During C macro expansion, is there a special case for macros that would expand to "/*"?

Question

Here's a relevant example. It's obviously not valid C, but I'm just dealing with the preprocessor here, so the code doesn't actually have to compile.

#define IDENTITY(x) x
#define PREPEND_ASTERISK(x) *x
#define PREPEND_SLASH(x) /x

IDENTITY(literal)
PREPEND_ASTERISK(literal)
PREPEND_SLASH(literal)
IDENTITY(*pointer)
PREPEND_ASTERISK(*pointer)
PREPEND_SLASH(*pointer)

Running gcc's preprocessor on it:

gcc -std=c99 -E macrotest.c

This yields:

(...)

literal
*literal
/literal
*pointer
**pointer
/ *pointer

Please note the extra space in the last line.

This looks like a feature to prevent macros from expanding to "/*" to me, which I'm sure is well-intentioned. But at a glance, I couldn't find anything pertaining to this behaviour in the C99 standard. Then again, I'm inexperienced at C. Can someone shed some light on this? Where is this specified? I would guess that a compiler adhering to C99 should not just insert extra spaces during macro expansion just because it would probably prevent programming mistakes.

Answer 1

The source code is already tokenized before being processed by CPP.

So what you have is a / and a * token that will not be combined implicitly to a /* "token" ( since /* is not really a preprocessor token I put it in "").

If you use -E to output preprocessed source CPP needs to insert a space in order to avoid /* being read by a subsequent compiler pass.

The same feature prevents from two e.g. + signs from different macros being combined into a ++ token on output.

The only way to really paste two preprocessor tokens together is with the ## operator:

#define P(x,y) x##y

...

P(foo,bar)   

results in the token foobar

P(+,+)

results in the token ++, but

P(/,*)       

is not valid since /* is not a valid preprocessor token.

Answer 2

The behavior of the pre-processor is standardized. In the summary at http://en.wikipedia.org/wiki/C_preprocessor , the results you are observing are the effect of:

"3: Tokenization - The preprocessor breaks the result into preprocessing tokens and whitespace. It replaces comments with whitespace".

This takes place before:

"4: Macro Expansion and Directive Handling".