CODEWITHSUNDEEP
cmacros
twfx
twfx

Reputation: 1694

macro definition

I tried to define a macro functioned as below. Call 1 has no problem, but Call 2 prompted compiler error because 3rd argument is not available. How to define a macro which support both call 1 and call 2? 

#define RDF_LOG(dbglevel, fmt, ...) (rdfDBG(dbglevel, " " fmt, __VA_ARGS__))
void rdfDBG(int dbglevel, const char *fmt, ...) { /* printf debug message */ }

RDF_LOG(kERROR, "Fail to open file %s\n", pinfile); /* Call 1 */
RDF_LOG(kERROR, "Insufficient Memory\n"); /* call 2 , compiler -> error: expected expression before ')' token */

Upvotes: 3

Views: 1735

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 754870

GCC Extensions

GCC has an extension to handle that (note the missing comma before the ...):

Incorrect (references __VA_ARGS__ which is not allowed in the GCC extension):

#define RDF_LOG(dbglevel, fmt ...) (rdfDBG(dbglevel, " " fmt, __VA_ARGS__))

Correct (not referencing __VA_ARGS__):

#define RDF_LOG(dbglevel, fmt...) (rdfDBG(dbglevel, " " fmt))
void rdfDBG(int dbglevel, const char *fmt, ...) { /* printf debug message */ }
enum { kERROR };

void x(const char *pinfile);
void x(const char *pinfile)
{
    RDF_LOG(kERROR, "Fail to open file %s\n", pinfile);
    RDF_LOG(kERROR, "Insufficient Memory\n");
}

You can tell I don't use the GCC extension - because I use some compilers that are not GCC.

There is also the second (GCC-specific) mechanism mentioned by Adam in his comment:

#define RDF_LOG(dbglevel, fmt, ...) (rdfDBG(dbglevel, " " fmt, ## __VA_ARGS__))
void rdfDBG(int dbglevel, const char *fmt, ...) { /* printf debug message */ }
enum { kERROR };

void x(const char *pinfile);
void x(const char *pinfile)
{
    RDF_LOG(kERROR, "Fail to open file %s\n", pinfile);
    RDF_LOG(kERROR, "Insufficient Memory\n");
}

Standard C99

Failing that, you have to use the C99 standard mechanism:

#define RDF_LOG(dbglevel, ...) (rdfDBG(dbglevel, " " __VA_ARGS__))
void rdfDBG(int dbglevel, const char *fmt, ...) { /* printf debug message */ }

RDF_LOG(kERROR, "Fail to open file %s\n", pinfile); /* Call 1 */
RDF_LOG(kERROR, "Insufficient Memory\n");

This basically cheats or circumvents the problem for this context. In the general case, C99 requires a comma and at least one argument.

Upvotes: 2

Keith Thompson
Keith Thompson

Reputation: 263617

You're getting an extra comma in the second macro expansion, because you have an unconditional comma after fmt in the macro definition.

Dropping the fmt parameter from the macro definition seems to fix the problem; the format string then becomes part of __VA_ARGS__:

#define RDF_LOG(dbglevel, ...) (rdfDBG(dbglevel, " " __VA_ARGS__))
void rdfDBG(int dbglevel, const char *fmt, ...) { /* printf debug message */ }

RDF_LOG(kERROR, "Fail to open file %s\n", pinfile); /* Call 1 */
RDF_LOG(kERROR, "Insufficient Memory\n");

This expands to:

void rdfDBG(int dbglevel, const char *fmt, ...) { }

(rdfDBG(kERROR, " " "Fail to open file %s\n", pinfile));
(rdfDBG(kERROR, " " "Insufficient Memory\n"));

Incidentally, it looks like the " " is intended to require the format to be a string literal (and my modified version preserves this). Are you sure you want to do that? Though it's rare, it can be useful to have a non-literal format string.

Upvotes: 4

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