What happens when I use the wrong format specifier?

Question

Just wondering what happens when I use the wrong format specifier in C?

For example:

x = 'A';
printf("%c\n", x);
printf("%d\n", x);

x = 65;
printf("%c\n", x);
printf("%d\n", x);

x = 128;
printf("%d\n", x);

Answer 1

what happens when I use the wrong format specifier in C?

Generally speaking, undefined behaviour.^*

However, recall that printf is a variadic function, and that the arguments to variadic functions undergo the default argument promotions. So for instance, a char is promoted to an int. So in practice, these will both give the same results:

char x = 'A';
printf("%c\n", x);

int y = 'A';
printf("%c\n", y);

whereas this is undefined behaviour:

long z = 'A';
printf("%c\n", z);

---

* See for example section 7.19.6.1 p9 of the C99 standard:

If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

Answer 2

since x is A, the first print f will print: 'A'.

The second will print the ascii value of A (look it up).

The 3rd one will print the ascii character for 65 (I think this is A or a, but its a letter).

The fourth one will print 65.

The 5th one will print 128.