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Comment by filereaper | Hacker News Reader

Enjoyed this article, anybody know the significance of adding the do..while(0) loop within the macro starting Linux 1.3?

Was curious if it guards against some C pre-processor issues.

  /** include/asm-i386/system.h */
  #define switch_to(tsk) do {
    [...]
  } while (0)

Just to add more context, this is a very common cpp (c pre-processor) idiom. You'll find it in most non-trivial C projects somewhere.

Abbreviating the C Preprocessor as cpp is very confusing imho.

Tell that to CPPFLAGS.

It's a common abbreviation older than C++. You used to even be able to run the c pre processor on arbitrary non-C files by using the cpp command.

You still can.

It enables you to invoke the macro as if it were an expression statement consisting of a function call, regardless of where it appears.

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")

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

Yeah, that's a good point too.