C macros

C Macros

#define defines a variable to a value (or nothing if it's not defined). It is a literal find and replace kinda stuff, so be aware of what's happening after rewrites. It can also be used in a functional fashion and has a bunch of operators.

#define macro_name([parameter_list]) replacement_text will use it as a function. Once again, keep in mind that it's a literal find and replace.

This would actually give 11 as if it's 3 + 2*3 + 2.

  #define square(x) x*x
  int main() {
      int y = square(3+2);
  }

Variadic macros

There are also variadic macros that take in multiple arguments.

#define ARGS(x, ...) do {\
   printf("%d\n", ##__VA_ARGS__); \
} while (0)

A common trick is to wrap a macro in a do-while loop as that eliminates issues with using semicolons and otherwise. We can also convert the arguments to a string in a macro by using #arg wherever we need it. To evaluate a macro result as a string, we need two macros

Nested macro behaviour

#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)

     → "foo"
xstr (foo)
     → xstr (4)
     → str (4)
     → "4"

s is stringized when used in str so not macro expanded first. but for xstr it is an ordinary argument so it is macro expanded before ~xstr~ itself is expanded. See here why.

Similarly, there is ## that is used for concatenation of two items in a macro. This is very often used to concatenate the arguments. Like the stringizing macro, this needs to be redirected to another macro for evaluation.