How can I clear an input buffer in C?

I have the following program:

int main(int argc, char *argv[])
{
    char ch1, ch2;
    printf("Input the first character:"); // Line 1
    scanf("%c", &ch1);
    printf("Input the second character:"); // Line 2
    ch2 = getchar();

    printf("ch1=%c, ASCII code = %d\n", ch1, ch1);
    printf("ch2=%c, ASCII code = %d\n", ch2, ch2);

    system("PAUSE");
    return 0;
}

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

OK, I got this. But my first question is: Why doesn't the second getchar() (ch2 = getchar();) read the Enter key (13), rather than the \n character?

Next, the author proposed two ways to solve such problems:

1. use fflush()

2. write a function like this:

void
clear (void)
{
    while ( getchar() != '\n' );
}

This code worked actually. But I cannot explain how it works. Because in the while statement, we use getchar() != '\n', that means read any single character except '\n'? If so, does the '\n' character still remain in the input buffer?

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?