Why scanf doesn't block the thread for input in this case? Objective-C

Question

Hi,

This must be very simple but since coming from Java world I feel a bit in the woods in this case.

while (operator != 'E') {
  NSLog(@"Enter:");
  scanf("%lf %c", &value2, &operator);

  switch (operator) {
   case 'S':
    [deskCalc setAccumulator:value2];
    break;
   case 'E':
    break;
   case '+':
    [deskCalc add:value2];
    break;
   case '-':
    [deskCalc subtract:value2];
    break;
   case '/':
    [deskCalc divide:value2];
    break;
   case '*':
    [deskCalc multiply:value2];
    break;
   default:
    NSLog(@"Unknown command");
    break;
  }

  NSLog(@"=%g", [deskCalc accumulator]);

 }

Now if I enter just 'e' or some other characters without entering the number first the program goes into loop like this:

2010-02-08 13:44:38.690 Calculator[89939:a0f] Enter:
2010-02-08 13:44:38.691 Calculator[89939:a0f] Unknown command
2010-02-08 13:44:38.691 Calculator[89939:a0f] =10
2010-02-08 13:44:38.692 Calculator[89939:a0f] Enter:
2010-02-08 13:44:38.692 Calculator[89939:a0f] Unknown command
2010-02-08 13:44:38.693 Calculator[89939:a0f] =10

Why scanf doesn't block the thread and ask for correct input again?

Thanks Noi

Answer 1

This is not an ObjC problem but a C problem.

The problem is not blocking the thread. In your scanf, you demand a floating point number (%lf) followed by some whitespaces (), and then a character (%c). But when you enter e, it is not a valid floating point number, so scanf fails immediately.

But the invalid character e is still left in the buffer. So, in the next operation with stdin, you ask for a floating point number again, and it fails and immediately return again because e is still not valid, and then the next scanf asks for a floating pointer number again, ad infinitum.

See http://stackoverflow.com/questions/1716013/scanf-causing-infinite-loop for detail.

The safer alternative is to use fgets to read the raw input, then parse the received string to get value2 and operator.