Carbon is the set of programming interfaces derived from earlier Mac OS APIs that can run on Mac OS X. Some of these APIs have been modified or extended to take advantage of Mac OS X features such as preemptive multitasking and protected memory.
In addition to being able to run on Mac OS X, Carbon applications built for Mac OS X can also run on Mac OS 8 and 9 when the CarbonLib system extension is installed. (As always, you should test for the existence of specific features before using them.)
Carbon includes about 70 percent of the existing Mac OS APIs, covering about 95 percent of the functions used by applications. Because it includes most of the functions you rely on today, converting to Carbon is a straightforward process. Apple provides tools and documentation to help you determine the changes you will need to make in your source code, as well as the header files and libraries necessary to build a Carbon application.
Mac OS X brings important new features and enhancements that developers have asked for, and Carbon allows you to take advantage of them while preserving your investment in Mac OS source code. As Apple moves the Mac OS forward, Carbon ensures you won’t be left behind.
Adapted from Carbon Porting Guide