Re: [9fans] acme, rio workalike available in plan 9 ports

[Christoph Hellwig <hch@lst.de>]

> Let's take program 'a', which depends on stat. In the new order of gcc,
> when built, 'a' will depend on stat from glib 2.0. A new stat comes along
> with fixes. It gets built into glibc 2.1. You install glibc 2.1. Program
> 'a', unless I rebuild or replace it, will be using the old stat. Of
> course, I might think that the shared library has fixed all binaries using
> stat, and I'm wrong -- or am I right? Is the V1 stat just a wrapper? who
> knows? And do you cover all the cases?

Stop here.  You don't get a new symbol version just because your new
version ends up in a new glibc.  so unless your fix changes the
interface to stat it will retain it's old symbol version.  Bumping the
symbol version is an explicit action of the program author.  And because
of the maintaince issue they rewrite the old version as a wrapper around
the new one in every case I've seen so far.  If this wasn't the case
_and_ the author wouldn't update the version that would be considered a
huge bug indeed.   But that's not what happens in real life.

I've looked for an example that shows this without too much code and
while we're at it shows some glibc braindamage (messing up kernel
syscalls when translating them to library calls), and that would be the
sched_getaffinity call, implemented in
sysdeps/unix/sysv/linux/sched_getaffinity.c.

We have a routine called __sched_getaffinity_new implementing a small
wrapper for the sched_getaffinity syscalls.  With

versioned_symbol (libc, __sched_getaffinity_new, sched_getaffinity,
                  GLIBC_2_3_4);

it's exported as sched_getaffinity for the symbol version GLIBC_2_3_4.

Then

int
attribute_compat_text_section
__sched_getaffinity_old (pid_t pid, cpu_set_t *cpuset)
{
  /* The old interface by default assumed a 1024 processor bitmap.  */
   return __sched_getaffinity_new (pid, 128, cpuset);
}
compat_symbol (libc, __sched_getaffinity_old, sched_getaffinity, GLIBC_2_3_3);

implements the older version ontop of the new one.