What does "#" mean before a macro?

Question

I need some explanation on this code

#define TRACE_PRINT5 (F, X)   fprintf(stderr, "%s" STRGY(__LINE__) ":(" #X "): ", X);

I know what does # on beginning, but I don't understand what does on ":(" #X"): "

Answer 1

This is called "stringizing"; When you add #, the macro will treat the argument like a string (e.g. getting the name of the thing passed in, instead of the value.)

So for example, if you write:

const char* someString = "abc";
TRACE_PRINT5(something, someString); // let's assume this is on line 10 for __LINE__

That should expand to:

const char* someString = "abc";
fprintf(stderr, "%s" "10" ":(" "someString" "): ", "abc");

(Assuming the STRGY macro just converts its argument to a string.)

Answer 2

When you put # before something in a macro it gets the value as string literal.

In your case takes X as a string literal. This means that if you pass the variable age with type int it will pass put "age" on the place of #X