When do we need to clear the scanf buffer?

Question

I always thought that the "'\n' in buffer" issue only occurs when we are reading characters, however, I stumbled upon this issue with the following code:

int main(int argc, char** argv){
    int height = 0, width = 0;

    while(height < 1 || height > 10|| width < 1 || width > 15){
        printf("Please insert the height(1~10) and width(1~15) of the parallelogram (integers): ");
        if(scanf("%d %d", &height, &width) != 2){
            height = width = 0;
        }
    }
    return 0;
}

As above, I'm only reading integers with scanf, but this piece of code still gets stuck in an infinite loop when I input something invalid. It's fixed if I clear up the buffer.

So my question is, is this "'\n' in buffer" issue a general thing? Or does it only happen for special usages? If it only happens for special usages, is there some general guideline I need to follow?

Answer 1

The general guideline is to not use *scanf() for user input. Your capabilities to recover gracefully from ill-formatted input are just too limited, the chance of errors too high (as can be seen by the sheer amounts of *scanf()-related questions here on SO). The *scanf() function family is best used for reading well-formatted input only (i.e. data that was previously written by your own application).

User input is line-based anyway, at least if you're relying on standard input functions.

So use fgets() to read a full line of input, then parse it in-memory. Functions like strtol() or strtod() can give very specific feedback at which point exactly they stopped parsing, you can skip back and try a different parse, you have all the string-handling functions of the standard at your disposal to pick apart your user's input. And if things go pear-shaped, you can repeat the whole line of input in your error message, adding whatever information on your parsing attempts you like.