If you have an #include file that should be #included only once, use
a sequence like

#ifndef H_FOOHEADER
#define H_FOOHEADER
... main header file contents
#endif

The #endif should be in the last line of the file, and this line should only
contain the #endif and whitespace and/or comments.
If this line is terminated by a single newline, which is the default on unix
systems, the preprocessor will recognize this construct and need not even
bother to skip through the main body of your header file when it reencounters
it.