从串口linux读取

Question

I'm trying to read from serial port, but always get 0 (zero) characters back. Already read the "Serial Programming Guide for POSIX Operating Systems", but can't find out why the program not waiting (blocking). The code:

#include <stdio.h>   /* Standard input/output definitions */
#include <string.h>  /* String function definitions */
#include <unistd.h>  /* UNIX standard function definitions */
#include <fcntl.h>   /* File control definitions */
#include <errno.h>   /* Error number definitions */
#include <termios.h> /* POSIX terminal control definitions */

void main()
{
    printf("Hello world\n");

    int fd; /* File descriptor for the port */
    int n;
    int bytes;

    char c;

    char buffer[10];
    char *bufptr;

    struct termios options;

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

    if (fd == -1) {
        perror("open_port: Unable to open /dev/ttyUSB0 - ");
    }
    else {
        fcntl(fd, F_SETFL, FNDELAY);
    }

  tcgetattr( fd, &options );

  /* SEt Baud Rate */

  cfsetispeed( &options, B9600 );
  cfsetospeed( &options, B9600 );

  //I don't know what this is exactly

  options.c_cflag |= ( CLOCAL | CREAD );

  // Set the Charactor size

  options.c_cflag &= ~CSIZE; /* Mask the character size bits */
  options.c_cflag |= CS8;    /* Select 8 data bits */

  // Set parity - No Parity (8N1)

  options.c_cflag &= ~PARENB;
  options.c_cflag &= ~CSTOPB;
  options.c_cflag &= ~CSIZE;
  options.c_cflag |= CS8;

  // Disable Hardware flowcontrol

  //  options.c_cflag &= ~CNEW_RTSCTS;  -- not supported

  // Enable Raw Input

  options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);

  // Disable Software Flow control

  options.c_iflag &= ~(IXON | IXOFF | IXANY);

  // Chose raw (not processed) output

  options.c_oflag &= ~OPOST;

  if ( tcsetattr( fd, TCSANOW, &options ) == -1 )
    printf ("Error with tcsetattr = %s\n", strerror ( errno ) );
  else
    printf ( "%s\n", "tcsetattr succeed" );

    fcntl(fd, F_SETFL, 0);

    // Write to the port
    n = write(fd, "1", 1);

    if (n < 0) {
        fputs("write() of 1 bytes failed!\n", stderr);
    }

    // Read from the port

    //fcntl(fd, F_SETFL, FNDELAY);

    bytes = read(fd, &buffer, sizeof(buffer));
    printf("number of bytes read is %d\n", bytes);
    printf("%s\n", buffer);
    //perror ("read error:");

    close(fd);
}

Answer 1

This information was originally from the Serial Programming Guide ¹ .

The reason you are getting a 0 return value is because of this line:

fcntl(fd, F_SETFL, FNDELAY);

If you want a normal blocking read, unset that flag.

_{1. http://www.easysw.com/~mike/serial/serial.html#2_5_4 (now defunct)}

Answer 2

You are using O_NDELAY

O_NONBLOCK or O_NDELAY

When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).

EDIT: You're doing the same thing in your fcntl() call, as well.