As mentioned earlier in this text, it is frequently convenient to place a list of declarations of user-defined data types and functions (as well as #defined names) in a separate "header" file which may be accessed by various other source files. This is accomplished with the FILE INCLUSION preprocessor directive, usually placed at the beginning of a source file. Its syntax is one of:
# include <standard library header file>
# include "user-defined header file"
This directs the preprocessor to replace the '#include' line with all the text contained in the named file. Using quotes instead of <> braces instructs the preprocessor to look for the file in the user's directory rather than that used by the system for standard header files. For example, a source file using the printf() standard library function declared in the standard header 'stdio.h' and the user-defined type personnel_rec defined in the user's file 'personnel.h' should begin with: