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comp.lang.c FAQ list · Question 11.17

Q: I'm trying to use the ANSI ``stringizing'' preprocessing operator `#' to insert the value of a symbolic constant into a message, but it keeps stringizing the macro's name rather than its value.

A: It turns out that the definition of # says that it's supposed to stringize a macro argument immediately, without further expanding it (if the argument happens to be the name of another macro). You can use something like the following two-step procedure to force a macro to be expanded as well as stringized:

#define Str(x) #x
#define Xstr(x) Str(x)
#define OP plus
char *opname = Xstr(OP);
This code sets opname to "plus" rather than "OP". (It works because the Xstr() macro expands its argument, and then Str() stringizes it.) [footnote]

An equivalent circumlocution is necessary with the token-pasting operator ## when the values (rather than the names) of two macros are to be concatenated.

Note that both # and ## operate only during preprocessor macro expansion. You cannot use them in normal source code, but only in macro definitions.

References: ISO Sec., Sec.

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