Loop until user inputs an Integer, but skip the read in if no integer exists on the terminal

Question

Im trying to create an application that will count pulses from an Oscilloscope, my problem is this:

I need a loop to run constantly, until a user input is entered. However I dont want getch() to be called unless there is an input on the terminal ready to be read. How would I go about checking for a character or integer existing on the terminal?

You may need to use OS specific functions. What platform are you running on? — George Houpis, Jan 19 '16 at 17:58
See http://stackoverflow.com/questions/6886579/get-input-from-keyboard-without-waiting-for-input — chux - Reinstate Monica, Jan 19 '16 at 18:04
I should have specified, im coding for a microcontroller using an OS thats un-named and not public, im hoping theres a standard C way of doing this — Sam Cornflake Scotford, Jan 19 '16 at 18:04
No standard C way. So unless you find an OS specific `bool DataInBuffer()` or tell us you OS, you are out of luck. — chux - Reinstate Monica, Jan 19 '16 at 18:06
Also check http://stackoverflow.com/questions/32607814/getting-an-input-from-the-user-without-having-to-wait-c-language — chux - Reinstate Monica, Jan 19 '16 at 18:09
I assume that you don't want to block if no input is there? Like, you have a loop which does all kinds of things, like a poor mans multi tasking, and you want to do other things until input is actually waiting? Then @rslemos' answer is for you, or a low-level register in your SoC. — Peter - Reinstate Monica, Jan 19 '16 at 18:12

score 4 · Answer 1 · answered Jan 19 '16 at 18:00

4

If you're coding for UNIX, you'll need to use either poll(2) or select(2).

answered Jan 19 '16 at 18:00

rslemos

2,454
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32

1

Possibly a `open` with the `O_NONBLOCK` flag and subsequent non-blocking `read`s are also an option, if the device supports that. – Peter - Reinstate Monica Jan 19 '16 at 18:15

score 2 · Answer 2 · answered Jan 19 '16 at 18:20

Since you mention the non-standard getch you might also have kbhit which tests if there is input waiting. But don't slug your oscilloscope with that every loop: you can ease the flow by checking occasionally.

#include <conio.h>
#include <stdio.h>

#define POLLMASK    0xFFFF

int main(void){

    int poll = 0;
    int ch = 0;

    while(ch != 27) {

        // ... oscilloscope details

        if ((poll++ & POLLMASK) == 0 && _kbhit()) {
            ch = _getch();
            // ... do something with input
        }
    }
    return 0;
}

Mason Watmough · Answer 3 · 2016-01-19T18:09:25.517

0

Use scanf() . It's standard, it already waits for a character in the buffer, and it's pretty much universal.

EDIT:

In the case of an unnamed OS, this is simply not to be done. Standard C has no way of reading raw data from a terminal and guaranteeing verbatim, unformatted results. It's entirely up to your terminal handler.

However, if your microcontroller has some sort of serial library, I would suggest doing something like this:

if characters in buffer is greater than 0, and the character just read is not a terminating character, report it. Otherwise, keep waiting for a character in the buffer.

edited Jan 19 '16 at 18:09

answered Jan 19 '16 at 18:01

Mason Watmough

495
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If you're using a different stream, use `fscanf()`. – Mason Watmough Jan 19 '16 at 18:01
2

If there is no input, `scanf()` will not return, but wait for input. – chux - Reinstate Monica Jan 19 '16 at 18:02
@chux I don't understand. Isn't that what the OP wants? To wait for input before it reads it? – Mason Watmough Jan 19 '16 at 18:03
OP wants to only call the input function if there is something there already: "i dont wish getch() to be called unless there is an input on the terminal ready to be read. " – chux - Reinstate Monica Jan 19 '16 at 18:05

score 0 · Answer 4 · answered Jan 19 '16 at 18:45

until a user input is entered. this sentence indicates the user will use the keyboard eventually, therefore, you can track keyboard callback event to track the user's input. There are several libraries that provide you with such keyboard events (e.g. GLUT, SDL)

Loop until user inputs an Integer, but skip the read in if no integer exists on the terminal

4 Answers4