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.TH "LIBUNWIND\-PTRACE" "3" "16 August 2007" "Programming Library " "Programming Library "
.SH NAME
libunwind\-ptrace
\-\- ptrace() support in libunwind 
.PP
.SH SYNOPSIS

.PP
#include <libunwind\-ptrace.h>
.br
.PP
unw_accessors_t
_UPT_accessors;
.br
.PP
void *_UPT_create(pid_t);
.br
void
_UPT_destroy(void *);
.br
.PP
int
_UPT_find_proc_info(unw_addr_space_t,
unw_word_t,
unw_proc_info_t *,
int,
void *);
.br
void
_UPT_put_unwind_info(unw_addr_space_t,
unw_proc_info_t *,
void *);
.br
int
_UPT_get_dyn_info_list_addr(unw_addr_space_t,
unw_word_t *,
void *);
.br
int
_UPT_access_mem(unw_addr_space_t,
unw_word_t,
unw_word_t *,
int,
void *);
.br
int
_UPT_access_reg(unw_addr_space_t,
unw_regnum_t,
unw_word_t *,
int,
void *);
.br
int
_UPT_access_fpreg(unw_addr_space_t,
unw_regnum_t,
unw_fpreg_t *,
int,
void *);
.br
int
_UPT_get_proc_name(unw_addr_space_t,
unw_word_t,
char *,
size_t,
unw_word_t *,
void *);
.br
int
_UPT_resume(unw_addr_space_t,
unw_cursor_t *,
void *);
.br
.PP
.SH DESCRIPTION

.PP
The ptrace(2)
system\-call makes it possible for a process to 
gain access to the machine\-state and virtual memory of \fIanother\fP
process. With the right set of call\-back routines, it is therefore 
possible to hook up libunwind
to another process via 
ptrace(2).
While it\&'s not very difficult to do so directly, 
libunwind
further facilitates this task by providing 
ready\-to\-use callbacks for this purpose. The routines and variables 
implementing this facility use a name\-prefix of _UPT,
which is 
stands for ``unwind\-via\-ptrace\&''\&. 
.PP
An application that wants to use the _UPT\-facility
first needs 
to create a new libunwind
address\-space that represents the 
target process. This is done by calling 
unw_create_addr_space().
In many cases, the application 
will simply want to pass the address of _UPT_accessors
as the 
first argument to this routine. Doing so will ensure that 
libunwind
will be able to properly unwind the target process. 
However, in special circumstances, an application may prefer to use 
only portions of the _UPT\-facility.
For this reason, the 
individual callback routines (_UPT_find_proc_info(),
_UPT_put_unwind_info(),
etc.) are also available for direct 
use. Of course, the addresses of these routines could also be picked 
up from _UPT_accessors,
but doing so would prevent static 
initialization. Also, when using _UPT_accessors,
\fIall\fP
the callback routines will be linked into the application, even if 
they are never actually called. 
.PP
Next, the application can turn on ptrace\-mode on the target process, 
either by forking a new process, invoking PTRACE_TRACEME,
and 
then starting the target program (via execve(2)),
or by 
directly attaching to an already running process (via 
PTRACE_ATTACH).
Either way, once the process\-ID (pid) of the 
target process is known, a _UPT\-info\-structure
can be created 
by calling _UPT_create(),
passing the pid of the target process 
as the only argument. The returned void\-pointer then needs to be 
passed as the ``argument\&'' pointer (third argument) to 
unw_init_remote().
.PP
The _UPT_resume()
routine can be used to resume execution of 
the target process. It simply invokes ptrace(2)
with a command 
value of PTRACE_CONT\&.
.PP
When the application is done using libunwind
on the target 
process, _UPT_destroy()
needs to be called, passing it the 
void\-pointer that was returned by the corresponding call to 
_UPT_create().
This ensures that all memory and other 
resources are freed up. 
.PP
.SH AVAILABILITY

.PP
Since ptrace(2)
works within a single machine only, the 
_UPT\-facility
by definition is not available in 
libunwind\-versions
configured for cross\-unwinding. 
.PP
.SH THREAD SAFETY

.PP
The _UPT\-facility
assumes that a single _UPT\-info
structure is never shared between threads. Because of this, no 
explicit locking is used. As long as only one thread uses 
a _UPT\-info
structure at any given time, this facility 
is thread\-safe. 
.PP
.SH RETURN VALUE

.PP
_UPT_create()
may return a NULL
pointer if it fails 
to create the _UPT\-info\-structure
for any reason. For the 
current implementation, the only reason this call may fail is when the 
system is out of memory. 
.PP
.SH FILES

.PP
.TP
libunwind\-ptrace.h
 Headerfile to include when using the 
interface defined by this library. 
.TP
\fB\-l\fPunwind\-ptrace \fB\-l\fPunwind\-generic
 Linker\-switches to add when building a program that uses the 
functions defined by this library. 
.PP
.SH SEE ALSO

.PP
execve(2), 
libunwind(3),
ptrace(2) 
.PP
.SH AUTHOR

.PP
David Mosberger\-Tang
.br
Email: \fBdmosberger@gmail.com\fP
.br
WWW: \fBhttp://www.nongnu.org/libunwind/\fP\&.
.\" NOTE: This file is generated, DO NOT EDIT.