How can I generate unique values in the C preprocessor?

I'm writing a bunch of related preprocessor macros, one of which generates labels which the other one jumps to. I use them in this fashion:

MAKE_FUNNY_JUMPING_LOOP(
  MAKE_LABEL();
  MAKE_LABEL();
)

I need some way to generate unique labels, one for each inner MAKE_LABEL call, with the preprocessor. I've tried using __LINE__, but since I call MAKE_LABEL inside another macro, they all have the same line and the labels collide.

What I'd like this to expand to is something like:

MAKE_FUNNY_JUMPING_LOOP(
  my_cool_label_1:  // from first inner macro
  ...
  my_cool_label_2:  // from second inner macro
  ...
)

Is there a way to generate hashes or auto-incrementing integers with the preprocessor?

If you're using GCC or MSVC, there is __COUNTER__.

Other than that, you could do something vomit-worthy, like:

#ifndef USED_1
#define USED_1
1
#else
#ifndef USED_2
#define USED_2
2
/* many many more */
#endif
#endif

I use this:

#define MERGE_(a,b)  a##b
#define LABEL_(a) MERGE_(unique_name_, a)
#define UNIQUE_NAME LABEL_(__LINE__)

int main()
{
    int UNIQUE_NAME = 1;
    return 0;
}

... and get the following:

int main()
{
    int unique_name_8 = 1;
    return 0;
}

As others noted, __COUNTER__ is the easy but nonstandard way of doing this.

If you need extra portability, or for other cool preprocessor tricks, the Boost Preprocessor library (which works for C as well as C++) will work. For example, the following header file will output a unique label wherever it's included.

#include <boost/preprocessor/arithmetic/inc.hpp>
#include <boost/preprocessor/slot/slot.hpp>

#if !defined(UNIQUE_LABEL)
#define UNIQUE_LABEL
#define BOOST_PP_VALUE 1
#include BOOST_PP_ASSIGN_SLOT(1)
#undef BOOST_PP_VALUE
#else
#define BOOST_PP_VALUE BOOST_PP_INC(BOOST_PP_SLOT(1))
#include BOOST_PP_ASSIGN_SLOT(1)
#undef BOOST_PP_VALUE
#endif


BOOST_PP_CAT(my_cool_label_, BOOST_PP_SLOT(1)):

Sample:

int main(int argc, char *argv[]) {
    #include "unique_label.h"
    printf("%x\n", 1234);
    #include "unique_label.h"
    printf("%x\n", 1234);
    #include "unique_label.h"
    return 0;
}

preprocesses to

int main(int argc, char *argv[]) {
    my_cool_label_1:
    printf("%x\n", 1234);
    my_cool_label_2:
    printf("%x\n", 1234);
    my_cool_label_3:
    return 0;
}

I can't think of a way to automatically generate them but you could pass a parameter to MAKE_LABEL:

#define MAKE_LABEL(n) my_cool_label_##n:

Then...

MAKE_FUNNY_JUMPING_LOOP(
  MAKE_LABEL(0);
  MAKE_LABEL(1);
)