Re: [seL4] Make seL4/libsel4 not depend upon libc

Hi Wink, The guards you've used look sufficient (though it's mildly concerning that any poor user who received your libsel4's assert.h before libc's has their asserts unconditionally become no-ops). We also previously explored the option of using different typedef names, but felt it led to unnecessary confusion when reading the stubs. I'm not debating whether these things can be checked for at compile time, but IMHO the necessary checks in a large build system are more complex than one might initially imagine. I don't have any objection to minimal systems and probably no one who works on microkernels does. Personally I think implementing a C application without including libc is signing yourself up to re-implement libc, but if you're OK with that than fair enough. I'm not opposed to the idea behind this proposal, of removing the libsel4->libc dependency, but since I am unlikely to be the one maintaining it, I'll leave it to others to make a call on whether to go in this direction. Cheers, Matt On 23/06/15 21:03, Wink Saville wrote:

Matt,

I've guarded the definitions with conditionals to try to avoid that, but maybe a better option might be to use different names completely. Also, we could test for compatibility at compile time and generate errors.

My rational for wanting this is that libc doesn't seem to be providing that much value as very few of its vsyscalls are implemented anyway. Thus its utility is actually small, IMHO. Also, I happen to be interested in minimal systems so unused and extraneous code is an anathema. Here, in libsel4, by adding these few LOC's we are removing "thousands" of LOC brought in by the need of a libc.

Anyway, I'd certainly like to keep the discussion going to revisit the the earlier decision, I just feel strongly the less code the better.

On Tue, Jun 23, 2015, 12:02 AM Matthew Fernandez mailto:matthew.fernandez@nicta.com.au> wrote:

Hi Wink,

As you've observed in that commit, removing the libc dependency leads you to re-implement pieces of it inline. We used to do something similar to this, but replaced this with an explicit dependency on libc to avoid complications with multiple definitions of typedefs and such. In the case of programmer error, these typedefs may not match and if the conflicting typedefs are never seen within the same translation unit your compiler will not notice either. The end result was subtle bugs arising from ABI incompatibility.

When we made the change, out rationale was that anyone using the syscall stubs would have libc available anyway. This certainly introduces a circular dependency, but it's not easy to remove without repeating parts of one library in the other or playing tricks with externs and weak symbols. In my experience, all of this is more fragile and less preferable than living with the circular dependency. However, my opinion may not be universally shared as we haven't had a discussion about this for some time.

Cheers, Matt

On 23/06/15 11:16, Wink Saville wrote: > I noticed that seL4/libsel4 was dependent upon libc and so I created a patch > (https://github.com/winksaville/seL4/commit/fc91a1f68c054fdb41aaa42a7b56a80f4...) which removes the dependency. If > there is interest I can submit a pull request, and of course will make any changes deemed necessary to make it suitable > for acceptance. > > Please advise, > > Wink > > > _______________________________________________ > Devel mailing list > Devel@sel4.systems > https://sel4.systems/lists/listinfo/devel >

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