How do I have a comma inside braces inside a macro argument when parentheses cause a syntax error?

Question

I've defined a few macros that make it simpler to define an array of structures, but I can't find a way to use them without generating errors. Here are the macros (and a few example structures to demonstrate why the macros might be used (the actual structures I'm populating are a little more complex)):

struct string_holder {
    const char *string;
};
struct string_array_holder {
    struct string_holder *holders;
};
#define DEFINE_STRING_ARRAY_HOLDER(name, values) \
    static struct string_holder name##__array[] = values; \
    static struct string_array_holder name = { name##__array }
#define WRAP_STRING(string) { string }

It works just fine when you use it to declare an array with one item:

DEFINE_STRING_ARRAY_HOLDER(my_string_array_holder, {
    WRAP_STRING("my string")
});

But when I use multiple items:

DEFINE_STRING_ARRAY_HOLDER(my_string_array_holder, {
    WRAP_STRING("hello"),
    WRAP_STRING("world")
});

I get this error:

error: too many arguments provided to function-like macro invocation

So it's interpreting the comma in the braces as an argument separator. I follow the advice from this question and put parentheses around the problematic argument:

DEFINE_STRING_ARRAY_HOLDER(my_string_array_holder, ({
    WRAP_STRING("hello"),
    WRAP_STRING("world")
}));

Now when I try to compile it, it interprets ({ ... }) as a statement expression and complains:

warning: use of GNU statement expression extension
(a bunch of syntax errors resulting from its interpretation as a statement expression)
error: statement expression not allowed at file scope

How can I either:

Use the macro without errors (preferred), or
Rewrite the macro[s] to work in these circumstances?

How about variadic macros? See: http://stackoverflow.com/q/679979/929459 — Dmitri, Feb 08 '12 at 04:02

Kijewski · Accepted Answer · 2018-06-06T14:48:37.830

Dmitri is right, variadic macros are the way to go.

I put some example code I use to test if given key is member of a list of values:

#define _IN(KEY, ...)                                             \
({                                                                \
  typedef __typeof(KEY) _t;                                       \
  const _t _key = (KEY);                                          \
  const _t _values[] = { __VA_ARGS__ };                           \
  _Bool _r = 0;                                                   \
  unsigned int _i;                                                \
  for (_i = 0; _i < sizeof(_values) / sizeof(_values[0]); ++_i) { \
    if (_key == _values[_i]) {                                    \
      _r = 1;                                                     \
      break;                                                      \
    }                                                             \
  }                                                               \
  _r;                                                             \
})

Mind the usage of __VA_ARGS__.

Update: A crude solution if you don't like __VA_ARGS__ at arbitrary places would be an "unwrapper" macro:

#define UNWRAP(...) __VA_ARGS__

You could use it like a prefix-operator. ;-)

#include <stdio.h>

/* "unwrapper": */
#define UNWRAP(...) __VA_ARGS__

/* your macros: */
#define WRAP(NAME, ELEMS) static const char *NAME[] = { UNWRAP ELEMS }

int main(void) {
  WRAP(some_test, ("a", "b", "c"));
  printf("The second elem in some_test is: '%s'\n", some_test[1]);
  return 0;
}

Actually, that `UNWRAP` thing is neat. Thanks! – icktoofay Feb 08 '12 at 04:38 — icktoofay, Feb 08 '12 at 04:38

score 1 · Answer 2 · edited May 23 '17 at 11:45

Yes, use __VA_ARGS__, but Kay's solution is much too complicated. Just doing:

#define DEFINE_STRING_ARRAY_HOLDER(name, ...)                 \
    static struct string_holder name##_array[] = __VA_ARGS__; \
    static struct string_array_holder name = { name##_array }

suffices. You may then just use this macro as you intended with:

DEFINE_STRING_ARRAY_HOLDER(my_string_array_holder, {
    WRAP_STRING("hello"),
    WRAP_STRING("world")
});

How do I have a comma inside braces inside a macro argument when parentheses cause a syntax error?

2 Answers2