Clearing the buffer when using Getchar (there must be a better way!)

Question

I am writing a function where I only need to obtain a single digit, I decided to use getchar() instead of scanf().

However since I only need one character I did not use an array to obtain it. This causes a problem because I dont have a loop to find the '/n' character. To make sure my buffer is cleared before using getchar() again, I wrote this:

while (getchar() != '\n'){}

It works as I need it to, but is this the "correct" way to go about this problem?

I tried using fflush(stdin), but it seems to not have any effect.

Thank you for all replies, I check the comments often and would love to answer questions.

The `while` runs into a non-terminating loop when there's no newline (e.g. on End Of File). — Jens, May 13 '17 at 14:16
@Jens By checking the value returned by the call of getchar. — Vlad from Moscow, May 13 '17 at 14:41
@VladfromMoscow Right; and here it is only checked for being a newline. Can you see how this can be a non-terminating loop? Hint: what happens when stdin is "foobar"? — Jens, May 13 '17 at 14:43

score 3 · Answer 1 · answered May 13 '17 at 14:25

Note that fflush(stdin) is undefined behavior according to the C Standard, though it is supported by a few implementations.

The idiomatic way to do this is:

int c;
while ((c = getchar()) != '\n' && c != EOF) {
    continue;
}

The check for EOF is needed to avoid an infinite loop in the event that there is an error in getchar(), or if the user signals EOF from the keyboard. The variable c needs to be an int to ensure that it can hold the value of EOF, which is typically -1.

Also be aware that this method requires that there be at least one character left in the input stream, otherwise getchar() will block, waiting for input. A previous call to getchar() or scanf() will leave at least a newline behind, but calls to fgets() may not.

Clearing the buffer when using Getchar (there must be a better way!)

1 Answers1