[ECOS] Re: uSTL hello world

[Jonathan Larmour]

Bart Veer wrote:
>>>>>>"Jifl" == Jonathan Larmour <jifl@jifvik.org> writes:
> 
> 
>     <snip>
>     
>     >> Jifl, is there a preferred way to ensure initialisation of the
>     >> stdio streams in the file descriptor table? I note that open()
>     >> in the file I/O package uses:
>     >> 
>     >> CYG_REFERENCE_OBJECT(stdin);
>     >> CYG_REFERENCE_OBJECT(stdout);
>     >> CYG_REFERENCE_OBJECT(stderr);
>     >> 
>     >> Is this addressing the same issue that we're observing with
>     >> uSTL?
> 
>     Jifl> I think there's a more fundamental problem, not specific to
>     Jifl> uSTL (so I think Uwe's workaround isn't really appropriate).
>     Jifl> That is that something like:
> 
>     Jifl>    write(STDOUT_FILENO, buf, n);
> 
>     Jifl> can't be guaranteed to just work, at present. That's a bug.
> 
>     Jifl> I think those CYG_REFERENCE_OBJECT lines (and surrounding
>     Jifl> ifdef) need to move out of open() and into cyg_fd_init().
>     Jifl> Theoretically cyg_fp_get() would be slightly better, but
>     Jifl> that is called a lot, and there is a trivial but non-zero
>     Jifl> overhead to CYG_REFERENCE_OBJECT. I'd appreciate Nick's
>     Jifl> opinion on this proposal if possible though. It does mean
>     Jifl> that including the fileio package at all in a configuration
>     Jifl> would then guarantee pulling in stdin/stdout/stderr, even
>     Jifl> irrespective of use. Maybe the number of people this would
>     Jifl> affect is too small to care about. Maybe there should be a
>     Jifl> default-enabled option, to allow it to be turned off to
>     Jifl> remove that dependency.
> 
>     Jifl> Taking the usual principle of "leave it to the user", I
>     Jifl> think I'd personally lean towards the latter.
> 
> I think there is another approach which might fit in better with
> linker garbage collection. Instead of #define'ing STDOUT_FILENO as 1,
> declare it as an extern const char. Then provide the variable
> STDOUT_FILENO in some module which causes the necessary initialization
> to happen. It is slightly more expensive: a bit of space for the const
> variable, an extra memory access for the write() call. However it
> would eliminate the need for CYG_REFERENCE_OBJECT's and it would allow
> linker garbage collection to work as normal.
> 
> Obviously code which uses write(1, buf, n) would still be broken, and
> http://www.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html
> requires that STDOUT_FILENO be defined as 1. Hence my proposal would
> not be fully standards-compliant.

While that's an interesting workaround, I think your last paragraph is the 
killer, chiefly because I think write(1, buf, n) should still be able to 
work. *However*, maybe the config option could instead be between 
standards compliant mode, and "not quite right, but may often still work" 
mode.

Unfortunately we have another problem. I've looked at the output of our 
recent compilers, and I see that CYG_REFERENCE_OBJECT no longer appears to 
be effective :-(. The reference gets removed by more aggressive compiler 
optimisation at -O2. This is a much wider problem than just stdin/out/err 
of course.

Solving that efficiently may be difficult (although I don't consider the 
current implementation to necessarily be efficient either, when it worked, 
anyway). Managing to get the reference into .text/.rodata would probably 
be better than .data/.bss. Even better would be if it were possible to 
play tricks to get it into a non-loadable section, although that starts to 
venture into the world of HAL-specific stuff. I see a .comment section 
though. Something along those lines might work (and by overridable by 
specific HALs if needed).

But I've experimented (admittedly briefly) and not got as far as even 
making the reference. I've tried using various attributes, nested 
functions, volatiles, embedding  asm(".word " #__object__ ";"), etc. It 
may be that we can only do this properly using HAL-specific inline asm 
:-(. The least worst idea I've had which actually creates a reference is 
to have a real function cyg_reference_object(void*) and call it with the 
address of the object to reference, but which returns immediately.

Any other ideas? At least there are not many uses of CYG_REFERENCE_OBJECT.

Jifl
-- 
--["No sense being pessimistic, it wouldn't work anyway"]-- Opinions==mine

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