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cmacrosc-preprocessorvariadic-macrosstringification
Marco Scannadinari
Marco Scannadinari

Reputation: 1884

Error when defining a stringising macro with __VA_ARGS__

I have been trying to implement a function macro in C that prepends "DEBUG: ", to the argument, and passes its arguments to printf:

#define DBG(format, ...) printf("DEBUG: " #format "\n", __VA_ARGS__)

This gives me this error in gcc:

src/include/debug.h:4:70: error: expected expression before ‘)’ token
#define DBG(format, ...) printf("DEBUG: " #format "\n", __VA_ARGS__)
                                                                   ^

Supposedly, it should stringise format, and pass its variable arguments to printf, but so far I can't get past this error.

EDIT

After giving up on stringising arguments, and double-hashing (##) __VA_ARGS__ I now have this error:

src/lib/cmdlineutils.c: In function ‘version’:
src/lib/cmdlineutils.c:56:17: warning: ISO C99 requires rest arguments to be used [enabled by default]
  DBG("version()");

Should I be placing a comma after the argument?

DBG("version()",);  // ?

For reference, DBG() now looks like this:

#define DBG(format, ...) printf("DEBUG: " format "\n", ##__VA_ARGS__)

Upvotes: 13

Views: 15916

Answers (4)

baqyoteto
baqyoteto

Reputation: 344

For C toolchains that support C++20 macros, __VA_OPT__ is a more portable solution.

#define DBG(format, ...) printf("DEBUG: " #format "\n" __VA_OPT__(,) __VA_ARGS__)

NOTE: There is no comma between "\n" and __VA_OPT__

From the GNU GCC manual:

C++20 introduces the VA_OPT function macro. This macro may only appear in the definition of a variadic macro. If the variable argument has any tokens, then a VA_OPT invocation expands to its argument; but if the variable argument does not have any tokens, the VA_OPT expands to nothing:

#define eprintf(format, ...) fprintf (stderr, format __VA_OPT__(,) __VA_ARGS__)

VA_OPT is also available in GNU C and GNU C++.

Upvotes: 0

stdcall
stdcall

Reputation: 28900

Why do you need to stringise format, it can stay as is, just treat it as a string when using the macro.

The error, as cnicutar propsed can be solved with adding '##' before the VA_ARGS

#define DBG(format, ...) printf("DEBUG: " format "\n", ##__VA_ARGS__)

Usage example:

DBG("%d - %s", a,b);

Upvotes: 0

cnicutar
cnicutar

Reputation: 182714

This happens unless there's at least one variable argument. You can try this GNU extension to fix it:

#define DBG(format, ...) printf("DEBUG: " #format "\n", ##__VA_ARGS__)
                                                        ^^

As explained in the GNU doc:

[if] the variable argument is left out when the macro is used, then the comma before the ‘##’ will be deleted.

Upvotes: 24

Will Custode
Will Custode

Reputation: 4614

Check out this on MSDN. It contains info on Variadic Macros, which is what you are using.

Upvotes: -1

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