A.12.4 File Inclusion

A control line of the form
  # include <filename>
causes the replacement of that line by the entire contents of the file filename. The characters in the name filename must not include > or newline, and the effect is undefined if it contains any of “, ‘, \, or /*. The named file is searched for in a sequence of implementation-defined places.
Similarly, a control line of the form
  # include “filename”
searches first in association with the original source file (a deliberately implementation-dependent phrase), and if that search fails, then as in the first form. The effect of using ‘, \, or /* in the filename remains undefined, but > is permitted.
Finally, a directive of the form
  # include token-sequence
not matching one of the previous forms is interpreted by expanding the token sequence as for normal text; one of the two forms with <…> or “…” must result, and is then treated as previously described.
#include files may be nested.