The central data structure of the unwind API is the unwind cursor.
This structure tracks the frame registers and the preserved registers.
The distinction between frame registers and preserved registers is
important: the former represent the *current* value of a register (as
it existed at the current IP); the latter represent the *saved* value
of a register (i.e., the value that existed on entry to the current
procedure).  The unwind API defines a handful of well-known frame
"registers":

        - ip: the instruction pointer (pc)
        - rp: the return pointer (rp, aka "return address" or "return link")
        - sp: the stack pointer (memory stack pointer, in the case of ia64)
        - fp: the frame pointer
        - first_ip: the starting address of the current "procedure"
        - handler: a pointer to an architecture & language-specific
          "personality" routine
        - lsda: a pointer to an architecture & language-specific
          data-area

The API defines no well-known preserved registers.  Each architecture
can define additional registers as needed.  Of course, a portable
application may only rely on well-known registers.  The names for
preserved registers are defined in the architecture-specific header
file <unwind-ARCH.h>.  For example, to get the IA-64-specific register
names, an application would do:

	#include <unwind-ia64.h>

The API is designed to handle two primary cases: unwinding within the
current (local) process and unwinding of another ("remote") process
(e.g., through ptrace()).  In the local case, the initial machine
state is captured by an unwind context (currently the same as
ucontext_t).  In the remote case, the initial machine state is
captured by an unwind accessor structure, which provides callback
routines for reading/writing memory and registers and for obtaining
unwind information.

Once a cursor has been initialized, you can step through the call
chain with the unw_step() routine.  The frame registers and the
preserved state can then be accessed with unw_get_reg() or modified
with unw_set_reg().  For floating-point registers, there are separate
unw_get_fpreg() and unw_set_fpreg() routines (on some arches, e.g.,
Alpha, these could be just aliases for unw_{g,s}et_reg()).  The
unw_resume() routine can be used to resume execution at an arbitrary
point in the call-chain (as identified by an unwind cursor).  This is
intended for exception handling and, at least for now, the intention
is to support this routine only for the local case.  Kevin, if you
feel gdb could benefit from such a routine, I'd be interested to hear
about it.

Note that it is perfectly legal to make copies of the unwind cursor.
This makes it possible, e.g., to obtain an unwind context, modify the
state in an earlier call frame, and then resume execution at the point
at which the unwind context was captured.

Here is a quick example of how to use the unwind API to do a simple
stack trace:

    unw_cursor_t cursor;
    unw_word_t ip, sp;
    ucontext_t uc;

    getcontext(&uc);
    unw_init_local(&cursor, &uc);
    do
      {
        unw_get_reg(&cursor, UNW_REG_IP, &ip);
        unw_get_reg(&cursor, UNW_REG_SP, &sp);
        printf ("ip=%016lx sp=%016lx\n", ip, sp);
      }
    while (unw_step (&cursor) > 0);

Note that this particular example should work on pretty much any
architecture, as it doesn't rely on any arch-specific registers.