如何使用C语言从COM端口读取数据

Question

I have been coding on C/C++ for a while but now I have faced a major problem which I can not resolve. I am trying to communicate with COM port. First I am sending data with WriteFile(), this part works.But when it comes to receiving an answer from the port with ReadFile(), I do not get anything.

Here is the code:

#include <stdlib.h>
#include <windows.h>
#include <stdio.h>
#include <conio.h>

int main(int argc, char *argv[])
{
   int n=0;
   DCB dcb={0};
   HANDLE hCom;
   BOOL fSuccess;
   char *pcCommPort = "COM3";
   DWORD dwBytesRead=0;
   DWORD dwRead;
   DWORD dwBytesWrite=0;
   DWORD dwWrite=0;
/***************************************CommTimeouts******************************************/
COMMTIMEOUTS timeouts={0};
timeouts.ReadIntervalTimeout=200;
//timeouts.ReadTotalTimeoutConstant=1;
//timeouts.ReadTotalTimeoutMultiplier=1;
timeouts.WriteTotalTimeoutConstant=2;
//timeouts.WriteTotalTimeoutMultiplier=1;
/*******************************************Handle*******************************************/
   hCom = CreateFile( pcCommPort,
                    GENERIC_READ | GENERIC_WRITE,
                    FILE_SHARE_READ,    // must be opened with exclusive-access
                    NULL, // no security attributes
                    OPEN_EXISTING, // must use OPEN_EXISTING
                    FILE_ATTRIBUTE_NORMAL,    // not overlapped I/O
                    NULL  // hTemplate must be NULL for comm devices
                    );
/***************************************SET*UP*COM*PORT**************************************/
   if (hCom == INVALID_HANDLE_VALUE)
   {
       printf ("CreateFile failed with error %d.\n", GetLastError());
       CloseHandle(hCom);
       return (1);
   }

   if(!SetCommTimeouts(hCom, &timeouts))
    {
        /*Well, then an error occurred*/
   }

   fSuccess = GetCommState(hCom, &dcb);

   if (!fSuccess)
   {

     /*More Error Handling*/
      printf ("GetCommState failed with error %d.\n", GetLastError());
      CloseHandle(hCom);

      return (2);
   }
   dcb.BaudRate = 9600;     // set the baud rate
   dcb.ByteSize = 8;             // data size, xmit, and rcv
   dcb.Parity = EVENPARITY;        // no parity bit
   dcb.StopBits = ONESTOPBIT;    // one stop bit
   fSuccess = SetCommState(hCom, &dcb);

   if (!fSuccess)
   {
      printf ("SetCommState failed. Error: %d.\n", GetLastError());
      CloseHandle(hCom);
      return (3);
   }

   printf ("Serial port %s successfully configured.\n", pcCommPort);
 //  return (0);
/*************************************Writing************************************************/
char bytes_to_send[] = {'36'};
if(!WriteFile(hCom, bytes_to_send, 2, &dwBytesWrite, NULL))
    {
        fprintf(stderr, "Error\n");
        CloseHandle(hCom);
        return 1;
    }
    printf("dwBytesWrite = %d | %x\n", dwBytesWrite, (dwBytesWrite));
/*************************************Reading************************************************/

char bytes_to_receive[7];

if(!ReadFile(hCom, bytes_to_receive, 7, &dwBytesRead, NULL)){
      printf ("SetCommState failed. Error: %d.\n", GetLastError());
      CloseHandle(hCom);
      return (4);
} else {
printf("Bytes read %d -> %d\n",dwBytesRead, bytes_to_receive);}


/********************************************************************************************/

CloseHandle(hCom);

return(0);

}

Answer 1

There is a file pointer, both for read and write. After the WriteFile it is at the end of the file. If you try to read from it, you will read at the end of the file. To read what you just wrote you have to reposition the file pointer at the start of the file, using the SetFilePointer function:

SetFilePointer(hCom, 0, NULL, FILE_BEGIN);