How does this clear the input buffer?

Question

I found this code that clears the input buffer, but I don't really understand how it works. Can anybody explain it in a simple way?

do{
    fgets(string,LENGTH,stdin);
} while (strlen(string) > 0 && string[strlen(string) - 1] != '\n');

Answer 1

It's a misnomer to say that it "clears the input buffer". What it does is advance to the next line of input.

The code is a do/while loop. It assumes a char array called string that is at least LENGTH in size.

The body of the loop reads as much of a line as it can, up to LENGTH - 1 characters:

fgets(string,LENGTH,stdin);

The return value from fgets() is ignored - this is a bug, because it can mean that string is not assigned to in a failure case (such as EOF) and we could loop forever.

The condition of the loop checks to see whether we have read up to the end-of-line. There's a test that the string has at least one character (which it will if fgets() was successful) and then compares the last character to a newline:

while (strlen(string) > 0 && string[strlen(string) - 1] != '\n')

If we see a newline, then fgets() has read the rest of the line; if not, it read LENGTH-1 characters from a longer input line, and there's still input to consume, so continue looping.

---

Fixed version:

while (fgets(string, LENGTH, stdin) && string[strlen(string) - 1] != '\n')
    ;

Simpler alternative:

scanf("%*[^\n]%*c");

Note that in both cases, any stream error will remain, to be detected on the next attempt at input.