Hosted and Free-Standing Environments

The dependency on the use of libraries to extend the language has an important effect on the practical use of C. Not only are the Standard I/O Library functions important to applications programmers, but there are a number of other functions that are widely taken almost for granted as being part of the language. String handling, sorting and comparison, character manipulation and similar services are invariably expected in all but the most specialized of applications areas.

Because of this unusually heavy dependency on libraries to do real work, it was most important that the Standard provided comprehensive definitions for the supporting functions too. The situation with the library functions was much more complicated than the relatively simple job of providing a tight definition for the language itself, because the library can be extended or modified by a knowledgeable user and was only partially defined in K&R. In practice, this led to numerous similar but different implementations of supporting libraries in common use. By far the hardest part of the work of the Committee was to reach a good definition of the library support that should be provided. In terms of benefit to the final user of C, it is this work that will prove to be by far and away the most valuable part of the Standard.

However, not all C programs are used for the same type of applications. The Standard Library is useful for ‘data processing’ types of applications, where file I/O and numeric and string oriented data are widely used. There is an equally important application area for C—the ‘embedded system’ area—which includes such things as process control, real-time and similar applications.

The Standard knows this and provides for it. A large part of the Standard is the definition of the library functions that must be supplied for hosted environments. A hosted environment is one that provides the standard libraries. The standard permits both hosted and freestanding environments. and goes to some length to differentiate between them. Who would want to go without libraries? Well, anybody writing ‘stand alone’ programs. Operating systems, embedded systems like machine controllers and firmware for instrumentation are all examples of the case where a hosted environment might be inappropriate. Programs written for a hosted environment have to be aware of the fact that the names of all the library functions are reserved for use by the implementation. There is no such restriction on the programmer working in a freestanding environment, although it isn't a good idea to go using names that are used in the standard library, simply because it will mislead readers of the program. Chapter 9 describes the names and uses of the library functions.