The APPENDDEF( ) m4 directive allows you to append new information to information that was previously defined. To illustrate, consider that the locations of your #include files are sometimes preset in the appropriate devtools/OS directory. For OS/UXPDS.V10, for example, the default is:

-I/usr/include -I/usr/ucbinclude

You can use this APPENDDEF( ) directive to add another directory to this list, without erasing what is already there:

APPENDDEF(`confINCDIRS', `-I/usr/local/include/db')

This causes the new directory to be appended to the declaration in the previous example:

-I/usr/include -I/usr/ucbinclude -I/usr/local/include/db

Even when you are not sure if a macro has been given a value by default, you can safely use this APPENDDEF( ) directive because no harm is caused by appending to an empty definition. See also PREPENDDEF( ).