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Date: Mon, 2 Sep 2019 19:01:18 -0400
From: Rich Felker <>
Subject: Re: About those weak aliases

On Mon, Sep 02, 2019 at 10:10:10PM +0200, Szabolcs Nagy wrote:
> * Markus Wichmann <> [2019-09-02 21:04:48 +0200]:
> > I'd like to know what those weak aliases are for in the many cases where
> > they are used to define a public interface. Or, more to the point, by
> > what criteria they are handed out, and by what logic the internal
> > symbols are used.
> > 
> > For instance, pthread_mutex_lock() et al. are weakly defined, but
> it's a weak alias for __pthread_mutex_lock which can be used
> to implement iso c apis (where pthread* is not reserved and
> thus may conflict with user defined symbols)
> __pthread_mutex_lock is not used internally right now, but
> e.g. __pthread_mutex_timedlock is.

Indeed, it looks like commit df7d0dfb9c686df31149d09008ba92834bed9803
added it with an expectation that C11 threads would use it, but
instead mtx_lock just calls mtx_timedlock with a null timeout. Having
it around may be useful at some point though so I don't think it makes
sense to add noise removing it and possibly adding it back later.

> (could be a strong alias, weakness of public api symbols
> doesn't matter, you can only observe the difference by
> getting a link error when static linking a conflicting
> definition, but that is non-standard: when the symbol is
> reserved for the implementation user code must not use it)

I don't follow here. There are very few if any places where strong
alias would be a valid substitute for weak. Where weak aliases provide
dummy implementations of functionality that's only needed if something
else is linked, strong would be a link error if both were linked.
Where weak aliases are used because the identifier being defined is
reserved to the application in some or all standard profiles, a strong
alias would produce a link error if the application actually made use
of its reservation and the file defining the alias got linked (and the
whole point is that this can and does happen).


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