Can't clear the stdin using fflush(stdin), after using getchar(), in an infinite for loop C prog

Question

I have just started off with C programming and while I was trying to write a programme to accept only y or n characters I came across that

#include <stdio.h>
#include <stdlib.h>

int main()
{
  char ch;
  printf("Do you want to continue\n");

  for (;;)
    {
      ch=getchar();
      if (ch=='Y' || ch=='y')
        {
            printf("Sure!\n");
            break;
        }
        else if (ch=='N'||ch=='n')
        {
            printf("Alright! All the best!\n");
            break;
        }
        else
        {
            printf("You need to say either Yes/No\n");
            fflush(stdin);
        }

    }
    return(0);
}

When I run this code, and type in any other character other than Y/y or N/n, I receive the last printf statement (You need to say either Yes/No) as output twice. I understand that this is happening because it considers enter, i.e, '\n' as another character. Using fflush doesn't help as it's an infinite loop. How else can I modify it so that the last statement is displayed only once?

Answer 1

You can use a loop to read any characters left using getchar():

  ch=getchar();
  int t;
  while ( (t=getchar())!='\n' && t!=EOF );

The type of ch should int as getchar() returns an int. You should also check if ch is EOF.

fflush(stdin) is undefined behaviour per C standard. Though, it's defined for certain platforms/compilers such as Linux and MSVC, you should avoid it in any portable code.