Question

If I get a handle to stdin from a console app like so:

HANDLE hStdIn = ::GetStdHandle(STD_INPUT_HANDLE);

I can then read data from it:

BYTE buff[32];
DWORD dwcbRead = 0;
BOOL bReadRes = ::ReadFile(hStdIn, buff, SIZEOF(buff), &dwcbRead, NULL);

My question is, how do I know how many bytes are available before I read them?

PS. ReadFile seems to block if there’s no data available to read.

Answer 1

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:

1. 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.

2. There is no one-to-one correspondence between input events and characters.

3. 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.