Preprocessor and subsitution of format string

Question

If I put:

#define SIZE 10

and after:

scanf("%SIZEs", s);

I have a runtime error. The preprocessor should substitute SIZE with 10 before to compile, so this should equal (in my opinion) to write

scanf("%10s", s);

so, where is the mistake?

Answer 1

It is not possible to replace the contents of the string literal by macro.
For example, it does the following:

#include <stdio.h>

#define S_(x) #x
#define S(x) S_(x)

#define SIZE 10

int main(void){
    char s[SIZE + 1];
    scanf("%" S(SIZE) "s", s);
    puts(s);
    return 0;
}

Answer 2

In your code, the problem is in

  scanf("%SIZEs", s);

Anything residing between the quotes " " of a format string (string literal, in general) will not get substituted through MACRO substitution.

So, your scanf() remains the same as you're written, after preprocessing, and %S (or, %SIZEs, in whole) being one invalid format specifier, you get into undefined behaviour and hence got the error.

You can try the workaround as

 scanf("%"SIZE"s", s);

This way, SIZE will be outside the quotes and will be considered for substitution in preprocessing stage.