The #include directive tells the preprocessor to treat the contents of a specified file as if those contents had appeared in the source program at the point where the directive appears.
You can organize constant and macro definitions into include files and then use #include directives to add these definitions to any source file. Include files are also useful for incorporating declarations of external variables and complex data types. You need to define and name the types only once in an include file created for that purpose.
The path-spec is a filename optionally preceded by a directory specification. The filename must name an existing file. The syntax of the path-spec depends on the operating system on which the program is compiled.
Both syntax forms cause replacement of that directive by the entire contents of the specified include file. The difference between the two forms is the order in which the preprocessor searches for header files when the path is incompletely specified.
| Syntax Form | Action | ||||||
|---|---|---|---|---|---|---|---|
| Quoted form | This form instructs the preprocessor to look for include files in the same directory of the file that contains the #include statement, and then in the directories of any files that include (#include) that file. The preprocessor then searches along the path specified by the /I compiler option, then along paths specified by the INCLUDE environment variable.Example #include "defs.h" | ||||||
| Angle-bracket form | This form instructs the preprocessor to search for include files first along the path specified by the /I compiler option, then, when compiling from the command line, along the path specified by the INCLUDE environment variable.Example #include <stdio.h> |
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