c - 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 conforming hosted implementation shall accept any strictly conforming program.

As I understand, this constitutes the typical application environment, with filesystems, allocated memory and threads...

A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers <float.h>, <iso646.h>, <limits.h>, <stdalign.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>, and <stdnoreturn.h>.

... and this constitutes the typical kernel and/or embedded, bare minimum environment that doesn't have standard filesystems, allocated memory or threads (among other things).

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program.

It seems as though this gives a hosted implementation the freedom to call itself a hosted or freestanding implementation, and when it comes to filesystems, allocated memory or threads (among other things), these can fall under the extension category so that it can merely implement an interface that returns a value indicating errors every time. Just to name a few:

• fopen, fgets and malloc can return NULL
• fprintf, fscanf, fputc and fgetc can return EOF
• thrd_create can return thrd_error (indicating that "the request could not be honored")

This implies that the distinction section four, paragraph six gives is virtually meaningless. Are there any requirements guaranteeing some actual level of functionality for these functions in hosted and freestanding implementations? For example, is it required that those functions above actually be able to return something other than their corresponding failure values?

Answer 1

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, if building for bare-metal, you should always pass -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.