Dan Kegel writes . . .

As Linux becomes more popular with embedded system developers, it's becoming increasingly important for gdb to be able to do remote debugging of multithreaded programs in an embedded Linux target. As of gdb 5.2, gdb's “gdbserver” remote probe did not support this.

At long last, this situation is improving. On 11 June 2002, Daniel Jacobowitz added support for remote debugging of multithreaded programs. As of June 15th, this is only in the CVS tree; hopefully it will make its way to a gdb release soon.

I have tested this feature on x86 and ppc405 linux, and it works — from an x86 Debian system, I successfully debugged multithreaded programs running on remote x86 and ppc405 Linux boxes. There are a few gotchas:

• It's not well-documented how to build gdbserver for cross-debugging (though it's not as hard as it used to be). I believe you have to configure and make gdb, then configure and make gdbserver. I'll try to post a recipe here soon.
• When cross-debugging (i.e. when the remote system is running a different OS or on a different processor type), you have to use commands like
  

  set solib-absolute-prefix /dev/null
  set solib-search-path /opt/hardhat/devkit/ppc/405/target/lib

  when you start gdb so it can find your shared libraries, or it will fail with strange errors. The GDB manual is silent on this important topic; Daniel Jacobowitz told me about it, and I also found some doc about it at Axis Communications.

• When you start gdb and attach to a remote gdbserver, you must 'continue' rather than 'run', as the program is already started.
• It's ok to run a stripped binary on the remote system as long as you keep the unstripped binary around for use by gdb on the local system. This is pretty important, since unstripped binaries compiled with -g can be too big to fit on some embedded devices.

Hats off to Daniel Jacobowitz of Montavista Software for getting this feature in, and for his active support of gdb on the gdb mailing list.

Related Web Pages

• GDB home page
• The gdb mailing list archive
• GDB doc page about remote debugging
• Axis Communications doc about setting solib-absolute-prefix
• GDB Remote Debugging Protocol
• Multi-arch gdb protocol proposal
• The CrossGCC mailing list archive
• Bill Gatliff
  • New CrossGCC FAQ
  • Crossgcc Wiki
  • BuildGdbServer Wiki page
  • gdbstubs project at sourceforge
• Dimi Shahbaz's script for building binutils/gcc3/glibc toolchains for sh4, ppc405, and ppc750
• Quality Quorum, Inc.
  • Early draft of formal spec for GDB Remote Protocol (including multithread support)
  • Example Implementation of gdb remote protocol for RTEMS
  • rproxy – an enhanced gdbserver (doesn't support multiple threads)

Copyright 2002 Dan Kegel; reproduced by LinuxDevices.com with permission. Last update: June 17, 2002 — for updates, visit Dan's website.

 
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