doc/libunwind.man

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.TH "LIBUNWIND" "3" "05 August 2004" "Programming Library " "Programming Library "
.SH NAME
libunwind
\-\- a (mostly) platform\-independent unwind API 
.PP
.SH SYNOPSIS

.PP
#include <libunwind.h>
.br
.PP
int
unw_getcontext(unw_context_t *);
.br
int
unw_init_local(unw_cursor_t *,
unw_context_t *);
.br
int
unw_init_remote(unw_cursor_t *,
unw_addr_space_t,
void *);
.br
int
unw_step(unw_cursor_t *);
.br
int
unw_get_reg(unw_cursor_t *,
unw_regnum_t,
unw_word_t *);
.br
int
unw_get_fpreg(unw_cursor_t *,
unw_regnum_t,
unw_fpreg_t *);
.br
int
unw_set_reg(unw_cursor_t *,
unw_regnum_t,
unw_word_t);
.br
int
unw_set_fpreg(unw_cursor_t *,
unw_regnum_t,
unw_fpreg_t);
.br
int
unw_resume(unw_cursor_t *);
.br
.PP
unw_addr_space_t
unw_local_addr_space;
.br
unw_addr_space_t
unw_create_addr_space(unw_accessors_t,
int);
.br
void
unw_destroy_addr_space(unw_addr_space_t);
.br
unw_accessors_t
unw_get_accessors(unw_addr_space_t);
.br
void
unw_flush_cache(unw_addr_space_t,
unw_word_t,
unw_word_t);
.br
int
unw_set_caching_policy(unw_addr_space_t,
unw_caching_policy_t);
.br
.PP
const char *unw_regname(unw_regnum_t);
.br
int
unw_get_proc_info(unw_cursor_t *,
unw_proc_info_t *);
.br
int
unw_get_save_loc(unw_cursor_t *,
int,
unw_save_loc_t *);
.br
int
unw_is_fpreg(unw_regnum_t);
.br
int
unw_is_signal_frame(unw_cursor_t *);
.br
int
unw_get_proc_name(unw_cursor_t *,
char *,
size_t,
unw_word_t *);
.br
.PP
void
_U_dyn_register(unw_dyn_info_t *);
.br
void
_U_dyn_cancel(unw_dyn_info_t *);
.br
.PP
.SH LOCAL UNWINDING

.PP
Libunwind
is very easy to use when unwinding a stack from 
within a running program. This is called \fIlocal\fP
unwinding. Say 
you want to unwind the stack while executing in some function 
F().
In this function, you would call unw_getcontext()
to get a snapshot of the CPU registers (machine\-state). Then you 
initialize an \fIunwind cursor\fP
based on this snapshot. This is 
done with a call to unw_init_local().
The cursor now points 
to the current frame, that is, the stack frame that corresponds to the 
current activation of function F().
The unwind cursor can then 
be moved ``up\&'' (towards earlier stack frames) by calling 
unw_step().
By repeatedly calling this routine, you can 
uncover the entire call\-chain that led to the activation of function 
F().
A positive return value from unw_step()
indicates 
that there are more frames in the chain, zero indicates that the end 
of the chain has been reached, and any negative value indicates that 
some sort of error has occurred. 
.PP
While it is not possible to directly move the unwind cursor in the 
``down\&'' direction (towards newer stack frames), this effect can be 
achieved by making copies of an unwind cursor. For example, a program 
that sometimes has to move ``down\&'' by one stack frame could maintain 
two cursor variables: ``curr\&''
and ``prev\&''\&.
The former 
would be used as the current cursor and prev
would be maintained 
as the ``previous frame\&'' cursor by copying the contents of curr
to prev
right before calling unw_step().
With this 
approach, the program could move one step ``down\&'' simply by copying 
back prev
to curr
whenever that is necessary. In the most 
extreme case, a program could maintain a separate cursor for each call 
frame and that way it could move up and down the callframe\-chain at 
will. 
.PP
Given an unwind cursor, it is possible to read and write the CPU 
registers that were preserved for the current stack frame (as 
identified by the cursor). Libunwind
provides several routines 
for this purpose: unw_get_reg()
reads an integer (general) 
register, unw_get_fpreg()
reads a floating\-point register, 
unw_set_reg()
writes an integer register, and 
unw_set_fpreg()
writes a floating\-point register. Note that, 
by definition, only the \fIpreserved\fP
machine state can be accessed 
during an unwind operation. Normally, this state consists of the 
\fIcallee\-saved\fP
(``preserved\&'') registers. However, in some 
special circumstances (e.g., in a signal handler trampoline), even the 
\fIcaller\-saved\fP
(``scratch\&'') registers are preserved in the stack 
frame and, in those cases, libunwind
will grant access to them 
as well. The exact set of registers that can be accessed via the 
cursor depends, of course, on the platform. However, there are two 
registers that can be read on all platforms: the instruction pointer 
(IP), sometimes also known as the ``program counter\&'', and the stack 
pointer (SP). In libunwind,
these registers are identified by 
the macros UNW_REG_IP
and UNW_REG_SP,
respectively. 
.PP
Besides just moving the unwind cursor and reading/writing saved 
registers, libunwind
also provides the ability to resume 
execution at an arbitrary stack frame. As you might guess, this is 
useful for implementing non\-local gotos and the exception handling 
needed by some high\-level languages such as Java. Resuming execution 
with a particular stack frame simply requires calling 
unw_resume()
and passing the cursor identifying the target 
frame as the only argument. 
.PP
Normally, libunwind
supports both local and remote unwinding 
(the latter will be explained in the next section). However, if you 
tell libunwind that your program only needs local unwinding, then a 
special implementation can be selected which may run much faster than 
the generic implementation which supports both kinds of unwinding. To 
select this optimized version, simply define the macro 
UNW_LOCAL_ONLY
before including the headerfile 
<libunwind.h>\&.
It is perfectly OK for a single program to 
employ both local\-only and generic unwinding. That is, whether or not 
UNW_LOCAL_ONLY
is defined is a choice that each source\-file 
(compilation\-unit) can make on its own. Independent of the setting(s) 
of UNW_LOCAL_ONLY,
you\&'ll always link the same library into 
the program (normally \fB\-l\fPunwind).
Furthermore, the 
portion of libunwind
that manages unwind\-info for dynamically 
generated code is not affected by the setting of 
UNW_LOCAL_ONLY\&.
.PP
If we put all of the above together, here is how we could use 
libunwind
to write a function ``show_backtrace()\&''
which prints a classic stack trace: 
.PP
.Vb
#define UNW_LOCAL_ONLY
#include <libunwind.h>

void show_backtrace (void) {
  unw_cursor_t cursor; unw_context_t uc;
  unw_word_t ip, sp;

  unw_getcontext(&uc);
  unw_init_local(&cursor, &uc);
  while (unw_step(&cursor) > 0) {
    unw_get_reg(&cursor, UNW_REG_IP, &ip);
    unw_get_reg(&cursor, UNW_REG_SP, &sp);
    printf ("ip = %lx, sp = %lx\\n", (long) ip, (long) sp);
  }
}
.Ve
.PP
.SH REMOTE UNWINDING

.PP
Libunwind
can also be used to unwind a stack in a ``remote\&'' 
process. Here, ``remote\&'' may mean another process on the same 
machine or even a process on a completely different machine from the 
one that is running libunwind\&.
Remote unwinding is typically 
used by debuggers and instruction\-set simulators, for example. 
.PP
Before you can unwind a remote process, you need to create a new 
address\-space object for that process. This is achieved with the 
unw_create_addr_space()
routine. The routine takes two 
arguments: a pointer to a set of \fIaccessor\fP
routines and an 
integer that specifies the byte\-order of the target process. The 
accessor routines provide libunwind
with the means to 
communicate with the remote process. In particular, there are 
callbacks to read and write the process\&'s memory, its registers, and 
to access unwind information which may be needed by libunwind\&.
.PP
With the address space created, unwinding can be initiated by a call 
to unw_init_remote().
This routine is very similar to 
unw_init_local(),
except that it takes an address\-space 
object and an opaque pointer as arguments. The routine uses these 
arguments to fetch the initial machine state. Libunwind
never 
uses the opaque pointer on its own, but instead just passes it on to 
the accessor (callback) routines. Typically, this pointer is used to 
select, e.g., the thread within a process that is to be unwound. 
.PP
Once a cursor has been initialized with unw_init_remote(),
unwinding works exactly like in the local case. That is, you can use 
unw_step()
to move ``up\&'' in the call\-chain, read and write 
registers, or resume execution at a particular stack frame by calling 
unw_resume\&.
.PP
.SH CROSS\-PLATFORM AND MULTI\-PLATFORM UNWINDING

.PP
Libunwind
has been designed to enable unwinding across 
platforms (architectures). Indeed, a single program can use 
libunwind
to unwind an arbitrary number of target platforms, 
all at the same time! 
.PP
We call the machine that is running libunwind
the \fIhost\fP
and the machine that is running the process being unwound the 
\fItarget\fP\&.
If the host and the target platform are the same, we 
call it \fInative\fP
unwinding. If they differ, we call it 
\fIcross\-platform\fP
unwinding. 
.PP
The principle behind supporting native, cross\-platform, and 
multi\-platform unwinding is very simple: for native unwinding, a 
program includes <libunwind.h>
and uses the linker switch 
\fB\-l\fPunwind\&.
For cross\-platform unwinding, a program 
includes <libunwind\-PLAT\&.h>
and uses the linker 
switch \fB\-l\fPunwind\-PLAT,
where PLAT
is the name 
of the target platform (e.g., ia64
for IA\-64, hppa\-elf
for ELF\-based HP PA\-RISC, or x86
for 80386). Multi\-platform 
unwinding works exactly like cross\-platform unwinding, the only 
limitation is that a single source file (compilation unit) can include 
at most one libunwind
header file. In other words, the 
platform\-specific support for each supported target needs to be 
isolated in separate source files\-\-\-a limitation that shouldn\&'t be an 
issue in practice. 
.PP
Note that, by definition, local unwinding is possible only for the 
native case. Attempting to call, e.g., unw_local_init()
when 
targeting a cross\-platform will result in a link\-time error 
(unresolved references). 
.PP
.SH THREAD\- AND SIGNAL\-SAFETY

.PP
All libunwind
routines are thread\-safe. What this means is 
that multiple threads may use libunwind
simulatenously. 
However, any given cursor may be accessed by only one thread at 
any given time. 
.PP
To ensure thread\-safety, some libunwind
routines may have to 
use locking. Such routines \fImust not\fP
be called from signal 
handlers (directly or indirectly) and are therefore \fInot\fP
signal\-safe. The manual page for each libunwind
routine 
identifies whether or not it is signal\-safe, but as a general rule, 
any routine that may be needed for \fIlocal\fP
unwinding is 
signal\-safe (e.g., unw_step()
for local unwinding is 
signal\-safe). For remote\-unwinding, \fInone\fP
of the 
libunwind
routines are guaranteed to be signal\-safe. 
.PP
.SH UNWINDING THROUGH DYNAMICALLY GENERATED CODE

.PP
Libunwind
provides the routines _U_dyn_register()
and 
_U_dyn_cancel()
to register/cancel the information required to 
unwind through code that has been generated at runtime (e.g., by a 
just\-in\-time (JIT) compiler). It is important to register the 
information for \fIall\fP
dynamically generated code because 
otherwise, a debugger may not be able to function properly or 
high\-level language exception handling may not work as expected. 
.PP
The interface for registering and canceling dynamic unwind info has 
been designed for maximum efficiency, so as to minimize the 
performance impact on JIT\-compilers. In particular, both routines are 
guaranteed to execute in ``constant time\&'' (O(1)) and the 
data\-structure encapsulating the dynamic unwind info has been designed 
to facilitate sharing, such that similar procedures can share much of 
the underlying information. 
.PP
For more information on the libunwind
support for dynamically 
generated code, see libunwind\-dynamic(3)\&.
.PP
.SH CACHING OF UNWIND INFO

.PP
To speed up execution, libunwind
may aggressively cache the 
information it needs to perform unwinding. If a process changes 
during its lifetime, this creates a risk of libunwind
using 
stale data. For example, this would happen if libunwind
were 
to cache information about a shared library which later on gets 
unloaded (e.g., via \fIdlclose\fP(3)).
.PP
To prevent the risk of using stale data, libunwind
provides two 
facilities: first, it is possible to flush the cached information 
associated with a specific address range in the target process (or the 
entire address space, if desired). This functionality is provided by 
unw_flush_cache().
The second facility is provided by 
unw_set_caching_policy(),
which lets a program 
select the exact caching policy in use for a given address\-space 
object. In particular, by selecting the policy 
UNW_CACHE_NONE,
it is possible to turn off caching 
completely, therefore eliminating the risk of stale data alltogether 
(at the cost of slower execution). By default, caching is enabled for 
local unwinding only. 
.PP
.SH FILES

.PP
.TP
libunwind.h
 Headerfile to include for native (same 
platform) unwinding. 
.TP
libunwind\-PLAT\&.h
 Headerfile to include when 
the unwind target runs on platform PLAT\&.
For example, to unwind 
an IA\-64 program, the header file libunwind\-ia64.h
should be 
included. 
.TP
\fB\-l\fPunwind
 Linker\-switch to add when building a 
program that does native (same platform) unwinding. 
.TP
\fB\-l\fPunwind\-PLAT
 Linker\-switch to add when 
building a program that unwinds a program on platform PLAT\&.
For example, to (cross\-)unwind an IA\-64 program, the linker switch 
\-lunwind\-ia64
should be added. Note: multiple such switches 
may need to be specified for programs that can unwind programs on 
multiple platforms. 
.PP
.SH SEE ALSO

.PP
libunwind\-dynamic(3),
libunwind\-ia64(3),
libunwind\-ptrace(3),
libunwind\-setjmp(3),
unw_create_addr_space(3),
unw_destroy_addr_space(3),
unw_flush_cache(3),
unw_get_accessors(3),
unw_get_fpreg(3),
unw_get_proc_info(3),
unw_get_proc_name(3),
unw_get_reg(3),
unw_getcontext(3),
unw_init_local(3),
unw_init_remote(3),
unw_is_fpreg(3),
unw_is_signal_frame(3),
unw_regname(3),
unw_resume(3),
unw_set_caching_policy(3),
unw_set_fpreg(3),
unw_set_reg(3),
unw_step(3),
_U_dyn_register(3),
_U_dyn_cancel(3)
.PP
.SH AUTHOR

.PP
David Mosberger\-Tang
.br 
Hewlett\-Packard Labs
.br 
Palo\-Alto, CA 94304
.br 
Email: \fBdavidm@hpl.hp.com\fP
.br
WWW: \fBhttp://www.hpl.hp.com/research/linux/libunwind/\fP\&.
.\" NOTE: This file is generated, DO NOT EDIT.