C puzzle...How Can I pass variadic arguments into a macro?

Question

I got stuck here...

#include <stdio.h>

#define DBG_LVL(lvl, stmt) \
do{ \
    if(lvl>1)  printf stmt; \
}while(0)

#define DBG_INFO(stmt)   DBG_LVL(1, stmt)
#define DBG_ERROR(stmt)  DBG_LVL(2, stmt)


int main()
{
    DBG_INFO(("hello, %s!\n", "world"));
    DBG_ERROR(("crazy, %s!\n", "world"));
    return 0;
}

As you can see, the code above uses macros like "DBG_INFO" or "DBG_ERROR" to control debug information level.

Now for some reason, I have to replace DBG_LVL() with a new function.

void myprint(int lvl, const char * format, ...);

The only difference is the debug level is taken as its fisrt parameter. I was thinking:

#define DBG_LVL(lvl, stmt) myprint(lvl, stmt)

Of course it failed, because the "stmt" expression includes parentheses around. Then I googled around trying to find a way to strip the parentheses, seems there's nothing could help. I also tried some tricks to pass parameters into "stmt", still failed... :(

Can you help me?

Always google for what you want, not what you think you want. Same goes for asking people questions. — Matt Joiner
– Matt Joiner, Commented Aug 26, 2011 at 23:45
Thanks. You are right. Maybe I should not put in too much subjective opinions when asking questions... — kumo
– kumo, Commented Aug 27, 2011 at 0:09

James McNellis · Accepted Answer · 2011-08-26 23:40:13Z

10

# define EXPAND_ARGS(...) __VA_ARGS__
# define DBG_LVL(lvl, stmt) myprint(lvl, EXPAND_ARGS stmt);

answered Aug 26, 2011 at 23:40

James McNellis

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1 Comment

kumo Over a year ago

Yes! It works. This trick is awesome! actually I studied _VA_ARGS_ for a while, but i did't notice it can be used this way... even worse, i tried #define xx((stmt)) stmt, haha, compiler complained a lot.....

bdonlan · Accepted Answer · 2011-08-26 23:41:26Z

2

Don't write this as a macro.

Write instead an ordinary varargs function:

void DBG_LVL(int level, char *fmt, ...)
{
    if (level < 1) return;

    va_list args;
    va_start(args, fmt);

    vaprintf(fmt, args);

    va_end(args);
}

For myprint(), define a similar vamyprint(int lvl, const char *format, va_list ap) as well, and forward the same way.

answered Aug 26, 2011 at 23:41

bdonlan

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1 Comment

kumo Over a year ago

Yes, that's one way. Thanks. :) but there's some difficulties, because these macros "DBG_INFO" "DBG_ERROR" were widely used in projects, over 20 thousands files... I can't just update all these projects for this reason.

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