Is there a meaningful distinction between freestanding and hosted implementations?

Question

The question I have is mostly related to section four, paragraph six.

The two forms of conforming implementation are hosted and freestanding. A confor

Answer 1

There are many kinds of C implementations, targeting many kinds of different execution platforms, many of which can provide a variety of useful features and guarantees that others cannot. The authors of the Standard decided that in most cases it should be sufficiently obvious what kinds of features and guarantees should be provided by implementations targeting various platforms and application fields, and how they should be provided, that there would be no need to have a standard concern itself with such details. On the other hand, the number of applications that would require things like file I/O and the number of platforms that could provide them were sufficient to justify recognizing as "special" those implementations which included such features.

In general, implementations which are intended designed for freestanding use will be usable on platforms that would be unable to usefully handle a hosted implementation. While the Standard imposes some requirements beyond what would be practical on some of the smaller C platforms, some almost-conforming implementations of C can be quite usefully employed on processors with only enough storage to hold 256 instructions and 16 bytes' worth of variables. If something like a digital kitchen thermometer/timer gadget doesn't have a file system or console, why should it waste storage on things like descriptors for stdout?

In addition, because the Standard defines no standard means by which freestanding applications can perform I/O, and because different platforms handle I/O differently, almost freestanding applications will be targeted for a particular target platform or range of platforms. A hosted implementation which doesn't expose the natural features or guarantees the underlying platform would provide could be useful for running programs that don't require such features or guarantees. It's not possible for an embedded program to do much of anything without using platform-specific features and guarantees, however, and thus a freestanding implementation which didn't allow a programmer access to such things would be unable to do much. Quality implementations should allow programmers to use any features or guarantees that may help them accomplish what they need to do, though some may require use of compilation options to ensure that they don't do anything wacky. For some reason, it has become fashionable to regard a decision by the Standards Committee that there might be some implementations and application fields where the value of a feature or guarantee wouldn't justify the cost, as an indication that programmers should not expect implementations to provide a feature or guarantee which would be useful in low-level programming and which the platform would provide as essentially zero cost.