Allow embedded timestamps by C/C++ macros to be set externally (3)

[Dhole <dhole at openmailbox dot org>]

Hi!

A few months ago I submited a patch to allow the embedded timestamps by
C/C++ macros to be set externally [2], which was already an improvement
over [1].  I was told to wait until the GCC 7 stage 1 started to send
this patch again.

I'm quoting from the original emails:

> As a reminder for the motivation behind this patch, we are working on
> the reproducible builds project which aims to provide users with a way
> to reproduce bit-for-bit identical binary packages from the source and
> build environment. The project involves Debian as well as several other
> free software projects. See <https://reproducible-builds.org/> for more
> information.

> In order to do this, we need to make the build processes
> deterministic.  As you can imagine, gcc is quite involved in producing
> Debian packages.  One issue we encounter in many packages that fail to
> build reproducibly is the use of the __DATE__, __TIME__ C macros [3],
> right now we have 442 affected packages that would need patching
> (either removing the macros, or passing a known date externally).

> A solution for toolchain packages that embed timestamps during the
> build process has been proposed for anyone interested and it consists
> of the following: The build environment can export an environment
> variable called SOURCE_DATE_EPOCH with a known timestamp in Unix epoch
> format (In our case, we use the last date of the package's debian
> changelog). The toolchain package running during the build can check
> if the exported variable is set and if so, instead of embedding the
> local date/time, embed the date/time from SOURCE_DATE_EPOCH.

> The proposal to use SOURCE_DATE_EPOCH has now been gathered in a more
> formal specification [4], so that any project can adhere to it to
> achieve reproducible builds when dealing with timestamps.

> It would be very beneficial to our project (and other free software
> projects working on reproducible builds) if gcc supported this
> feature.

I'm attaching a patch for the svn/trunk GCC repository (now GCC 7) that
enables this feature: it modifies the behavior of the macros __DATE__
and __TIME__ when the environment variable SOURCE_DATE_EPOCH is
exported.

Documentation of the environment variable is also provided.

Note: I have already gone through the copyright assignment process :)


[1] https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02210.html
[2] https://gcc.gnu.org/ml/gcc-patches/2015-11/msg01890.html
[3] https://wiki.debian.org/ReproducibleBuilds/TimestampsFromCPPMacros
[4] https://reproducible-builds.org/specs/source-date-epoch/

Best regards,
-- 
Dhole

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