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The central data structure of the unwind API is the unwind cursor.
This structure tracks the register contents. 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
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;
unw_context_t uc;
unw_init_local(&cursor, &uc);
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.
* Multiarchitecture support
If libunwind is configured for a target other than the local (native)
host, the library is installed as libunwind-$ARCH, where $ARCH is
the target architecture name (e.g., ia32, ia64, or alpha). Similarly,
the header file is installed as libunwind-$ARCH.
With this setup, an application should:
- include <libunwind.h>, and
- link against -lunwind
if the application needs to use the unwinder of the host. An
application wanting to use the unwinder for a different target (e.g.,
a cross-debugger) should:
- include <libunwind-$ARCH.h>, and
- link against -lunwind-$ARCH
The global symbols exported by -lunwind-$ARCH are unique such that the
same application can be linked against the separate unwind libraries
of multiple targets. However, a single compilation unit can include
the header file for only one target. For example, foo.c might include
<libunwind-ia64.h> and bar.c might include <libunwind.h> and the
entire application would have to be linked against both -lunwind and
Note: the unwind header files of all targets have a common dependency
on libunwind-common.h. To avoid version conflicts, it is necessary to
ensure that the unwind libraries for all targets were derived from the
same release of libunwind. That is, if the unwind library for one
target is upgraded to a newer version, the libraries for all other
targets also need to be upgraded.
Note 2: The assumption is that a cross-unwinder can handle all
interesting flavors of a target. For example, the unwinder for the
ia64 target is expected to be able to handle both Linux and HP-UX.
* IA-64 Specific Information
Apart from the normal frame-registers, the IA-64 implementation of
libunwind provides the means to access the current value of the
register backing store pointer (bsp). One quirk with this
frame-register is that it corresponds to the address that would be in
register ar.bsp after flushing the current register stack to the
backing store (i.e., as if a "flushrs" instruction had been executed).
Of course, given this value and the contents of the current frame
marker (CFM), it's easy to calculate the original value of ar.bsp:
unw_word_t cfm, bsp, bsp_after_flushrs, sof;
unw_get_reg (&cursor, UNW_IA64_BSP, &bsp_after_flushrs);
unw_get_reg (&cursor, UNW_IA64_CFM, &cfm);
bsp = ia64_rse_skip_regs (bsp_after_flushrs, -(cfm & 0x7f));
** Dynamic Unwind Info