what is the output of this and explain how it works??

Question

#include <stdio.h>
#define TO_STR(x) #x
int main() {
printf("%s\n"TO_STR(123\\45));
return 0;
}

Answer 1

Stringizing Operator #
What comes after # will be converted into a string, it means it will be enclosed in double quotes.

So TO_STR(123\\45) will be replaced by "123\\45"
And ends up like this:
printf("%s\n" "123\\45");

Adjacent strings are concatenated so its the same as
printf("%s\n123\\45");

But it wont compile since %s specifier requires an argument which is not provided.

Answer 2

i think the preprocessor substitute every occurrence of TO_STR(x) with #x
So your program actually runs:
printf("%s\n"#123\\45);
However I never saw a #something into a printf, so I don't know exactly what is going to happen