[PATCH, v3] Potential solution to librt issue.

[Martin Sebor]

Paolo Bonzini wrote:
>> Is that libpthread thing confirmed?
> 
> Yes:
> 
> $ echo 'int main() {}' > f.c
> $ gcc f.c -lrt
> $ ldd a.out
>         linux-gate.so.1 =>  (0xffffe000)
>         librt.so.1 => /lib/librt.so.1 (0xb7f4a000)
>         libc.so.6 => /lib/libc.so.6 (0xb7e0c000)
>         libpthread.so.0 => /lib/libpthread.so.0 (0xb7df3000)
>         /lib/ld-linux.so.2 (0xb7f64000)
> 
> I'm not saying it is a "no-no" to use clock_gettime -- but I guess 
> you'll agree it is a "no-no" to break performance of single-threaded 
> programs because of it, and it is lucky that Dan brought up the point so 
> soon.
> 
> Possibilities include:
> 
> 1) implementing clock_gettime on Linux using the system call interface. 
>  I guess that the clock_gettime syscall itself does not need pthreads.
> 
> 2) do some trick with specs.  For example, -Wl,--as-needed -lrt does not 
> bring in pthreads unless necessary:
> 
> $ gcc f.c -Wl,--as-needed -lrt
> $ ldd a.out
>         linux-gate.so.1 =>  (0xffffe000)
>         libc.so.6 => /lib/libc.so.6 (0xb7da9000)
>         /lib/ld-linux.so.2 (0xb7ef8000)
> 
> Is <chrono> a template?  If yes, i.e. if there is no code in 
> libstdc++.so that needs clock_gettime, you could add the 
> "-Wl,--as-needed -lrt -Wl,--no-as-needed" flags to the specs for C++ on 
> Linux.
> 
> Neither solution is particularly appealing to me. :-(

Would declaring a weakref for clock_gettime in libstdc++ work?

Martin

> 
> In the meanwhile, can anyone check the dependencies of -lrt on other 
> systems?
> 
> Paolo