unwind, exc_virtual_unwind, RtlVirtualUnwind, exc_find_frame_ptr,
exc_remote_virtual_unwind - routines to unwind a context
PRUNTIME_FUNCTION prf ); void exc_virtual_unwind(
PCONTEXT pcontext ); exc_address RtlVirtualUnwind(
PCONTEXT_POINTERS ppointers ); exc_address exc_find_frame_ptr(
PCONTEXT pnext_context ); unsigned long exc_remote_vir‐
int (*fetch_from_process) (void *handle, void *addr, void *buf‐
fer, long size),
PCONTEXT pcontext );
Pointer to a struct sigcontext (see signal(2)) used to represent a pro‐
cedure's context. Pointer to a struct sigcontext (see signal(2)) used
to represent the context of a procedure's caller. If you specify a
nonzero pnext_context argument, the pcontext argument is ignored.
Pointer to run-time function (code-range descriptor) for the PC stored
in the sc_pc field of the activation context record; a call to
exc_lookup_function_entry() returns this value. Although this argument
can be zero, by providing this argument, a caller already having this
information would save an extra search for the run-time function. Copy
of the sc_pc field of the activation context record. Pointer to struc‐
ture containing addresses corresponding to the locations that were used
to restore registers; if zero, this argument is ignored.
The following descriptions apply to parameters for the exc_remote_vir‐
tual_unwind() routine. How these parameters are interpreted depends on
whether the process involved in the operation is local or remote.
Remote unwind: A pointer to an arbitrary block of memory that is set up
and managed by the user application and passed through the exception
handling routines. This allows the author of the function identified
by the fetch_from_process parameter to maintain any necessary state.
The state maintained here will most likely include the identity of the
process to be unwound.
Local unwind: This parameter is ignored. Remote unwind: The
address of an application-specific function, written by the
author of the application that calls exc_remote_vir‐
tual_unwind(). If the value of the fetch_from_process parameter
is NULL, the unwind takes place in the local process, not a
The function identified by the fetch_from_process parameter
takes the following arguments: The handle parameter that was
passed into exc_remote_virtual_unwind(). The starting address
in the remote process from which to copy memory. A buffer in
the local process into which data from the remote process is
copied. The number of bytes to copy from addr to buffer. The
region of memory pointed to by buffer must be at least size
bytes in length.
The function identified by the fetch_from_process parameter
returns 0 (zero) to indicate success and nonzero to indicate a
failure in accessing the remote address space. A failure will
also cause an exception status to be raised.
Local unwind: The fetch_from_process parameter is ignored.
Remote unwind: The address of the cell exc_crd_list_head in the
remote process. The means by which this address is communicated
to the local (tracing) process is application specific.
Local unwind: This parameter is ignored. Remote unwind: This
parameter is ignored.
Local unwind: The address of a code-range descriptor describing
the context from which to begin the unwind. If NULL, the code-
range descriptor is looked up based on the PC contained in the
pcontext parameter. This is often passed as NULL for the initial
invocation of exc_remote_virtual_unwind() and is always passed
as NULL for iterated invocations during the stack walk. A
pointer to a struct sigcontext representing the context of the
procedure (local or remote) to be unwound.
At a minimum, this context structure must contain the following
context for the routine to be unwound: FP register ($15), SP
register ($30), RA register ($26 in a standard call), and the
PC. Additionally, the context may contain the preserved regis‐
This argument needs to be set up only once; consecutive calls to
exc_remote_virtual_unwind() manipulate this structure to repre‐
sent the state of successively earlier procedures in the call
All of the unwind routines perform a virtual unwind. Unlike the rou‐
tines described in exc_resume(3), these routines do not actually unwind
a procedure call by modifying the real registers and other machine
state. Instead, these routines modify the structure pointed to by the
pcontext argument so that it represents the previous procedure in the
call stack. The routines use procedure information supplied in the
structure pointed to by the prf argument to decide how to virtually
unwind the context (for instance, how to modify the registers and other
machine state). This information is placed in the object by the assem‐
bler and linker and conforms to the calling standard for Alpha systems.
If the specify a procedure's context in pcontext and the pnext_context
argument is nonzero, exc_find_frame_ptr generates a copy of the pcon‐
text argument. The original copy of the context is not modified. If
you supply a pointer to the context of the caller in the pnext_context
argument, exc_find_frame_ptr() returns the stack pointer associated
with that context as the frame pointer of the current context.
The other unwind routines modify the structure pointed to by the pcon‐
text argument in order to represent the context of the caller.
Remote unwinding by the exc_remote_virtual_unwind() function can
involve a local or remote process -- unlike the unwind(), exc_vir‐
tual_unwind(), and RtlVirtualUnwind() functions that operate only on
local processes. Remote unwinding is controlled by the
fetch_from_process parameter. This parameter is a pointer to (or handle
for) an application-supplied function that knows how to access the mem‐
ory of the process (local or remote) being acted on: If the user appli‐
cation passes a NULL value in the fetch_from_process parameter, the
local process is performing an unwind on its own stack. This allows the
unwind routine to make certain optimizations in its processing. If the
user application passes the address of a routine in the
fetch_from_process parameter, the unwind routine is not allowed to
assume anything about the process it is accessing, and only the fetch
routine is allowed to know the identity of the process being unwound.
It might be the local process, it might be another process in the sys‐
tem, it might be a process on another system on the network, or it
might be a corefile from a long-deceased process.
To summarize, remote unwinding is the capability to unwind the stack of
a process that is not necessarily the process doing the unwind.
The exc_remote_virtual_unwind() function returns 1 to indicate that the
program being unwound was in the prologue or epilogue of the frame for
the current context; otherwise, it returns 0.
All of the unwind routines typically use masks and stack offsets found
in procedure related data structures (described in the Calling Standard
for Alpha Systems) to restore registers. Those data structures also can
contain enough information for these routines to adequately deal with
prologues, epilogues, and signal frames.
Users writing assembly language routines should consult the Assembly
Language Programmer's Guide to determine which directives are required
to provide enough information for these routines to correctly unwind
The origins of the various unwind routines are as follows: unwind()
originated in the ULTRIX libexc and has an interface compatible with
the original one, as long as the ULTRIX caller treated the prf argument
as an opaque pointer. The prf structure has been changed to conform to
the calling standard for Alpha systems, and any callers that explicitly
access its fields will encounter incompatibilities. exc_vir‐
tual_unwind() originated in libexc. RtlVirtualUnwind() is a Microsoft
Windows NT run-time library interface. It returns a copy of the updated
sc_pc field of the sigcontext when leaving the routine. The routine
also updates the structure pointed to by the ppointers argument.
/usr/ccs/lib/cmplrs/cc/libexc.a -- exception handling library
/usr/include/excpt.h -- include file
/usr/include/pdsc.h -- include file
/usr/include/signal.h -- include file
/usr/include/machine/fpu.h -- include file
Functions: exception_intro(3), exception_dispatcher(3),
exc_lookup_function_entry(3), signal(2), sigaction(2), setjmp(3),
exc_unwind(3), __exc_last_chance(3), ieee(3)
Files: excpt(4), c_excpt(4), signal(4), pdsc(4)
Assembly Language Programmer's Guide
Calling Standard for Alpha Systems