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Re: Bug#1030785: -ffile-prefix-map option and reproducibility

On Tue, Feb 14, 2023 at 09:04:47AM +0100, Stéphane Glondu wrote:
> Hi,
> Le 08/02/2023 à 10:58, Emilio Pozuelo Monfort a écrit :
> > What is the purpose of having the build flags in a file in the .deb?
> ocamlc can act as a driver for the C compiler, to compile C stubs with the
> "right" flags. These flags are basically the CFLAGS ocaml was compiled with,
> plus some additional OCaml-specific flags (like -fwrapv).
> But I wonder now: shouldn't the CFLAGS part be queried from the environment
> at use time, rather than at OCaml compilation time? This setting of a non-C
> compiler calling a C compiler must happen often... I don't know what's the
> practice elsewhere.

I can't speak of many other systems, but at least with Perl's XS
(the standard way to write Perl modules parts of which are compiled C code)
and two of the ways this can be done in Python, it is the same:
the C compiler's name and flags are recorded at the time the "wrapper" is
built, so that they can be used later. At least IMHO, this has two main
- people (or programs) that want to build a Perl/Python/OCaml/whatever
  module do not have to know anything about C compilers, flags, optimization,
  language-specific libraries[1] , etc. The consumers tell their own language
  ecosystem "look, I need to compile something that has C parts in it, go do
  something about it", and the wrapper for the C compiler knows exactly what
  to do
- everything is compiled using the same compiler[2], the same optimization
  flags, the same libraries, etc., so one module that has C parts in it can
  call the C functions from another module directly with no fear of any kind
  of calling convention mismatch or whatever
- to reinforce the previous point: everything is compiled using the same
  recorded settings *no matter what* environment variables or paths there
  may be in the current user's execution environment, so that nothing will
  break if somebody has the wrong environment variable defined when they
  build a module a couple of months later. Of course, there are some cases
  when some such systems allow additional flags to be specified or
  overridden, but IMHO it is good that this must be done explicitly and is
  most often not done at all, so that everything is built in the same way
  and it can all work together

[1] At least with Perl and Python, C code very often invokes functions from
    the Perl/Python standard library, the C code does not know how to create
    a list or how to invoke a Perl/Python function, so it has to use
    the language's internal libraries for that.

[2] Well, okay, that's not strictly true, since the binary called "gcc" now
    may not be the same that was provided by the C compiler package a couple
    of months ago, but it ought to be guaranteed to generate compatible code
    and object files.

So yeah, I'd say that "record the C compiler flags at build time" is
pretty much standard practice for such interface providers.


Peter Pentchev  roam@ringlet.net roam@debian.org pp@storpool.com
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