Monitoring Communications Events

The following example code opens the serial port for overlapped I/O, creates an event mask to monitor CTS and DSR signals, and then waits for an event to occur. The WaitCommEvent function should be executed as an overlapped operation so the other threads of the process can perform I/O operations during the wait.

C++
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#include <windows.h>
#include <tchar.h>
#include <assert.h>
#include <stdio.h>

void _tmain(
            int argc, 
            TCHAR *argv[]
           )
{
    HANDLE hCom;
    OVERLAPPED o;
    BOOL fSuccess;
    DWORD dwEvtMask;

    hCom = CreateFile( TEXT("\\\\.\\COM1"),
        GENERIC_READ | GENERIC_WRITE,
        0,    // exclusive access 
        NULL, // default security attributes 
        OPEN_EXISTING,
        FILE_FLAG_OVERLAPPED,
        NULL 
        );

    if (hCom == INVALID_HANDLE_VALUE) 
    {
        // Handle the error. 
        printf("CreateFile failed with error %d.\n", GetLastError());
        return;
    }

    // Set the event mask. 

    fSuccess = SetCommMask(hCom, EV_CTS | EV_DSR);

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

    // Create an event object for use by WaitCommEvent. 

    o.hEvent = CreateEvent(
        NULL,   // default security attributes 
        TRUE,   // manual-reset event 
        FALSE,  // not signaled 
        NULL    // no name
         );
    

    // Initialize the rest of the OVERLAPPED structure to zero.
    o.Internal = 0;
    o.InternalHigh = 0;
    o.Offset = 0;
    o.OffsetHigh = 0;

    assert(o.hEvent);

    if (WaitCommEvent(hCom, &dwEvtMask, &o)) 
    {
        if (dwEvtMask & EV_DSR) 
        {
             // To do.
        }

        if (dwEvtMask & EV_CTS) 
        {
            // To do. 
        }
    }
    else
    {
        DWORD dwRet = GetLastError();
        if( ERROR_IO_PENDING == dwRet)
        {
            printf("I/O is pending...\n");

            // To do.
        }
        else 
            printf("Wait failed with error %d.\n", GetLastError());
    }
}

 

 

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