The environment variable DC_PATH is used to determine the search path for the #include directive. On Unix system, different directories are colon separated, DC_PATH=/home/path1:path2:/path3. The environment variable can be set in your .login, .cshrc, or in a number of other places.
On OS/2 and DOS file system the path variable are semi-colon separated, DC_PATH=c:homepath1;d:path2;c:path3. Environment variables can be set in config.sys files, autoexec.bat, and, in WINNT, in the system configuration.