Re: Same binary, different md5sums

[Mattias BrÃndstrÃm <thebrasse at brasse dot org>]

Andrew Haley wrote:
> Mattias BrÃndstrÃm writes:
>  > Andrew Haley wrote:
>  > > Mattias BrÃndstrÃm writes:
>  > > 
>  > >  > 
>  > >  > Why does g++ generate binaries with different checksums when I compile 
>  > >  > my project at two different occasions with the same commands?
>  > >  > 
>  > >  > I realize that this is a fairly broad question, but I would like to know 
>  > >  > if there is anything in particular I should take into account when 
>  > >  > trying to generate binaries with the same checksum.
>  > > 
>  > > There could be all manner of reasons.  To begin with, diff the
>  > > assembly language source produced by g++ -save-temps.  That'll get us
>  > > started.
>  > > 
>  > 
>  > Thanks for the pointer! I have now compiled my project with -save-temps, 
>  > as you suggested, and ended up with some .s files that actually differ. 
>  > Here is a portion of one the diffs:
>  > 
>  > <       call
>  > _ZN53_GLOBAL__N_locale_CompactLocale.cpp_9D2164D3_6D4895CA10is_integerERKN6ttutil4misc7DecimalE@PLT
>  > ---
>  >  >       call
>  > _ZN53_GLOBAL__N_locale_CompactLocale.cpp_9D2164D3_A9AF82EA10is_integerERKN6ttutil4misc7DecimalE@PLT
>  > 
>  > To me it seems that this difference is related to name mangling, one 
>  > function gets a different mangled name each time. In this case it is a 
>  > function ( is_integer(ttutil::misc::Decimal&) ) in an anonymous namespace.
>  > 
>  > My guess is that since the namespace is anonymous g++ creates a random 
>  > name for it that will hopefully not clash with another randomly 
>  > generated namespace name? Could this be the reason?
>  > 
>  > Any ideas on how to get around this?
>
> Try   -frandom-seed=<string>      Make compile reproducible using <string>

Nice! I had missed that compile option. The man page says that the seed
should be different for every file I compile. Do you know what kind of
trouble i might get into if I have the same seed for every file in my
project?

Regards,
Mattias