPAR;
      tp.c_lflag = 0;
      cfsetospeed(&tp, baud);
      cfsetispeed(&tp, baud);
      if (tcsetattr(fd, TCSANOW, &tp) < 0) {
        perror("Couldn't set term attributes");
        exit(-1);
      }
    
      memset(buffer, '\0', sizeof(buffer));
      read(fd, buffer, 10);
    
      printf("Buffer: %s\n", buffer);
    
      return 1;
    }
    and here is the output I get, it is very random output:
    Code:
    hedpe@hedlinux serial $ ./serial
    Buffer: <?3x
    hedpe@hedlinux serial $ ./serial
    Buffer: `
             ~0
    hedpe@hedlinux serial $ ./serial
    Buffer: <
    any help would be greatly appreciated...thanks!
    George

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    It looks like buffer hasn't been initialized. But memset() initializes it.

    Try this code instead of your printf() to see if anything is working:

    Code:
    int x;
    for(x = 0; x < sizeof(buffer); x ++) {
        putchar(buffer[x]);
    }
    printf("\n");
    That ouput is if you send the same data over the port, right?

  3. #3
    ATH0 quzah's Avatar
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    Or better yet, why don't you pay attention to the return value of read?

    Quzah.
    Hope is the first step on the road to disappointment.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Now why didn't I see that.

  5. #5
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    the return value of read is never -1 or 0

    any other ideas?

  6. #6
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    Quote Originally Posted by gnychis
    the return value of read is never -1 or 0

    any other ideas?

    If you want the bit rate to be 4800, then use B4800, (whose integer value may not be 4800).

    Code:
      speed_t baud = B4800;
    Also, you must set the CREAD bit in c_cflag (the value integer 4800 that you had used just happened to set the CREAD bit, otherwise nothing would have been received).

    so the setting for c_cflag would be something like this:

    Code:
     tp.c_cflag = CREAD|CS8|CLOCAL|baud;
    Regards,

    Dave
    Last edited by Dave Evans; 06-02-2005 at 09:54 AM.

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