|How to read input from serial port?

I will have 2 machines hooked up via the COM1 ports.
1 machine running windows having a hyperterminal session opened and the other machine running Linux having my program running.
I want to able to type a character into the hyperterminal window and have the character displayed on the Linux machine.

Im very new to C programming but heres what i have for my program so far, just not sure how to read the input and display it:

#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*/

int open_port(void)
      int fd; /* File descriptor for the port */

      fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
            if(fd == 0)
                  perror("open_port: Unable to open       /dev/ttyS0 - ");
                  fcntl(fd, F_SETFL, 0);
            return (fd);

/*Configire port*/

struct termios options;

/*Get the current options for the port*/

tcgetattr(fd, &options);

/*Set the Baud rates to 19200*/

cfsetispeed(&options, B19200);
cfsetospeed(&options, B19200);

/*Enable received and set local mode*/

options.c_cflag |= (CLOCAL | CREAD);

/*Set new options for port*/

tcsetattr(fd, TCSANOW, &options);

/*Set data bits*/

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

/*Set RAW input*/

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

/*Set Raw output*/

options.c_oflag &= ~OPOST;

/*Set timeout to 1 sec*/

options.c_cc[VMIN] = 0;
options.c_cc{VTIME] = 10;

/*Code to read input goes here*/


Can anyone advise on this?

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Daniel JungesConnect With a Mentor Commented:
/* A sample program to illustrate setting up a serial port. */

#include <windows.h>

int main(int argc, char *argv[])
   DCB dcb;
   HANDLE hCom;
   BOOL fSuccess;
   char *pcCommPort = "COM2";

   hCom = CreateFile( pcCommPort,
                    GENERIC_READ | GENERIC_WRITE,
                    0,    // must be opened with exclusive-access
                    NULL, // no security attributes
                    OPEN_EXISTING, // must use OPEN_EXISTING
                    0,    // not overlapped I/O
                    NULL  // hTemplate must be NULL for comm devices

       // Handle the error.
       printf ("CreateFile failed with error %d.\n", GetLastError());
       return (1);

   // Build on the current configuration, and skip setting the size
   // of the input and output buffers with SetupComm.

   fSuccess = GetCommState(hCom, &dcb);

   if (!fSuccess)
      // Handle the error.
      printf ("GetCommState failed with error %d.\n", GetLastError());
      return (2);

   // Fill in DCB: 57,600 bps, 8 data bits, no parity, and 1 stop bit.

   dcb.BaudRate = CBR_57600;     // set the baud rate
   dcb.ByteSize = 8;             // data size, xmit, and rcv
   dcb.Parity = NOPARITY;        // no parity bit
   dcb.StopBits = ONESTOPBIT;    // one stop bit

   fSuccess = SetCommState(hCom, &dcb);

   if (!fSuccess)
      // Handle the error.
      printf ("SetCommState failed with error %d.\n", GetLastError());
      return (3);

   printf ("Serial port %s successfully reconfigured.\n", pcCommPort);
   return (0);
Kent OlsenConnect With a Mentor Data Warehouse Architect / DBACommented:
Hi aaronmcl,

Use the read() and write() functions to perform the actual I/O.

Also, if the two PC serial ports are directly connected, you'll want what is called a "crossover" cable that reverses the input/output signals.  If the two PCs are connected via a hub, switch, or router you should be fine.

Good Luck,
Daniel JungesCommented:

sorry, i forgot the read part:

unsigned long readed = 0;
char buff[128];
ReadFile(hCom, buff, 128, &readed, 0);
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aaronmclAuthor Commented:
Its alright guys got it sorted myself.
Thanks for the help anyways

Hi all,

I am trying to communicate with the serial port. when I try to open the port using open system call, open("/dev/ttys0", O_RDONLY)...I am getting the Iput/output error. What could be the reason for this?
After opening the file using open system call,
fd = open("filename", O_RDWR );

 Is it possible to convert the fd, i.e... the file descriptor(int) to FILE* so that I can do a formatted read like fread, fscanf etc....
Daniel JungesCommented:
Dear gurup_k, this question is already closed.
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