Preprocessor #ifndef

Question

Assume I have a.h which includes the following:

Assume I also have b.

Answer 1

All C standard headers must be made such that they can be included several times and in any order:

Standard headers may be included in any order; each may be included more than once in a given scope, with no effect different from being included only once

Answer 2

If stdbool.h itself has include guards (#ifndef) then everything will be fine. Otherwise you may indeed end up including some headers twice. Will it cause a problem? It depends. If the twice-included header contains only declarations then everything will compile - it will just take few nanoseconds longer. Imagine this:

int the_answer(void); // <-- from first inclusion
int the_answer(void); // <-- from from second inclusion - this is OK
                      //       at least as long as declarations are the same

int main()
{
    return the_answer();
}

If on the other hand there will be definitions it will cause an error:

int the_answer(void)  // <-- from first inclusion - OK so far
{
    return 42;
}

int the_answer(void)  // <-- from second inclusion
{                     //     error: redefinition of 'the_answer'
    return 42;
}

int main()
{
    return the_answer();
}

Answer 3

It's normal that most header start with

#ifndef _HEADERFILENAME_H_
#define _HEADERFILENAME_H_

and end with the following line:

#endif

If you include a header two times, the second time your programm won't include the full header again because of the #ifndef, #define and #endif