How to use debug version of libc

I think that the version of libc with debug symbols is in /usr/lib/debug/lib. I tried setting my LD_LIBRARY_PATH variable to have this at the front of the path but that did not seem to make a difference.

These are not the droids you are looking for.

The libraries in /usr/lib/debug are not real libraries. Rather, they contain only debug info, but do not contain .text nor .data sections of the real libc.so.6. You can read about the separate debuginfo files here.

The files in /usr/lib/debug come from libc6-dbg package, and GDB will load them automatically, so long as they match your installed libc6 version. If your libc6 and libc6-dbg do not match, you should get a warning from GDB.

You can observe the files GDB is attempting to read by setting set verbose on. Here is what you should see when libc6 and libc6-dbg do match:

(gdb) set verbose on
(gdb) run
thread_db_load_search returning 0
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/lib/ld-2.11.1.so...done.
thread_db_load_search returning 0
done.
thread_db_load_search returning 0
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from system-supplied DSO at 0x7ffff7ffb000...done.
WARNING: no debugging symbols found in system-supplied DSO at 0x7ffff7ffb000.
thread_db_load_search returning 0
Reading in symbols for dl-debug.c...done.
Reading in symbols for rtld.c...done.
Reading symbols from /lib/librt.so.1...Reading symbols from /usr/lib/debug/lib/librt-2.11.1.so...done.
thread_db_load_search returning 0
... etc ...

Update:

For instance I see
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done

That implies that your GDB is not searching /usr/lib/debug. One way that could happen is if you set debug-file-directory in your .gdbinit incorrectly.

Here is the default setting:

(gdb) show debug-file-directory
The directory where separate debug symbols are searched for is "/usr/lib/debug".

Make sure you've installed the debug symbols for libc:

sudo apt-get install libc6-dbg

And if you're on an x64 system debugging x86 code:

sudo apt-get install libc6:i386
sudo apt-get install libc6-dbg:i386