binary compatibility query

[Kris Kennaway]

Carl Shapiro wrote:
> FreeBSD Hackers,
> 
> I have a general question about the compatibility of FreeBSD binaries
> within major releases.  If I build a binary for a given release of
> FreeBSD can I make a reasonable guarantee that the binary will run on
> both previous and subsequent minor releases of the same major release?
> In other words, if I build on FreeBSD 6.3 and do not rely on anything
> unique to 6.3 (such as the presence of specific version strings) how
> certain can I be that the code will or will not run on 6.2, 6.1 etc.?
> 
> Also, is this documented anywhere on the FreeBSD web site?  The
> closest thing I could find is the following guidance for driver
> vendors which falls just short of answering my question:
> 
> http://wiki.freebsd.org/VendorInformation
> 
> (Too bad the fancy illustration is missing.)

Binaries compiled on a certain version of FreeBSD will continue to run 
on later versions, but are not guaranteed to run on earlier versions 
(and in fact *will* not run depending on the binary).  This is because 
over time the system libraries and kernel grow new features which may be 
used by applications, so they will therefore fail to run if executed on 
old systems that do not provide these features.

If your goal is to provide an application that runs on a range of 
FreeBSD versions, then either build it for the oldest of these versions, 
or provide multiple versions if there is a reason to do so (e.g. if 
there have been major improvements in the OS that are relevant to your 
application).

Kris