Is there a meaningful distinction between freestanding and hosted implementations?

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The cited paragraph already states it quite well.

A hosted execution environment is also a freestanding, but not vice versa. A compiler need only provide a freestanding implementation. gcc, for example, is strictly speaking freestanding only, as the standard library is not included. However, it assumes it is available when compiling for a hosted environment (the default), assuming the lib is available on the system (like glibc). See here

Simply put, freestanding is just the language. It is not required to support any libraries and just a few headers (mostly for common types and implementation specific stuff like numerical limits, etc.). This implies the standard library need not exist – nor do the corresponding headers. Reason is a freestanding environment most likely will not have such facilities like files, display, etc. It is used for kernels, bare-metal embedded, etc.

Note that gcc for instance will, if compiling for a hosted environment (-fhosted), assume functions used in the standard library have the corresponding meaning and might apply very aggressive optimizations (it has many of those functions built-in). For freestanding, it actually does not, so you can use a function strcmp for instance with completely different semantics. However, it assumes the mem…-functions to exist, as these are used for normal code, e.g. struct assignment.

So, when compiling for bare-metal without a standard library (or a non-standard standard library), you must use -ffreestanding.

If a hosted implementation calls itself freestanding, it is obviously not a hosted implementation anymore. Once it calls itself hosted, however, it has to provide all facilities required by the standard and is not allowed to just implement dummies, but has to provide the semantics as defined in the standard.

Just to state that clear: The cited section allows a freestanding environment to omit all functions of the library, except for the few listed headers. So you are free to supply any other library and use the same names, but do anything you like. As that would be not the standard library, there is no need for compliance.

5.1.2.1 further states that “Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.”. That does support my statement. Sidenote: It also does not require main() as program entry point.

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