In Debian, to get ?debugging information for glibc, you need to install the libc0.3-dbg package. At the place ?GDB looks for debugging symbols by default (/usr/lib/debug/lib/), Debian's libc0.3-dbg stores only the frame unwind information used for backtracing. If you want to step into glibc while debugging, you need to add LD_LIBRARY_PATH=/usr/lib/debug to debugged program's environment (set env VAR value from the GDB command line). If that still does not work, try LD_PRELOAD=/usr/lib/debug/ instead.