When writing your C program, you can include files in two ways.
The first way is to surround the file you want to include with the angled brackets < and >.

This method of inclusion tells the preprocessor to look for the file in the predefined default

location.

This predefined default location is often an INCLUDE environment variable that denotes the

path to your include files.

For instance, given the INCLUDE variable

INCLUDE=C:COMPILERINCLUDE;S:SOURCEHEADERS;

using the #include version of file inclusion, the compiler first checks the

C:COMPILERINCLUDE

directory for the specified file. If the file is not found there, the compiler then checks the

S:SOURCEHEADERS directory. If the file is still not found, the preprocessor checks the

current directory.

The second way to include files is to surround the file you want to include with double

quotation marks. This method of inclusion tells the preprocessor to look for the file in the

current directory first, then look for it in the predefined locations you have set up. Using the

#include file version of file inclusion and applying it to the preceding example, the

preprocessor first checks the current directory for the specified file. If the file is not found in

the current directory, the C:COMPILERINCLUDE directory is searched. If the file is still not

found, the preprocessor checks the S:SOURCEHEADERS directory.

The #include method of file inclusion is often used to include standard headers such as

stdio.h or

stdlib.h.

This is because these headers are rarely (if ever) modified, and they should always be read

from your compiler’s standard include file directory.

The #include file method of file inclusion is often used to include nonstandard header files

that you have created for use in your program. This is because these headers are often

modified in the current directory, and you will want the preprocessor to use your newly

modified version of the header rather than the older, unmodified version.