原型
BOOL WINAPI WaitCommEvent(
__in HANDLE hFile,
__out LPDWORD lpEvtMask,
__in LPOVERLAPPED lpOverlapped
);
主要參數
hFile：指向通信設備的一個句柄，該句柄應該是由 CreateFile函式返回的。
lpEvtMask：一個指向DWORD的指針。如果發生錯誤， pEvtMask指向0，否則指向以下的某一事件
| EV_DSR 0x0010 | The DSR (data-set-ready) signal changed state. DSR(數據裝置就緒)信號改變狀態。 |
| EV_ERR 0x0080 | A line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY. |
| EV_RING 0x0100 | A ring indicator was detected. |
| EV_RLSD 0x0020 | The RLSD (receive-line-signal-detect) signal changed state. |
| EV_RXCHAR 0x0001 | A character was received and placed in the input buffer.(在輸入緩衝區中接收並放置一個字元) |
| EV_RXFLAG 0x0002 | The event character was received and placed in the input buffer. The event character is specified in the device's DCBstructure, which is applied to a serial port by using the SetCommStatefunction. |
| EV_TXEMPTY 0x0004 | The last character in the output buffer was sent.（輸出緩衝區的數據全部傳送出去) |
lpOverlapped：指向 OVERLAPPED結構體的一個指針。如果 hFile是用異步方式打開的 （在CreateFile（）函式中，第三個參數設定為FILE_FLAG_OVERLAPPED） ， lpOverlapped不能指向一個空 OVERLAPPED結構體，而是與Readfile（）和WriteFile（）中的 OVERLAPPED參數為同一個參數。如果 hFile是用異步方式打開的，而 lpOverlapped指向一個空的 OVERLAPPED結構體，那么函式會錯誤地報告，等待的操作已經完成（而此時等待的操作可能還沒有完成）。
如果 hFile是用異步方式打開的，而 lpOverlapped指向一個非空的 OVERLAPPED結構體，那么函式WaitCommEvent被默認為異步操作，馬上返回。這時， OVERLAPPED結構體必須包含一個由 CreateEvent（）函式返回的手動重置事件對象的句柄hEven。
如果 hFile是用同步方式打開的，那么函式WaitCommEvent不會返回，直到要等待的事件發生。
返回值：
如果函式成功，返回非零值，否則返回0。要得到錯誤信息，可以調用 GetLastError函式。
備註：
WaitCommEvent函式為指定的通信資源監聽一系列的Event,這些Event可以由 SetcommMask和 GetcommMask函式來設定和查詢。
如果異步操作不能馬上完成，那么該函式會返回一個FALSE，同時 GetLastError函式可以截獲錯誤碼ERROR_IO_PENDING（#define ERROR_IO_PENDING 997），表示操作轉到後台運行。在WaitCommEvent函式返回之前，系統將 OVERLAPPED結構中的hEven句柄設定為無信號狀態；當WaitCommEvent函式所等待的任何一個Event發生後，系統將 OVERLAPPED結構中的hEven句柄設定為有信號狀態，同時將所發生事件賦給 lpEvtMask。
父進程可以根據 lpEvtMask來做出相應的事件處理，然後也可以調用 GetOverlappedResult函式來判斷WaitCommEvent的操作是否成功。
如果WaitCommEvent函式在後台運行的時候，進程企圖想通過 SetcommMask函式來改變當前設備的Event，那么WaitCommEvent函式馬上返回， lpEvtMask指向0。
舉例：
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 WaitCommEventfunction 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( 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());
}
}
