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.
Regards
Juan.
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