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Comment by apaprocki | Hacker News Reader

It might as well talk about #pragma once if it is explaining include guards.

Fun fact: Oracle (Sun) Studio doesn't support #pragma once. Instead, it actually looks for proper include guards in the header and if they exist, it optimizes away re-includes of the file if the condition is false.

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gcc does this as well (though it also supports #pragma once). I believe the current convention is to prefer #ifndef include guards as it is more portable and most compilers can detect and avoid re-including files with them.

I assumed that was common practice.

Nope, and to make matters worse -- IBM xlC doesn't optimize at the include sites. So adding include-site guards actually speeds up compilation times in a non-insignificant way. e.g.,

  #ifndef FOO_H
  #include <foo.h>
  #endif

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