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Re: Dummy pthread functions in libc considered harmful

On Mon, Aug 31, 2015 at 05:54:46PM +0300, Alexander Monakov wrote:
> On Tue, 25 Aug 2015, Rich Felker wrote:
> > > (however when libpthread is not linked in, you know that pthread_mutex_lock is
> > > not operating on shared memory: you'd need pthread_mutexattr_setpshared, which
> > > is not provided in
> > 
> > This is false. The process that maps and locks the process-shared
> > mutex need not be the one that called pthread_mutex_init. I ran into
> > this issue when optimizing static linking; you can't omit
> > process-shared code when the program doesn't reference
> > pthread_mutexattr_setpshared, because it could get the mutex from
> > another program -- that's the whole point of having it be
> > process-shared.
> As a result, a libc "stub" mutex must:
>  - contain a full implementation for process-shared mutexes
>  - for non-shared mutexes in single-threaded processes, modify them
>    consistently with non-stub implementation (but modifications need not be
>    atomic)
> Does Glibc do the former? (the bug report is about Glibc not doing the latter)

I may be wrong, but my impression is that glibc does not support this
usage for process-shared mutexes (omitting -lpthread) and is relying
on the POSIX rule that you may need -lpthread to get pthread_*
functions, and interpreting this to mean that it's okay to have a
broken/non-conforming version of pthread_mutex_* in If so, I
think this is technically correct, but risky since applications may be
assuming that, if the symbol exists (e.g. via configure checks) then
it works.


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