How to clear input buffer in C?

The program will not work properly because at Line 1, when the user presses Enter, it will leave in the input buffer 2 character: Enter key (ASCII code 13) and \n (ASCII code 10). Therefore, at Line 2, it will read the \n and will not wait for the user to enter a character.

The behavior you see at line 2 is correct, but that's not quite the correct explanation. With text-mode streams, it doesn't matter what line-endings your platform uses (whether carriage return (0x0D) + linefeed (0x0A), a bare CR, or a bare LF). The C runtime library will take care of that for you: your program will see just '\n' for newlines.

If you typed a character and pressed enter, then that input character would be read by line 1, and then '\n' would be read by line 2. See I'm using scanf %c to read a Y/N response, but later input gets skipped. from the comp.lang.c FAQ.

As for the proposed solutions, see (again from the comp.lang.c FAQ):

• How can I flush pending input so that a user's typeahead isn't read at the next prompt? Will fflush(stdin) work?
• If fflush won't work, what can I use to flush input?

which basically state that the only portable approach is to do:

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

Your getchar() != '\n' loop works because once you call getchar(), the returned character already has been removed from the input stream.

Also, I feel obligated to discourage you from using scanf entirely: Why does everyone say not to use scanf? What should I use instead?

You can do it (also) this way:

fseek(stdin,0,SEEK_END);

A portable way to clear up to the end of a line that you've already tried to read partially is:

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

This reads and discards characters until it gets \n which signals the end of the file. It also checks against EOF in case the input stream gets closed before the end of the line. The type of c must be int (or larger) in order to be able to hold the value EOF.

There is no portable way to find out if there are any more lines after the current line (if there aren't, then getchar will block for input).