Handling GNU flags for tar & SOURCE_DATE_EPOCH

[Ryan Carsten Schmidt]

On May 1, 2025, at 16:16, Mojca Miklavec wrote:
> 
> I ran into software doing the following:
> 
> set( TARFLAGS cfz )
> if( DEFINED ENV{SOURCE_DATE_EPOCH} )
>    set( TARFLAGS --sort=name --mtime=@$ENV{SOURCE_DATE_EPOCH}
> --clamp-mtime --owner=0 --group=0 --numeric-owner
> --pax-option=exthdr.name=%d/PaxHeaders/%f,delete=atime,delete=ctime
> -czf )
> endif()
> 
> Basically: if SOURCE_DATE_EPOCH is defined, run
>    tar --sort=name --mtime=...
> which fails on macOS because --sort, --mtime etc. are all GNU extensions.
> 
> What's the best way to handle this?
> 
> 1.) Can the code be rewritten in such a way that it will work with
> native tar, and yet result in reproducible builds?
> 
> 2.) If that is not possible, I guess that using gnu tar as build
> dependency is the best bet?
> 
> 3.) Can SOURCE_DATE_EPOCH be easily unset during configure/build step?
> 
> Thank you,
>    Mojca

Report to the developers of that software that they have GNU-tar-specific assumptions in their code and ask them to rewrite it in a portable way that will also work with BSD tar.