ipkiss ipkiss - 2 months ago 7
C Question

How to clear 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 have explained:
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.

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

Next, the author proposed 2 ways to solve such probrems:


  1. use
    fflush()

  2. write a function like this:

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



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

Answer

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):

The 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?

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