Question

stdin polling on Windows

Asked by: crazycomputers

I have been trying, without success, to find some function on Windows that will simply tell me if a call to getc(stdin) will not block (i.e. there is one or more characters waiting on stdin.)  This is very simple on *nix systems.  Also, the polling solution should be able to work regardless of whether the program is running in a Win32 console or was started by another process that uses stdio to communicate to it.

The main loop of my program is best represented by this pseudo-code:

for (;;) {
    while (stdin_ready()) {
        read_one_char_from_stdin();
        do_something_with_char();
    }

    while (socket_ready())
        handle_socket();
}

On both platforms, the socket_ready() code uses select() to poll the socket.  On *nix, stdin_ready() also uses select(), but I cannot do this on Windows, as select() is provided by the Winsock library, so it only works on sockets.

A solution may use isatty() if it is 100% necessary to split code based upon whether the program is being run in a console.  However, I will prefer solutions that "do not care" about where the program is run.  Basically, I'm looking for a solution that uses stdio in the purest form.

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Asked On
2005-03-09 at 12:20:07ID21344305
Tags

stdin

,

windows

Topic

C Programming Language

Participating Experts
3
Points
500
Comments
16

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Answers

 

by: jkrPosted on 2005-03-09 at 12:37:28ID: 13500159

On Windows, you'd either use 'PeekConsoleInput()' or 'GetNumberOfConsoleInputEvents()' to achieve that.

 

by: jkrPosted on 2005-03-09 at 12:41:48ID: 13500190

Ooops, forgot the sample

BOOL stdin_ready() {

DWORD dwNumEvents;

GetNumberOfConsoleInputEvents ( GetStdHandle ( STD_INPUT_HANDLE), &dwNumEvents);

if ( dwNumEvents) return TRUE;

return FALSE;
};

 

by: grg99Posted on 2005-03-09 at 12:42:35ID: 13500199

You could always spin off a thread that just does a getc() and stuffs the character away somewhere.

 

by: sunnycoderPosted on 2005-03-09 at 20:05:14ID: 13502964

There is a function getc_unlocked() defined in stdio.h and it conforms to POSIX.1 ... I am not sure if windows has it since windows does not conform to POSIX ... still give it a try ... If they do not have getc_unlocked(), you may find something equivalent like _getc_unlocked() in their header files

 

by: crazycomputersPosted on 2005-03-10 at 00:13:07ID: 13503862

jkr: I have tried using your example, and it does compile, but I have not been close enough to a Windows machine to test it.  I will get back to you when I can test.

sunnycoder: Locking isn't the same thing as blocking.

 

by: crazycomputersPosted on 2005-03-10 at 00:15:32ID: 13503877

Oh, and...

grg99: Sorry, I don't do threads in C... especially when I would be writing a bunch of extra code when one function call would work.

 

by: sunnycoderPosted on 2005-03-10 at 00:29:27ID: 13503927

My bad ... I should have read carefully

 

by: crazycomputersPosted on 2005-03-12 at 01:25:32ID: 13523629

Okay, I have tested on my brother's XP box.  I was not able to get GetNumberOfConsoleInputEvents to work correctly; it was returning (via the pointer) a large number (4 million and something) every time it was called, regardless of how many characters were waiting on stdin.  This would ONLY happen if I had typed something; if I left it alone, the network was handled fine.  As soon as I typed something, GetNumberOfConsoleInputEvents would continue to give out the same large number after every call.

I am using select() for the network, and I am not sure if that is even working correctly.  Maybe a broader question is what is the "standard" way to write a console app for windows that needs to juggle stdin and a socket?  The program is a "telnet helper" -- a program calls it in place of opening a telnet connection, and uses its stdio as if it were coming from the server.  This means that my program has to handle the socket instead, but means that I get to do all sorts of interesting manipulations and change the data going through.

So... if you know of "the right way" to do something like that on Windows, please advise.

 

by: jkrPosted on 2005-03-12 at 11:08:37ID: 13525508

What was teh exact code you were using?

 

by: crazycomputersPosted on 2005-03-12 at 21:20:46ID: 13527374

       if (!isatty_stdin_cache) {
                DWORD ne;
                GetNumberOfConsoleInputEvents(GetStdHandle(STD_INPUT_HANDLE), &ne);
                return (ne > 0);
        }

 

by: crazycomputersPosted on 2005-04-08 at 21:47:47ID: 13742477

I've had my Windows developer work on this a bit more, and he says he got it working using multithreading.  I have not been able to test yet; once I do test, if it works the points will go to grg99.  If it doesn't, then I will recommend PAQ w/refund.

Thanks all for your input.

 

by: crazycomputersPosted on 2005-04-18 at 10:02:28ID: 13808168

A comment was added within 10 days, actually.

 

by: crazycomputersPosted on 2005-04-18 at 10:31:13ID: 13808438

The threading approach seems to work, so I've awarded the points to grg99.

20120131-EE-VQP-002

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