referrence:

http://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html#Argument-Prescan

Procedure

macro_expand (MACRO(ARGV))
{
	if ((MACRO contain "#") || (MACRO contain "##"))
	{
		stringified ARGV || ARGV pasted with other tokens
	}
	else if ( ARGV is still an macro, which ARGV == NEW_MACRO (NEW_ARGV))
	{
		/* 
		that is to say macro_expand (ARGV) 
		ARGV == NEW_MACRO (NEW_ARGV)
		*/
		macro_expand (NEW_MACRO (NEW_ARGV))
	}
	else /* ARGV is a plain argument */ 
	{
		expand the MACRO with argument ARGV
	}
}

three examples

First example:

	#define AFTERX(x) X_ ## x
	#define XAFTERX(x) AFTERX(x)
	#define TABLESIZE 1024
	#define BUFSIZE TABLESIZE
then AFTERX(BUFSIZE) expands to X_BUFSIZE, 
and XAFTERX(BUFSIZE) expands to X_1024. (Not to X_TABLESIZE. Prescan always does a complete expansion.)

Second example:

If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.

	#define xstr(s) str(s)
	#define str(s) #s
	#define foo 4
	str (foo)
		==> "foo"
	xstr (foo)
		==> xstr (4)
		==> str (4)
		==> "4"

s is stringified when it is used in str, so it is not macro-expanded first. 
But s is an ordinary argument to xstr, so it is completely macro-expanded before xstr itself is expanded (see Argument Prescan). 
Therefore, by the time str gets to its argument, it has already been macro-expanded.

Third example:

#define a(x) b(x) + 1
#define b(x) c(x)
#define c(x)  #x
#define TABLE SIZE + 2
#define SIZE  5
 

/* right */
a(TABLE)
	==> a(SIZE + 2)
	==> a(5 + 2)
	==> b(5 + 2) + 1
	==> c(5 + 2) + 1
	==> "5 + 2" + 1
	
/* wrong */
a(TABLE) 
	==> b(TABLE) + 1
	==> c(TABLE) + 1
	==> "TABLE" + 1