What happens?

[KBriggs]

#include <stdio.h>

int main()
{
  int x;
  do
  {
    scanf("%d",&x);
    printf("The number is %d\n",x);
  } while (x != 0);
}

This is just a really simple do-while loop. What I am curious about is what happens when you enter a non-integer number. For example if you enter a character, say 'r' from the keyboard. If you do this at the first call, it prints

The number is 0

and then exits, which just means that x happened to be 0 already. If x wasn't 0, or if you enter a valid number first, say 4 on the first iteration, and then 'r' on the next, it will go into an infinite loop and print

 
The number is 4
The number is 4
The number is 4
The number is 4
The number is 4
.
.
.

forever.

What exactly is happening? Why does it skip the scanf and not allow another entry when a non-integer character is input? I suspect that I am in undefined behaviour land here, but does anyone know?

[sean]

scanf reads data in from the operating system's input buffer. When you enter 4, it reads the 4 and passed it to your program, but the 4 is still in your input buffer, so the next time you call scanf, it doesn't wait for you to enter anything - to scanf it looks like you just entered '4'. You'll want to read up on how to flush the buffer (the FAQ entry on this site is pretty good). It's not exactly "undefined" - I'd say it's more OS-dependant. You won't find a standard solution that works across the board.

[flexo87]

You should also check the value that scanf returns. It in fact tells you how many parameters have been correctly read. if you put 'x' and your scanf waits for an integer (%d) it will return 0, so if you check scanf result you can avoid some strange behavior.

[BEN10]

scanf reads data in from the operating system's input buffer. When you enter 4, it reads the 4 and passed it to your program, but the 4 is still in your input buffer, so the next time you call scanf, it doesn't wait for you to enter anything - to scanf it looks like you just entered '4'. You'll want to read up on how to flush the buffer (the FAQ entry on this site is pretty good). It's not exactly "undefined" - I'd say it's more OS-dependant. You won't find a standard solution that works across the board.

What you are saying here, I think, is not applicable in this case. For eg. if I input 4, then according to you 4 should be in the buffer and it should continuosly print 4(as the call to next scanf also reads it), but what's happening here is it asks for the next input and so on until we enter a 0. The main problem is when we enter an integer and after that any character then it prints the integer indefinitely(i.e it is printing the last correct input indefinitely).
I actually myself is confused what is happening in this case but to avoid this thing what flexo has advised is better. There should be an error check whether the input is actually an integer or not.

[BEN10]

This is how I've modified the code.

#include <stdio.h>
int main()
{
  int x,i;
  do
  {
    i=scanf("%d",&x);
	if(i==1)
    printf("The number is %d\n",x);
	else
		break;
  } while (x != 0);
}

[KBriggs]

Thanks, but I know how to do most of that - this isn't a coding problem, just my own curiosity. My big question is this: after entering a non-integer, why does scanf not pause execution anymore until I enter a new entry?

[bithub]

The problem is that scanf() is failing to parse out any characters from the input buffer. It leaves those characters in the input buffer which is what is causing your infinite loop. This is why it is a good idea to check the return value of scanf().

[KBriggs]

Cool, thanks This kindof thing keeps cropping up in my codes when I try to use do-while loops to get input - ill use that check from now on.

[KBriggs]

Sorry to double post, but this actually came up now. How do you flush the standard input? fflush(stdin) is undefined is it not?

[Elysia]

scanf reads data in from the operating system's input buffer. When you enter 4, it reads the 4 and passed it to your program, but the 4 is still in your input buffer, so the next time you call scanf, it doesn't wait for you to enter anything - to scanf it looks like you just entered '4'. You'll want to read up on how to flush the buffer (the FAQ entry on this site is pretty good). It's not exactly "undefined" - I'd say it's more OS-dependant. You won't find a standard solution that works across the board.

But there is a standard solution.

Sorry to double post, but this actually came up now. How do you flush the standard input? fflush(stdin) is undefined is it not?

Flushing buffer
One of many examples.
And yes, fflush(stdin) is undefined.