RFC: libc6 policy supplement 2nd try
Here is a reworked proposal:
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Debian library policy supplement draft for libc5->libc6 migration
This document is meant to tell what a Debian package providing a
library should do to support both libc6 (glibc2) and libc5.
Note that these requirements are for Debian 2.0 (codename hamm).
Contents
1. Run time packages
2. Development packages
3. Requirements on libraries for Debian 2.0
4. Requirements on compiler packages
1. Run time packages
A package providing a shared library has to support both C library
packages, libc5 and libc6 based libraries. This must be done using
two Debian packages, each depending on the correct C library
package.
The package naming convention currently suggests to name these
packages as follows [1]. Some packages (mostly from base) may use
locations in /lib.
based on | package name | library location
--------------------------------------------
libc6 | libfoo | /usr/lib/libfoo.so.<ver>
libc5 | libfoo-g | /usr/lib/libc5-compat/libfoo.so.<ver> [2]
If a library runtime package contains files that are needed by
both versions of the library, a new package should be made for
just these files that both other packages depend on.
This package naming convention does _not_ apply if a package uses
different sonames for libc5 and libc6 based packages
There are two exceptions from this rule. The shared linker
ld-linux.so.1 and the C library files libc.so.5 and libm.so.5
should still be located in /lib, not in /lib/libc5-compat.
Packages based on X have to use /usr/X11R6 as prefix, not /usr.
Note that the X libraries are designed to work with both C libraries.
2. Development packages
The Debian policy requires that all files needed for compiling/linking
other packages with the library are in a separate package, the
development package. Up to now this package simply was called
libfoo-dev. As packages based on libc5 and libc6 usually cannot
use the same development files there has to be a clear statement
how to separate these. So for now the following packages are
required:
based on | package name | hierarchy locations
---------------------------------------------------------------
libc6 | libfoo-dev | /usr/{lib,include}
libc5 | libfoo-g-altdev | /usr/i486-linuxlibc1/{lib,include}
Note that the choice to base Debian 2.0 on libc6 fixed the fact
that the main locations will be used for libc6 packages. The
alternate locations are used for libc5 based packages.
This decision does not necessarily mean that by default the
compiler uses the libc6 packages, please read section 4 for more
information. Using a four-way approach for library locations
(standard and alternate locations for libc6 and libc5 based
packages) will make Debian systems inconsistent with each other,
something we should avoid at (nearly) all costs.
3. Requirements on libraries for Debian 2.0
Libraries (regardless of which library they're compiled against) need
to have runtime dependencies on one of libc, libdl or libm to enable
the shared linker to determin which library to use for a binary.
In general we want libraries compiled for libc6 to be thread-safe.
This means that the following changes must be made to the library
packages:
- compile the library using -D_RENTRANT or -D_THREAD_SAFE
- there may be no permanent data residing in the library memory that
can be different for different threads.
this means in the first place no static or global variables that
are not in some way protected from access by a different threads.
- all write access to files from a library must be both protected
using some file locking mechanism in addition to using mutexes.
- library functions must be protected from being used at the same
time by two threads sharing the same memory space. This is done
using mutexes.
As these usually are all nontrivial changes to a library if it isn't
thread-safe already (in which case just using -D_RENTRANT should
be used in addition to whatever the library uses to support threads),
I suggest that noone starts doing this without getting in contact with
the upstream maintainer(s).
There will be a list available that lists all libraries part of
Debian and their current status regarding compliance with these
standard requirements. This list will be posted regularly to
debian-devel by Helmut Geyer <Helmut.Geyer@iwr.uni-heidelberg.de>.
4. Requirements on compiler packages
The compiler and binutils packages have to provide working
development environments for both C libraries. Basically (that is
from the compiler standpoint) there is no real difference between
the two environments, only some paths and automatic definitions
have to be changed. All this can be done (and is in fact done) by
supplying a different specs file in a different location.
The gcc packages do this as follows:
The gcc package uses libc6 by default and is installed in /usr/bin.
The alt-gcc package uses libc5 by default and is located in
/usr/i486-linuxlibc1/bin. By prepending this to the path this can
be made the default.
These requirements are fulfilled by the current gcc packages.
Remarks:
[1] I'm suggesting a different naming convention than David Engel because
there may be libraries out there having a 'g' as last component of the
soname. An additional hyphen doesn't hurt much and makes the scheme
easier to read.
[2] The location ../libc5-compat was introduced in the ldso package. As
ldso is a package on all linus distributions we'll keep it for
compatibility with other distributons even though
/usr/i486-linuxlibc1/lib would be more consistent.
--
Helmut Geyer Helmut.Geyer@iwr.uni-heidelberg.de
public PGP key available : finger geyer@saturn.iwr.uni-heidelberg.de
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