dlsym(RTLD_NEXT, "msgctl") does not return the default version

Question

If a library defines multiple version of the same symbol, dlsym(RTLD_NEXT, "symbol") returns the older symbol even though the older symbol is not the default symbol.

For example, libpthread defines two versions of pthread_cond_broadcast:

$:> nm -g /lib64/libpthread-2.15.so|grep pthread_cond_broadcast
000000000000bfc0 T pthread_cond_broadcast@@GLIBC_2.3.2
000000000000c310 T pthread_cond_broadcast@GLIBC_2.2.5

• "GLIBC_2.3.2" is the default version that you get when linking with libpthread (without any dlsym involvement). (Notice the "@@" which signifies the default symbol)
• "GLIBC_2.2.5" is an older version

Now, if I use dlsym(RTLD_NEXT, "pthread_cond_broadcast"), I always get the GLIBC_2.2.5 version and not the GLIBC_2.3.2 version. Of course one can use dlvsym to get the default version, but that gets complicated if one needs to do it for a large number of symbols and a lot of them have different new/old versions.

I do understand that RTLD_NEXT shouldn't always return the latest symbol to maintain compatibility, but why not return the default symbol?

Does anyone know about the rationale behind this?

Answer 1

This has been reported as a glibc bug:

• dlsym(handle, "foo") and dlsym(RTLD_NEXT, "foo") return different result with versioned "foo"

As far as I know, it has not been fixed yet.

Answer 2

The whole point of RTLD_NEXT is to return the symbol after the first, default one. Aren't you just looking for RTLD_DEFAULT?