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Asked: 2026-05-20T19:35:36+00:00

My C headers usually resemble the following style to avoid multiple inclusion: #ifndef <FILENAME>_H

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My C headers usually resemble the following style to avoid multiple inclusion:

#ifndef <FILENAME>_H
#define <FILENAME>_H

// define public data structures / prototypes, macros etc.

#endif  /* !<FILENAME>_H */

However, in his Notes on Programming in C, Rob Pike makes the following argument about header files:

There’s a little dance involving #ifdef‘s that can prevent a file being read twice, but it’s usually done wrong in practice – the #ifdef‘s are in the file itself, not the file that includes it. The result is often thousands of needless lines of code passing through the lexical analyzer, which is (in good compilers) the most expensive phase.

On the one hand, Pike is the only programmer I actually admire. On the other hand, putting several #ifdefs in multiple source files instead of putting one #ifdef in a single header file feels needlessly awkward.

What is the best way to handle the problem of multiple inclusion?

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1 Answer

  1. Editorial Team

    In my opinion, use the method that requires less of your time (which likely means putting the #ifdefs in the header files). I don’t really mind if the compiler has to work harder if my resulting code is cleaner. If, perhaps, you are working on a multi-million line code base that you constantly have to fully rebuild, maybe the extra savings is worth it. But in most cases, I suspect that the extra cost is not usually noticeable.

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