关闭并立即重新打开 COM 端口失败：为什么？
我试图通过在启动允许用户执行 com-porty 操作的对话框窗口之前测试 COM 端口的“可打开性”来进行“飞行前检查”。
下面是代码序列的概述：
handle = CreateFile("\\\\.\\COM4:", GENERIC_READ | GENERIC_WRITE, 0,NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED,NULL);
if (handle != INVALID_HANDLE_VALUE)
{
CloseHandle(handle);
DoTheWork("\\\\.\\COM4:");
}
else
{
ShowMessage("I'm sorry Dave, I can't do that");
}
...
void DoTheWork(char * port)
{
handle = CreateFile(port, GENERIC_READ | GENERIC_WRITE, 0,NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED,NULL);
/// do lots of stuff
CloseHandle(port);
}
问题是：“DoTheWork”是一个经过尝试和测试的函数，并且它自己可以正确执行。仅当在先前的 CreateFile/CloseHandle 调用之后立即调用且第二个 CreateFile 返回 E_ACCESSDENIED 时，它才会失败。
更糟糕的是，如果我在调试器中慢慢地单步执行代码，它就可以正常工作。
似乎我在第一个 closeHandle 之后需要一个 Sleep() ，但这感觉就像一个黑客 - 我无法知道它必须有多长。
I'm trying to do 'pre-flight checks' by testing a COM port's 'openability' before launching a dialog window which allows the user to do com-porty things.
Here's the code sequence, in outline:
handle = CreateFile("\\\\.\\COM4:", GENERIC_READ | GENERIC_WRITE, 0,NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED,NULL);
if (handle != INVALID_HANDLE_VALUE)
{
CloseHandle(handle);
DoTheWork("\\\\.\\COM4:");
}
else
{
ShowMessage("I'm sorry Dave, I can't do that");
}
...
void DoTheWork(char * port)
{
handle = CreateFile(port, GENERIC_READ | GENERIC_WRITE, 0,NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED,NULL);
/// do lots of stuff
CloseHandle(port);
}
Here's the problem: "DoTheWork" is a tried and tested function, and performs correctly on it's own. It only fails when called immediately after the earlier CreateFile/CloseHandle calls, when the second CreateFile returns E_ACCESSDENIED.
Worse yet, if I step through the code slowly in the debugger, It works just fine.
It seems like I need a Sleep() after the first closeHandle, but that feels like a hack - and i have no way of knowing how long it must be.
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系统需要一些时间来关闭资源。可能有一些神秘的方法来测试它是否已被释放，但我不知道那是什么。我所知道的是，如果您检查注册表项：
您将看到哪些串行端口可用，而无需打开它们，这应该可以解决您的问题。
The system takes a bit of time to close the resource. There's probably some arcane way to test whether it's been freed, but I don't know what that is. What I do know is that if you check the registry key:
You'll see which serial ports are available without having to open them, which should solve your problem.
在关闭句柄之前尝试调用 PurgeComm() 或 FlushFileBuffers()。
Try calling PurgeComm() or FlushFileBuffers() on the handle before closing it.
好吧，经过更多的搜寻，我发现了这个，它涉及 Windows CE 而不是 Win32。
我想这同样适用于 Win32，但我还没有找到任何记录证明。
Well, after more trawling, I found this, which relates to Windows CE rather than Win32.
I guess the same applies to Win32, but I haven't found any documented proof.