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Asked: 2026-05-10T22:26:36+00:00

How does one prevent an inclusion cycle in C? ie. You shouldn’t have a.h

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How does one prevent an inclusion cycle in C? ie. You shouldn’t have a.h #include ‘b.h’, which #include’s ‘c.h’ which #include’s ‘a.h’. I’m looking for a way of preventing this from happening using some sort of C directive.

I had originally thought this would’ve prevented this from happening:

Contents of a.h:

#ifndef __A_H #define __A_H  #include 'b.h'  #endif // __A_H

Contents of b.h:

#ifndef __B_H #define __B_H  #include 'c.h'  #endif // __B_H

Contents of c.h:

#ifndef __C_H #define __C_H  #include 'a.h'  #endif // __C_H

But it doesn’t seem to work.

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1 Answer

  1. It does work allright: the files are repeatedly included, but the sections protected by #ifdndef/#define/#endif are not repeated, and that breaks the cycle.

    Use your compiler to produce the preprocessed output and look at it for yourself. With GNU CC, you need to use ‘-E’ option on the .c[pp] file, like this:

    gcc -E $(CFLAGS) -o foo.i foo.cpp
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