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