在进程间使用事件对象
我试图在 win32 环境中使用事件对象来同步两个进程。下面是两个程序的简化代码。
// process1
int main()
{
HANDLE h = CreateEvent(NULL, FALSE, FALSE, TEXT("Hello"));
WaitForSingleObject(h, INFINITE);
// RunProcess(L"process2.exe", L"");
}
// process2
int main()
{
HANDLE h = OpenEvent(EVENT_MODIFY_STATE, FALSE, TEXT("Hello"));
SetEvent(h);
}
它非常简单，并且当两个进程独立启动时效果很好。然而，当进程 1 将进程 2 作为子进程启动时（在上面的代码中进行了注释），它不起作用 - SetEvent 调用失败。这个问题的原因和解决方法是什么？
I'm trying to use event object in win32 environment to synchronize two processes. Below are the simplified code of two programs.
// process1
int main()
{
HANDLE h = CreateEvent(NULL, FALSE, FALSE, TEXT("Hello"));
WaitForSingleObject(h, INFINITE);
// RunProcess(L"process2.exe", L"");
}
// process2
int main()
{
HANDLE h = OpenEvent(EVENT_MODIFY_STATE, FALSE, TEXT("Hello"));
SetEvent(h);
}
It's quite simple, and works well when two processes are launched independently. However it does not work when the process 1 launches process 2 as a child process (which is commented in the above code) - the SetEvent call fails. What is the reason and solution of this problem?
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您的代码需要检查并处理错误。如果失败，
CreateEvent和OpenEvent都会返回NULL，在这种情况下，您需要使用GetLastError检查错误。您对
WaitForSingleObject和SetEvent的调用也应该根据 MSDN 文档进行检查。在父进程中需要执行的操作顺序是：
否则你会遇到@Mark Tolonen 指出的问题。
最好在等待时设置超时，以处理子进程无法启动、意外退出或挂起的情况。
如果您打算使用这种父/子关系，另一种方法是允许继承事件句柄。然后，该事件不需要命名，其他人也无法在对您的应用程序进行 DoS 攻击时“抢占”该事件。您可以将句柄值作为命令行参数传递给子级。您可以使用
CreateEvent的eventAttributes参数上的bInheritHandle字段来执行此操作。Your code needs to check and handle errors. Both
CreateEventandOpenEventwill returnNULLif they fail, in that case you need to check the error usingGetLastError.Your calls to
WaitForSingleObjectandSetEventshould be checked per the MSDN docs as well.The order in which you need to do things in the parent process is:
Otherwise you will hit the problem called out by @Mark Tolonen.
It would also be best to have a timeout on your wait, to handle the case where the child process fails to start, exits unexpectedly, or hangs.
An alternative approach if you intend to use this parent/child relationship would be to allow inheritance of the event handle. Then the event does not need to be named, and nobody else can 'squat' on it in a DoS attack on your apps. You can pass the handle value to the child as a command-line parameter. You do this using the
bInheritHandlefield on theeventAttributesparameter toCreateEvent.你确定吗？如前所述，如果 process1 在当前位置创建 process2，它永远不会创建 process2，因为它将永远等待事件被触发。先创建process2，然后等待事件被设置。
Are you sure? As written, if process1 creates process2 in the current location, it will never create process2 because it will wait forever for the event to be fired. Create process2 first, then wait for the event to be set.
您有一个 NULL 安全描述符，文档说它不允许子进程继承句柄，具体来说：
如果此参数为 NULL，则子进程无法继承句柄也许您需要创建正确的安全描述符？
You have a NULL security descriptor, which the documentation says cannot allow the handle to be inherited by children processes, specifically:
If this parameter is NULL, the handle cannot be inherited by child processesMaybe you need to create a proper security descriptor?