Getting error while making debug print macro in c

Question

Code written as follows,

#define print_err(fmt, ...) \
    do { if (DEBUG_ERR) fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, \
                            __LINE__, __func__, ##__VA_ARGS__); } while (0)

I am getting error, if not define the DEBUG_ERR macro. Compilation was done using gcc 5.4.0. There is no problem if the macro DEBUG_ERR is defined.

include/tmp.h:54:18: error: ‘DEBUG_ERR’ undeclared (first use in this function)
         do { if (DEBUG_ERR) fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, \*

See [C `#define` macro for debug printing](https://stackoverflow.com/questions/1644868/c-define-macro-for-debug-printing) — Jonathan Leffler, Aug 02 '17 at 07:12
`fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, __LINE__, __func__, ##__VA_ARGS__);` is not very flexible nor is it robust. The macro `#define(fmt,...)` can only accept string literals for the format string. It will fail to compile should it be passed a `char *` or `char[]` used to construct a format string at run-time. — Andrew Henle, Aug 02 '17 at 08:58

score 2 · Accepted Answer · answered Aug 02 '17 at 06:46

The if instruction inside your macro is a C instruction, not a preprocessor instruction (you can't nest preprocessor instructions)

So DEBUG_ERR must be defined even if debug must not be activated. Set it to 0 and hopefully the compiler wil optimize your statement out.

What you could do instead is test the DEBUG_ERR macro outside and define empty macro if not defined:

#ifdef DEBUG_ERR
#define print_err(fmt, ...) \ do { fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, \ __LINE__, __func__, ##__VA_ARGS__); } while (0)
#else
#define print_err(fmt, ...) do {} while(0)
#endif

score 0 · Answer 2 · answered Aug 02 '17 at 06:47

What you can do also is define DEBUG_ERR as another macro or not. Then you can get rid of the if in print_err

#ifdef DEBUG_ERR
#define print_err(fmt, ...) \
    do { fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, \
         __LINE__, __func__, ##__VA_ARGS__); } while (0)
#else
#define print_err(fmt, ...)
#endif

score 0 · Answer 3 · answered Aug 02 '17 at 06:55

To make a conditional on the definition/nondefinition of a preprocessor #define, you have to use #ifdef or #if defined.

See, for example, this description of #if defined

If you want to use it in a code conditional, you can define it as 0 or 1 and then you can use it in an "if".

So you can do either:

#ifdef DEBUG_ERR
#define print_err(fmt, ...) \
            do {  fprintf(stderr, "%s:%d:%s(): " fmt, __FILE__, \
                           __LINE__, __func__, ##__VA_ARGS__); } while (0)
#else
#define print_err(fmt, ...)
#endif

Or do as you have done but define DEBUG_ERR to 1 or 0 instead of defining it/leaving it undefined.

I have generally used the first option.

Getting error while making debug print macro in c

3 Answers3