multi file C program, how best to implement optional logging?

Question

I have a multi file C program. I\'d like the user to be able to specify different debugging levels at run time.

What is the best way to implement this?

I was thi

Answer 1

Jonathan's suggestion is good but since C99 we have variadic macros so one doesn't need to use double braces for debug macro.

There is a light version of logging header I use:

#define LOG_FATAL    (1)
#define LOG_ERR      (2)
#define LOG_WARN     (3)
#define LOG_INFO     (4)
#define LOG_DBG      (5)

#define LOG(level, ...) do {  \
                            if (level <= debug_level) { \
                                fprintf(dbgstream,"%s:%d:", __FILE__, __LINE__); \
                                fprintf(dbgstream, __VA_ARGS__); \
                                fprintf(dbgstream, "\n"); \
                                fflush(dbgstream); \
                            } \
                        } while (0)
extern FILE *dbgstream;
extern int  debug_level;

So wherever I need to log something, I just add line like

LOG(LOG_ERR, "I/O error %s occured while opening file %s", strerror(errno), filename);

During program initialisation you need to specify values for dbgstream (usually defaults to stderr) and debug_level.

For real projects instead of calling fprintf many times I just call my function from LOG macro and pass __FILE__, __LINE__ and __VA_ARGS_ as argument - that function also prints date, time and pid in log line, and don't do fflush() every time - only when buffering counter exceeds preset value - itsignificantly increases logging performance.

But please be aware that some compilers may not support variadic macros as it was introduced only in C99.