How to use Variadic macros with fprintf

Question

I am trying to print logs into a file by writing macros. My macro looks like as shown below:

#define LOG(fmt,...){\
    FILE *F;\
    F = fopen("output.txt","a");\
    fprintf(F,fmt " %s %d",__VA_ARGS__,__FILE__,__LINE__);}

And I plan to call LOG in the following format:

LOG("values are : %d %d",num1,num2);

But when I compile I get the error

error: expected expression before ‘,’ token
     fprintf(F,fmt " %s %d",__VA_ARGS__,__FILE__,__LINE__);}

Can someone please explain where I am going wrong?

You know `__FILE__` is a `char[]`, not a `int`, right? And unrelated to your problem, I sincerely hope there is an `fclose()` somewhere in the **real** code, as otherwise you're leaking stream pointers like a sieve leaks rain water. — WhozCraig, Oct 10 '14 at 17:45
Check out this question. Works both in C and C++. http://stackoverflow.com/questions/1056411/how-to-pass-variable-number-of-arguments-to-printf-sprintf — Niklas Hansson, Oct 10 '14 at 17:47
@WhozCraig Yeah sorry for the mistake there. There is fclose in the main program. In the hurry of copy paste missed it :-) — user1692342, Oct 10 '14 at 17:49
I know this is potentially a dumb question, but I gotta ask. your toolchain supports variadic macros, right? — WhozCraig, Oct 10 '14 at 17:51
You probably can't afford to leak open file streams at the rate that code would do so. You should check that the `fopen()` worked and close the file before you finish. Or, more sensibly, you should call a function which handles stuff like caching the open file stream and manages the formatting more simply. — Jonathan Leffler, Oct 10 '14 at 18:26
See also [C `#define` macro for debug printing](http://stackoverflow.com/questions/1644868/c-define-macro-for-debug-printing/1644898#1644898). — Jonathan Leffler, Oct 10 '14 at 18:33

score 3 · Answer 1 · edited May 23 '17 at 12:23

3

#define LOG(fmt,...){\
    FILE *F;\
    F = fopen("output.txt","a");\
    fprintf(F,fmt " %d %d",__VA_ARGS__,__FILE__,__LINE__);}

Multiple issues

You never fclose F.
__FILE__ is a string.
If you want to be able to call it without parameters __VA_ARGS__ must go at the end...

... or use this little hack:

#define LOG(fmt,...){\
    FILE *F;\
    F = fopen("output.txt","a");\
    fprintf(F,fmt " %s %d", ##__VA_ARGS__, __FILE__,__LINE__);}

(##__VA_ARGS__ is a GCC extension that removes the preceding comma if there are no args).

Also check out this answer about "custom" printf-like functions.

edited May 23 '17 at 12:23

Community

1
1

answered Oct 10 '14 at 17:52

dom0

7,356
3
28
50

1

I tried your code: THe following error occurs: error: expected expression before ‘,’ token #define VA_ARG(...) , ##__VA_ARGS__ note: in expansion of macro ‘VA_ARG’ fprintf(F,fmt " %s %d", VA_ARG(__VA_ARGS__),__FILE__,__LINE__);} – user1692342 Oct 10 '14 at 17:57
Oh yeah, I forgot. That's an GCC extension. My bad. Also there was another mistake in the code I posted, fixed now. – dom0 Oct 10 '14 at 18:06
It works If i have arguments passed :) If there are no arguments then junk values are printed :( – user1692342 Oct 10 '14 at 18:15

2501 · Accepted Answer · 2014-10-10T18:27:27.243

3

First of all you have to wrap you macro into a do-while loop, so it will be handled with expressions correctly.

#define LOG( fmt , ... ) do{  }while(0)

Then you have to make sure that fopen() call succeeds and that you close the file after usage.

FILE* f = fopen( "output.txt" , "a" ) ;
if( !f )
    break ;    //break works because you are in a loop
fclose( f ) ;    //also flushes the stream

Then you include the print in the full macro.

#define LOG( fmt , ... )    \
        do{ \
            FILE* f = fopen( "output.txt" , "a" ) ; \
            if( !f )    \
                break ; \
            fprintf(f, fmt" %s %d\n",__VA_ARGS__,__FILE__,__LINE__);    \
            fclose( f ) ;   \
        }while( 0 )

The call is in the form:

LOG("values are : %d %d",4444,55555);

where you have to input at least one correct optional parameter, with the corresponding flag in the string.

edited Oct 10 '14 at 18:27

answered Oct 10 '14 at 18:20

2501

25,460
4
47
87

1

I replaced __VA_ARGS__ with ##__VA_ARGS__ and it started working. Thanks :) I didnt understand why a loop is required. What happens when the loop isnt there? – user1692342 Oct 10 '14 at 18:23
@user1692342 You code was already correct( with some problems ), ##__VA_ARGS__ is a compiler extension. – 2501 Oct 10 '14 at 18:24
1

@user1692342 The macro might not get handled correctly. Try a couple examples with and without do-while using if statements and you will see that the code will be broken in certain cases. – 2501 Oct 10 '14 at 18:25
"##__VA_ARGS__ is a compiler extension." -- wow, good to know. I thought preprocessor behavior was more or less universal, and the environment probing was mostly there to enable platform-agnostic code for the compiler. When you say it's a compiler extension, do you mean that a preprocessed file containing `##a b c` becomes `a` during compilation, or were you using 'compiler' colloquially to mean the whole pipeline? – John P Sep 11 '17 at 01:50

How to use Variadic macros with fprintf

2 Answers2