(K&R, §A12.3) The C compiler can accept a # token not followed by an argument in a macro definition. For example:

#define add1(x) #x #1  // OK, but probably not what you wanted:
                       // add1(abc) creates "abc"#1
#define add2(x) #x "2" // OK: add2(abc) creates "abc2"

To turn this feature on, turn off the ANSI Strict setting in the C/C++ Language Panel. If the ANSI Strict setting is on, then this extension to standard C is disabled.

See also: "ANSI Strict."