3.1.3 Extended search paths

Following the standard Unix convention for search paths, several directories can be specified together in an environment variable as a colon separated list:

DIR1:DIR2:DIR3:...

The directories are then searched in order from left to right. A single dot '.' can be used to specify the current directory.⁽⁹⁾

For example, the following settings create default include and link paths for packages installed in the current directory '.' and the 'include' and 'lib' directories under '/opt/gdbm-1.8.3' and '/net' respectively:

$ C_INCLUDE_PATH=.:/opt/gdbm-1.8.3/include:/net/include
$ LIBRARY_PATH=.:/opt/gdbm-1.8.3/lib:/net/lib

For C++ programs, use the environment variable CPLUS_INCLUDE_PATH instead of C_INCLUDE_PATH.

To specify multiple search path directories on the command line, the options -I and -L can be repeated. For example, the following command,

$ gcc -I. -I/opt/gdbm-1.8.3/include -I/net/include
   -L. -L/opt/gdbm-1.8.3/lib -L/net/lib .....

is equivalent to the environment variable settings given above.

When environment variables and command-line options are used together the compiler searches the directories in the following order:

1. command-line options -I and -L, from left to right
2. directories specified by environment variables, such as C_INCLUDE_PATH (for C programs), CPLUS_INCLUDE_PATH (for C++ programs) and LIBRARY_PATH
3. default system directories

In day-to-day usage, directories are usually added to the search paths with the options -I and -L.