Modeling With Data

Tip 25: Variadic macros

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level: not hard
purpose: delightful tricks to follow in the coming week

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

I broadly consider variable-length functions in C to be broken--I have a personal rules about the three forms I'm willing to use, which I'll maybe expound upon later.

But variable-length macro arguments are easy. The keyword is __VA_ARGS__, and it expands to whatever set of elements were given.

For example, here's a fast way to implement a customized variant of printf for reporting errors:

#define print_a_warning(...) {\
    printf("Glitch detected in %s at line %s:%i: ", __FUNCTION__, __FILE__, __LINE__); \
    printf(__VA_ARGS__); }

//usage:
print_a_warning("x has value %g, but it should be between zero and one.\n", x);

The __FUNCTION__, __FILE__, and __LINE__ macros get filled in with what you'd expect.

You can probably guess how the ellipsis (...) and __VA_ARGS__ work: whatever is between the parens gets plugged in at the __VA_ARGS__ mark.

You can have arguments before the ellipsis if you want. A full example:

#define print_a_warning(dostop, ...) { \
    printf("Glitch detected in %s at line %s:%i: ", __FUNCTION__, __FILE__, __LINE__);\
    printf(__VA_ARGS__);    \
    if (dostop=='s') abort(); \
}

int main(int argc, char ** argv){
    if (argc <= 1) print_a_warning('s', "argc has value %i, but it should be two. Halting. \n", argc);
    if (argc > 2) print_a_warning('c', "argc has value %i, but it should be two. Using ony the first arg. \n", argc);

    printf("arg one = %s\n", argv[1]);
}

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