Unimplemented syscalls in RIOT

Hello RIOTers,

I have run into some issues while working on lua support, involving missing system calls and linker errors while trying to use C standard functions, such as rename() and clock().

For example: A program calls rename(). Rename is defined by newlib and calls link(), which calls _link_r (the reentrant version), which is not defined, so linking fails. A similar thing happens with clock() (calls times() which calls _times_r).

The immediate reason is that, as of PR #8795, RIOT is linked with -lnosys, meaning newlib no longer provide stubs. Of course, for us this is a good thing (that's why the PR got merged). The problems arise when we try to compile packages, that is, code that we don't "control" and that we'd like to modify as little as possible.

sys/newlib_syscalls_default/syscalls.c implements several stubs. For some functions it implements the right thing if the correct module is enable, like filesystem calls if we have VFS.

There are several options for fixing this problem of missing syscalls:

1. Patch the packages to avoid those functions. It's a lot of work, and does not add any value to RIOT. 2. Add empty stubs that always fail (and set errno to ENOSYS). This makes programs compile, but then it may run into errors at runtime and the developer will be probably unaware that the code is using unsupported functionality. 3. Add empty stubs as part of a module, so that the fake functions must be explicitly included. 4. Make other riot modules provide the standard C interface. For example, xtimer may provide for clock().

Going back to my case of Lua, I initially thought of doing (2), but after a conversation with José Alamos he convinced me it's not a good idea. I'd suggest doing (3) and (4), that is, having parts of the standard library be implemented by the relevant RIOT module (clock() in xtimer, link()/rename() in vfs) while also having a stub modules for those cases in which it is acceptable to have unimplemented functionality.

I look forward to you comments.