[CentOS] Building for older versions

Mon Nov 23 16:57:13 UTC 2015
Michael Eager <eager at eagerm.com>

On 11/23/2015 07:43 AM, Chris Adams wrote:
> Once upon a time, Michael Eager <eager at eagerm.com> said:
>> I'm trying to build an application on CentOS 7 which
>> can run on older versions of CentOS.  I'm running into
>> problems with versioning of memcpy in Glibc.  Executables
>> built on CentOS 7 require memcpy from glibc-2.14, which
>> causes the program not to load on systems with older
>> versions of glibc.
> Most shared libraries are "upwards compatible" but not "backwards
> compatible" - builds against an old version will run with the new
> version, but not the other way around.  You've found this with glibc,
> but you could also run into it with other libraries.

The situation with memcpy is a bit different.  This isn't really a
forward/backward interface compatibility issue.

There was a patch applied to memcpy to improve performance on
some architectures, but it also changed the order in which data was
moved.  Some programs were dependent on this and they broke with the
new implementation.  These programs did not conform to the non-overlapping
data requirements in memcpy's specification.  Programs which did conform
worked with both the new and old implementation.

To prevent non-conforming programs from using the new version of
memcpy, they tagged it with glibc-2.14.  Unfortunately, this also
makes conforming programs, which work with either the old or new
implementation from running on systems which have older versions of

>> My online search suggests to add an asm() with a .symver
>> option to select memcpy from glibc-2.2.5 in each of the
>> source files which reference memcpy().  This isn't practical
>> with a program with tens of thousands of source files.
>> Does anyone have a reasonable solution?
> Would it be practical to use mock and build on the oldest version you
> want to support?  This is how EPEL packages are built for example.  It
> is targeted at building RPMs, but you can manually use copy-in and
> copy-out to do other things.

I'll look into mock.

Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077