Question

I’m curious as to why I see nearly all C macros formatted like this:

#ifndef FOO
#   define FOO
#endif

Or this:

#ifndef FOO
#define FOO
#endif

But never this:

#ifndef FOO
    #define FOO
#endif

(moreover, vim’s = operator only seems to count the first two as correct.)

Is this due to portability issues among compilers, or is it just a standard practice?

Answer 1

IIRC, older C preprocessors required the # to be the first character on the line (though I’ve never actually encountered one that had this requirement).

I never seen your code like your first example. I usually wrote preprocessor directives as in your second example. I found that it visually interfered with the indentation of the actual code less (not that I write in C anymore).

The GNU C Preprocessor manual says:

Preprocessing directives are lines in
your program that start with ‘#’.
Whitespace is allowed before and after
the ‘#’.