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.
#include <windows.h>
#include <assert.h>
#include <stdio.h>
void main( )
{
HANDLE hCom;
OVERLAPPED o;
BOOL fSuccess;
DWORD dwEvtMask;
hCom = CreateFile( "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
FALSE, // auto reset event
FALSE, // not signaled
NULL // no name
);
// Intialize 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("WaitCommEvent failed with error %d.\n", GetLastError());
}
}
