how to get backtrace of a coroutine's state from a ucontext_t?

[finger at spacecurve dot com]

Hi.  I've got an in-house coroutine library that is built using
{get,make,swap}context.  I would like to be able to get a backtrace of
one of these coroutines (one not currently running).

Any pointers on doing this?

Relevant versions:
x86_64
gdb 7.7
gcc 4.7.3
centos 6.5
glibc from centos6.5 (glibc-2.12-1.132.el6.x86_64)
kernel from centos6.5 (2.6.32-431.el6.x86_64)

I'm not able to even figure out how to get the "frame" command to work
by passing a frame address.  Without coroutines (to simplify), I can do
an "info frame" to get what gdb claims is the frame address.  That
frequently doesn't work, I'm guessing because we're building with
frame-pointer omit.

I am more than happy to try and script something using python.  But I don't
see any python api docs that are pointing me in the right direction.  I was
looking for something that would let me backtrace a frame at a time given
an $sp/$pc.  Or something along those lines.  Not sure if there's nothing
available for manually building frame info so that gdb can backtrace, or if
I'm just not finding it.

Thanks
Jay Finger