Macros and Enums

Names of macros defining constants and labels in enums are capitalized.

#define CONSTANT 0x12345

and

enum sample
{
	FIRST = 1,
	SECOND,
	THIRD
};

Enums are preferred when defining several related constants.

CAPITALIZED macro names are appreciated but macros resembling functions may be named in lower case.

Generally, inline functions are preferable to macros resembling functions.

Macros with multiple statements should be enclosed in a do - while block:

#define macrofun(a, b, c) \
	do \
	{ \
		if (a == 5) \
			do_this(b, c); \
	} while (condition)

---

Things to avoid when using macros:

1) Macros that affect control flow:

#define FOO(x) \
	do \
	{ \
		if (bar(x) < 0) \
			return (-1); \
	} while (condition)

This is a very bad idea.
It looks like a function call but exits the calling function; don't break the internal parsers of those who will read the code.

2) Macros that depend on having a local variable with a magic name:

#define FOO(val) bar(index, val)

might look like a good thing, but it's confusing as hell when one reads the code and it's prone to breakage from seemingly innocent changes.

3) Forgetting about precedence: macros defining constants using expressions must enclose the expression in parentheses. Beware of similar issues with macros using parameters.

#define CONSTANT 0x4000
#define CONSTEXP (CONSTANT | 3)

4) Namespace collisions when defining local variables in macros resembling functions:

#define FOO(x) \
({ \
	typeof(x) ret; \
	ret = calc_ret(x); \
	(ret); \
})

ret is a common name for a local variable. __foo_ret is less likely to collide with an existing variable.