# and ## macros in C [duplicate]

Answer 1

# is the "stringizing" operator; it turns its argument into a string literal.

## is the "token-pasting" operator; it joins its two arguments into a single token, not necessarily a string literal.

An example:

#include <stdio.h>

#define foo(m, n) m ## n

int main(void) {
    char *kl = "token pasting";
    printf("%s\n", foo(k, l));
}

which prints:

token pasting