stdin如果我像这样从控制台应用程序获得句柄：
HANDLE hStdIn = ::GetStdHandle(STD_INPUT_HANDLE);
然后我可以从中读取数据：
BYTE buff[32];
DWORD dwcbRead = 0;
BOOL bReadRes = ::ReadFile(hStdIn, buff, SIZEOF(buff), &dwcbRead, NULL);
我的问题是，在读取它们之前我怎么知道有多少字节可用？
PS。ReadFile如果没有可读取的数据，似乎会阻塞。
Use ReadConsoleInput to read raw input events and PeekConsoleInput to examine them without removing from the input queue. There is a bunch of caveats here:
Your standard input might be redirected, then you'll have to determine its type and act accordingly. If it's a file, it won't block and you just go ahead and read. If it's a pipe, PeekNamedPipe provides some help.
There is no one-to-one correspondence between input events and characters.
If ENABLE_LINE_MODE is set on the console, ReadFile/ReadConsole would block if there is no newline yet entered; additionally, line editing facilities are unavailable before you actually call ReadConsole, and when you call ReadConsole, it will block.
I would recommend doing ReadFile or ReadConsole (or trying the latter with fallback to the former) in a separate thread. Your main thread may do something useful and eventually check (or wait for) readyness of the reading thread.
对于控制台输入，您不知道用户或机器将提供多少字符。
例如，我的程序要求你输入一个句子。你在想哪句话？你会打哪一个？句子中有多少个字母？
如果你真的想知道，我强烈推荐一门阅读思想的课程，或者在非人类输入的情况下，一门预测未来的课程。
对于文件，您可以检查大小。
是的，你可以这样做，是的，它会坐在那里等待你的输入来填充缓冲区。如果这不是你想要的，那么不要使用 ReadFile。
还有其他用于读取控制台 I/O 的功能，包括为您提供待处理“事件”数量的功能。