for or while loop inside #define directive

Question

How can I write a for/while loop inside a #define directive in C?

Answer 1

Short answer is "don't". But if you have to, for the love of all that's sacred don't do this:

#define FOREACH(start, end)                         \
      for (; (start) < (end); (start)++)            \
      {                                             \
        // do something interesting                 \
      }                                             

Bad juju all the way around. Note that start must correspond to an lvalue; you would not be able to call this as FOREACH(1,10), or FOREACH((a+b), c), or FOREACH(x++,y++). All of those would lead to a compile-time error (the operand of ++ must be an lvalue, and none of 1, a+b, or x++ qualify). Calling it as FOREACH(x, y++) will do something you really don't want it to do. Similarly, you wouldn't want to call it as FOREACH(x, y()).

You can guard against these problems to an extent by doing something like

#define FOREACH(start, end)                        \
do {                                               \
  int i;                                           \
  int j = end;                                     \
  for (i = start; i < j; i++)  {                   \ 
    // do something interesting                    \
  }                                                \
} while (0)

Essentially, you're creating local variables corresponding to your macro arguments. This protects against start not being an lvalue, and against end having a side effect that gets applied or being a function that gets called every iteration.

But if you're trying to encapsulate a loop that gets called frequently, put it in its own separate function. It's safer and easier to understand and maintain.

Answer 2

You're probably looking for \ to continue a macro definition across several lines:

#define LOOP(start, end)                  \
  for (int i = (start); i < (end); i++) { \
    printf("%d\n", i);                    \
  }

Answer 3

Since C doesn't require statements to be on separate lines, you can simply smush together into one long line:

#define M while (...) { ...; ...; }

Or you could escape newlines in the macro definition:

#define M \
  while (...) { \
    ...; \
    ...; \
  }

Answer 4

#define something for(;;) printf("hooray, i'm in infinite loop!");


int main() { something }