[loeg]

In particular, a single statement. I'm sure the link covers it, but:

    if (foo)
      MULTI_LINE_MACRO;

Breaks without some wrapper like if (1) { A; B; } or do { A; B; } while (0):

    if (foo)
      A;
      B;  // oops, unconditional (e.g., "goto fail")

[monocasa]

It also gives you a nice scope to keep local variables in, but there are other ways to accomplish that too.

[loeg]

Yeah, that's a good point too.