Reading and writing from serial port,through loopback in c in linux

Question

I am on a program to read and write from and to serial port using serial crosscable and loopback by soring 2nd and 3rd pin of cable. I am able to write but not able to read. in read output it is showing 0 as the no. of bytes read by it. It is not showing error as -1 .

#include <stdio.h> // standard input / output functions
#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 definitionss
#include <time.h>   // time calls
#include <sys/ioctl.h>


int open_port(void)
  {
    int fd; // file description for the serial port

  fd = open("/dev/ttyS1", O_RDWR | O_NOCTTY | O_NDELAY | O_NONBLOCK);

  if(fd == -1) // if open is unsucessful
 {
 perror("open_port: Unable to open /dev/ttyS0 - ");
 }
else
{
fcntl(fd, F_SETFL, 0);
}
 printf("%d",fd);
return(fd);
 }
  int configure_port(int fd)      // configure the port
{
struct termios port_settings;      // structure to store the port settings in

cfsetispeed(&port_settings, B9600);    // set baud rates
cfsetospeed(&port_settings, B9600);
port_settings.c_cflag |= ( CLOCAL | CREAD );

port_settings.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
port_settings.c_cflag &= ~CSTOPB;
port_settings.c_cflag &= ~CSIZE;
port_settings.c_cflag |= CS8;
tcflush( fd, TCIOFLUSH );
tcsetattr(fd, TCSANOW, &port_settings);    // apply the settings to the port
return(fd);

  }
int main()
{
int fd= open_port();
int d=configure_port(fd);
printf("%d",d);
int bytes;

char mk[10];
scanf("%s",&mk);
int w=write(fd,mk,strlen(mk));

int y=ioctl(fd,FIONREAD,&bytes);

printf("%d\n",w);
perror("write");
printf("%d",y);
char buffer[80];
char *data;
int nbytes;

data=buffer;
nbytes=read(fd,data,5);
printf("the outputis \n%d\n\n",nbytes);
perror("read");
while(nbytes > 0)
{printf("datmukun %d\n\n",nbytes);
data+=nbytes;
if (data[-1]=='\n'||data[-1]=='\r')
break;
}
return 0;
}

Answer 1

Depending on your TTY-driver the O_NDELAY and O_NONBLOCK can cause read to behave in a non-blocking manner. Therefore, it is very likely that the data has not been received by the time you call read . If you remove those flags, you should block in read until at least one character is available.

Answer 2

you must wait for a while for the data to become available. you can do this like the following:

• very simply and just for test purpose by creating a small delay using a while loop and a sleep() function inside it and possibly a counter to try for a number of times.
• you can use select() function on your file descriptor to let your process go to sleep for a while and get notified when the data is available or the timeout has reached.