What do I need to debug pthreads?

Question

I want to debug pthreads on my custom linux distribution but I am missing something. My host is Ubuntu 12.04, my target is an i486 custom embedded Linux built with a crosstool-N

Answer 1

This:

dlopen failed on 'libthread_db.so.1' - /lib/libthread_db.so.1: undefined symbol: ps_lgetfpregs
GDB will not be able to debug pthreads.

means that the libthread_db.so.1 library was not able to find the symbol ps_lgetfpregs in gdb.

Why?

Because I built gdb using Crosstoolg-NG with the "Build a static native gdb" option and this adds the -static option to gcc.

The native gdb is built with the -rdynamic option and this populates the .dynsym symbol table in the ELF file with all symbols, even unused ones. libread_db uses this symbol table to find ps_lgetfpregs from gdb.

But -static strips the .dynsym table from the ELF file.

At this point there are two options:

1. Don't build a static native gdb if you want to debug threads.
2. Build a static gdb and a static libthread_db (not tested)

Edit:

By the way, this does not explain why Breakpad in unable to debug multithreaded applications on my target.