The default configuration file, (/usr/lpp/cbclib/xlc/etc/xlc.cfg), specifies information that the compiler uses when you invoke it. This file defines values used by the compiler to compile C or C++ programs. You can make entries to this file to support specific compilation requirements or to support other C or C++ compilation environments.
Options specified in the configuration file override the default settings of the option. Similarly, options specified in the configuration file are in turn overridden by options set in the source file and on the command line.