Clean debug output function?

Question

I want to write an function that will print out error messages/warnings in my program together with the file & line number. There are these two macros in C:

__FILE__
__LINE__

but there's a problem in my opinion... When I'm writing a function like this:

#include <stdio.h>
#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x)
#define AT __FILE__ ":" TOSTRING(__LINE__)
void error(const char *location, const char *msg)
{
#ifdef DEBUG
  printf("Error at %s: %s\n", location, msg);
#endif
}
int main(int , char**)
{
  error(AT, "fake error");
  return 0;
}

There are still a lot of useless function calls & trash (the values of __FILE__ and __LINE__ at every call) in the binary file, even if I undefine DEBUG for the release build. So how can I accomplish this more elegant? I want something like this:

#include <stdio.h>
#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x)
#define AT __FILE__ ":" TOSTRING(__LINE__)
void error(const char *location, const char *msg)
{
  printf("Error at %s: %s\n", location, msg);
}
int main(int , char**)
{
#ifdef DEBUG
  error(AT, "fake error");
#endif
  return 0;
}

But not writing #ifdef DEBUG and #endif before and after every function call - that would be too huge for such an task. And remove every error(AT, "fake error"); call manually isn't really elegant as well...

Any ideas? Maybe inline the function (would not help, wouldn't it)? Some macro or an change of this construct?

Daniel Gehriger · Accepted Answer · 2011-01-19 12:16:22Z

3

Just turn error into a macro:

#ifdef DEBUG
# define ERROR(l, m) error(l, m)
#else
# define ERROR(l, m)
#endif

then, in your functions, write

ERROR(AT, "fake error");

Of course, you could also simplify ERROR and get rid of AT as the first parameter, directly specifying this information in the macro definition.

answered Jan 19, 2011 at 12:16

Daniel Gehriger

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Comments

caf · Accepted Answer · 2011-01-19 13:17:45Z

2

Wrapping error() in a macro is indeed the right way to go - I'd write it like so:

#ifdef DEBUG
static void error(const char *file, long line, const char *msg)
{
  fprintf(stderr, "Error (%s:%ld): %s\n", file, line, msg);
}
#define error(msg) (error)(__FILE__, __LINE__, msg)
#else
#define error(msg) ((void)0)
#endif

edited Jan 19, 2011 at 13:17

answered Jan 19, 2011 at 12:45

caf

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Comments

Rozuur · Accepted Answer · 2012-12-18 19:33:52Z

2

you can use this variadic macro to mimic printf which writes file and line no with desired out put.

#define eprintf(...) do {fprintf(stderr, "%s:%d: ", __FILE__, __LINE__);\
 fprintf(stderr, __VA_ARGS__);} while(0)

edit : modified eprintf from the suggestions of jcsalomon

edited Dec 18, 2012 at 19:33

answered Jan 19, 2011 at 12:30

Rozuur

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3 Comments

J. C. Salomon Over a year ago

This is for C99 or C++0x only, and would be better defined #define eprintf(...) do {fprintf(stderr, "%s:%d: ", __FILE__, __LINE__); fprintf(stderr, __VA_ARGS__)} while(0); note the statements wrapped in a do{…}while(0), and (more to the point) fprintf(stderr, __VA_ARGS__) which lets you pass a raw string without extra argument.

Rozuur Over a year ago

ya i know, tats why i wrote it will be similar to above statement.

lfxgroove Over a year ago

Just tried it and there's a missing ; after __VA_ARGS__) otherwise it's a really nice solution!

CashCow · Accepted Answer · 2011-01-19 12:17:03Z

1

#ifdef DEBUG
#define ERROR(msg) error(AT, msg)
#else
#define ERROR(msg)
#endif

int main( int argc, char * argv[] )
{
   ERROR( "fake error");
   return 0;
}

answered Jan 19, 2011 at 12:17

CashCow

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Comments

T33C · Accepted Answer · 2011-01-19 12:18:13Z

1

replace error with #define

something like:

#ifdef DEBUG
#define error(a,b) printf("Error at %s: %s\n", (a), (b))
#else
#define error(a,b) 
#endif

You could also move AT into error instead of passing it each time

answered Jan 19, 2011 at 12:18

T33C

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