Years ago, I found a macro substitution issue when I was using stringify macro. See the exacmple code listed as below:

#include <stdio.h>

#define HAHA lala
#define _(x) #x
#define TEST _(HAHA)

int main(int argc, char *argv[])
{ 
	printf("%s\n", TEST);
	return 0;
}

What I expected the output is "lala". But I got "HAHA". Why the HAHA was not substituted in the TEST macro ?

So that I wrote an another example:

#define <stdio.h>

#define HAHA lala
#define _(x) #x
#define STR(x) _(x)
#define TEST STR(HAHA)

int main(int argc, char *argv[])
{
	printf("%s\n", TEST);
	return 0;
}

Finally, the output is "lala". I did not understand util I found this document

Argument Prescan

Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringified or pasted with other tokens.

That is, if we defined a macro with argument(s), the argument will be expanded before substitution if this argument is an macro itself. One of the exceptions is the argument is used in # or ##.