What does scanf do to the value it reads?

Question

Picture this: single variable, and scanf is set to expect a value of type int.

What happens internally when, say, a character is entered?

Does scanf just ignore it, or run a check on whether its a number - how does it know?

Answer 1

A lot can go wrong when reading from stdin, check the "Return Value" section in the cplusplus docs: http://www.cplusplus.com/reference/cstdio/scanf/

We have the feof and ferror functions, checking the return value for EOF, checking the errno variable for EILSEQ.
It's a can of worms.

Anyway in your case: Nothing will be read. `scanf` will tell you how many characters it read, so you can check that for 0.

int a = 1234;
int n = scanf("%d", &a);
if(n == 0) {
printf("You did an oopsie!\n.");
// a is still 1234 here, it wasn't touched
}

And yes, scanf runs checks - if you check the docs I linked above you can see which format specifier allows which inputs exactly!