C48. C #include Directive

The #include Directive

The #include directive has two primary forms. The first form is used for header files that belong to C's own library:

#include

The second form is used for all other header files, including any that we write:

#include "filename"

The difference between the two is a subtle one having to do with how the com­piler locates the header file. Here are the rules that most compilers follow:#include : Search the directory (or directories) in which system header files reside. (On UNIX systems, for example, system header files are usually kept in the directory /usr/include.)

#include "filename": Search the current directory, then search the direc­tory (or directories) in which system header files reside.

The places to be searched for header files can usually be altered, often by a com­mand-line option such as -I path.