In that case it's just to convert the value of a macro into a string constant cat > a.c #define X(s) #s #define S(s) X(s) char *f(void) return { S(

In that case it's just to convert the value of a macro into a string constant cat > a.c #define X(s) #s #define S(s) X(s) char *f(void) return { S(A); } ^D cc -E a.c char *f(void) return { "A"; } cc -DA=B -E a.c char *f(void) return { "B"; } More interestingly, you can use this trick to create code where some user-specified word appears as a string and as the name of the function. Exercice: write a macro M(x) such that compiling the code M(foo) M(bar) results in void print_foo(void) { puts("foo"); } voir print_bar(void) { puts("bar"); }