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<chapter id="manual-core-adv" xreflabel="Valgrind's core: advanced topics">
<title>Using and understanding the Valgrind core: Advanced Topics</title>

<para>This chapter describes advanced aspects of the Valgrind core
services, which are mostly of interest to power users who wish to
customise and modify Valgrind's default behaviours in certain useful
ways.  The subjects covered are:</para>

<itemizedlist>
  <listitem><para>The "Client Request" mechanism</para></listitem>
  <listitem><para>Debugging your program using Valgrind's gdbserver
      and GDB</para></listitem>
  <listitem><para>Function Wrapping</para></listitem>
</itemizedlist>



<sect1 id="manual-core-adv.clientreq" 
       xreflabel="The Client Request mechanism">
<title>The Client Request mechanism</title>

<para>Valgrind has a trapdoor mechanism via which the client
program can pass all manner of requests and queries to Valgrind
and the current tool.  Internally, this is used extensively 
to make various things work, although that's not visible from the
outside.</para>

<para>For your convenience, a subset of these so-called client
requests is provided to allow you to tell Valgrind facts about
the behaviour of your program, and also to make queries.
In particular, your program can tell Valgrind about things that it
otherwise would not know, leading to better results.
</para>

<para>Clients need to include a header file to make this work.
Which header file depends on which client requests you use.  Some
client requests are handled by the core, and are defined in the
header file <filename>valgrind/valgrind.h</filename>.  Tool-specific
header files are named after the tool, e.g.
<filename>valgrind/memcheck.h</filename>.  Each tool-specific header file
includes <filename>valgrind/valgrind.h</filename> so you don't need to
include it in your client if you include a tool-specific header.  All header
files can be found in the <literal>include/valgrind</literal> directory of
wherever Valgrind was installed.</para>

<para>The macros in these header files have the magical property
that they generate code in-line which Valgrind can spot.
However, the code does nothing when not run on Valgrind, so you
are not forced to run your program under Valgrind just because you
use the macros in this file.  Also, you are not required to link your
program with any extra supporting libraries.</para>

<para>The code added to your binary has negligible performance impact:
on x86, amd64, ppc32, ppc64 and ARM, the overhead is 6 simple integer
instructions and is probably undetectable except in tight loops.
However, if you really wish to compile out the client requests, you
can compile with <option>-DNVALGRIND</option> (analogous to
<option>-DNDEBUG</option>'s effect on
<function>assert</function>).
</para>

<para>You are encouraged to copy the <filename>valgrind/*.h</filename> headers
into your project's include directory, so your program doesn't have a
compile-time dependency on Valgrind being installed.  The Valgrind headers,
unlike most of the rest of the code, are under a BSD-style license so you may
include them without worrying about license incompatibility.</para>

<para>Here is a brief description of the macros available in
<filename>valgrind.h</filename>, which work with more than one
tool (see the tool-specific documentation for explanations of the
tool-specific macros).</para>

 <variablelist>

  <varlistentry>
   <term><command><computeroutput>RUNNING_ON_VALGRIND</computeroutput></command>:</term>
   <listitem>
    <para>Returns 1 if running on Valgrind, 0 if running on the
    real CPU.  If you are running Valgrind on itself, returns the
    number of layers of Valgrind emulation you're running on.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_DISCARD_TRANSLATIONS</computeroutput>:</command></term>
   <listitem>
    <para>Discards translations of code in the specified address
    range.  Useful if you are debugging a JIT compiler or some other
    dynamic code generation system.  After this call, attempts to
    execute code in the invalidated address range will cause
    Valgrind to make new translations of that code, which is
    probably the semantics you want.  Note that code invalidations
    are expensive because finding all the relevant translations
    quickly is very difficult, so try not to call it often.
    Note that you can be clever about
    this: you only need to call it when an area which previously
    contained code is overwritten with new code.  You can choose
    to write code into fresh memory, and just call this
    occasionally to discard large chunks of old code all at
    once.</para>
    <para>
    Alternatively, for transparent self-modifying-code support,
    use<option>--smc-check=all</option>, or run
    on ppc32/Linux, ppc64/Linux or ARM/Linux.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_COUNT_ERRORS</computeroutput>:</command></term>
   <listitem>
    <para>Returns the number of errors found so far by Valgrind.  Can be
    useful in test harness code when combined with the
    <option>--log-fd=-1</option> option; this runs Valgrind silently,
    but the client program can detect when errors occur.  Only useful
    for tools that report errors, e.g. it's useful for Memcheck, but for
    Cachegrind it will always return zero because Cachegrind doesn't
    report errors.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_MALLOCLIKE_BLOCK</computeroutput>:</command></term>
   <listitem>
    <para>If your program manages its own memory instead of using
    the standard <function>malloc</function> /
    <function>new</function> /
    <function>new[]</function>, tools that track
    information about heap blocks will not do nearly as good a
    job.  For example, Memcheck won't detect nearly as many
    errors, and the error messages won't be as informative.  To
    improve this situation, use this macro just after your custom
    allocator allocates some new memory.  See the comments in
    <filename>valgrind.h</filename> for information on how to use
    it.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_FREELIKE_BLOCK</computeroutput>:</command></term>
   <listitem>
    <para>This should be used in conjunction with
    <computeroutput>VALGRIND_MALLOCLIKE_BLOCK</computeroutput>.
    Again, see <filename>valgrind.h</filename> for
    information on how to use it.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_RESIZEINPLACE_BLOCK</computeroutput>:</command></term>
   <listitem>
    <para>Informs a Valgrind tool that the size of an allocated block has been
    modified but not its address. See <filename>valgrind.h</filename> for
    more information on how to use it.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>
   <command><computeroutput>VALGRIND_CREATE_MEMPOOL</computeroutput></command>,
   <command><computeroutput>VALGRIND_DESTROY_MEMPOOL</computeroutput></command>,
   <command><computeroutput>VALGRIND_MEMPOOL_ALLOC</computeroutput></command>,
   <command><computeroutput>VALGRIND_MEMPOOL_FREE</computeroutput></command>,
   <command><computeroutput>VALGRIND_MOVE_MEMPOOL</computeroutput></command>,
   <command><computeroutput>VALGRIND_MEMPOOL_CHANGE</computeroutput></command>,
   <command><computeroutput>VALGRIND_MEMPOOL_EXISTS</computeroutput></command>:
   </term>
   <listitem>
    <para>These are similar to 
    <computeroutput>VALGRIND_MALLOCLIKE_BLOCK</computeroutput> and
    <computeroutput>VALGRIND_FREELIKE_BLOCK</computeroutput>
    but are tailored towards code that uses memory pools.  See 
    <xref linkend="mc-manual.mempools"/> for a detailed description.</para>
   </listitem>
  </varlistentry>
  
  <varlistentry>
   <term><command><computeroutput>VALGRIND_NON_SIMD_CALL[0123]</computeroutput>:</command></term>
   <listitem>
    <para>Executes a function in the client program on the
    <emphasis>real</emphasis> CPU, not the virtual CPU that Valgrind
    normally runs code on.  The function must take an integer (holding a
    thread ID) as the first argument and then 0, 1, 2 or 3 more arguments
    (depending on which client request is used).  These are used in various
    ways internally to Valgrind.  They might be useful to client
    programs.</para> 

    <para><command>Warning:</command> Only use these if you
    <emphasis>really</emphasis> know what you are doing.  They aren't 
    entirely reliable, and can cause Valgrind to crash.  See
    <filename>valgrind.h</filename> for more details.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_PRINTF(format, ...)</computeroutput>:</command></term>
   <listitem>
    <para>Print a printf-style message to the Valgrind log file.  The
    message is prefixed with the PID between a pair of
    <computeroutput>**</computeroutput> markers.  (Like all client requests,
    nothing is output if the client program is not running under Valgrind.)
    Output is not produced until a newline is encountered, or subsequent
    Valgrind output is printed; this allows you to build up a single line of
    output over multiple calls.  Returns the number of characters output,
    excluding the PID prefix.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_PRINTF_BACKTRACE(format, ...)</computeroutput>:</command></term>
   <listitem>
    <para>Like <computeroutput>VALGRIND_PRINTF</computeroutput> (in
    particular, the return value is identical), but prints a stack backtrace
    immediately afterwards.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_MONITOR_COMMAND(command)</computeroutput>:</command></term>
   <listitem>
    <para>Execute the given monitor command (a string).
    Returns 0 if command is recognised. Returns 1 if command is not recognised.
    Note that some monitor commands provide access to a functionality
    also accessible via a specific client request. For example,
    memcheck leak search can be requested from the client program
    using VALGRIND_DO_LEAK_CHECK or via the monitor command "leak_search".
    Note that the syntax of the command string is only verified at
    run-time. So, if it exists, it is preferable to use a specific
    client request to have better compile time verifications of the
    arguments.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_CLO_CHANGE(option)</computeroutput>:</command></term>
   <listitem>
     <para>Changes the value of a dynamically changeable option (a string).
       See <xref linkend="manual-core.dynopts"/>.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_STACK_REGISTER(start, end)</computeroutput>:</command></term>
   <listitem>
    <para>Registers a new stack.  Informs Valgrind that the memory range
    between start and end is a unique stack.  Returns a stack identifier
    that can be used with other
    <computeroutput>VALGRIND_STACK_*</computeroutput> calls.</para>
    <para>Valgrind will use this information to determine if a change
    to the stack pointer is an item pushed onto the stack or a change
    over to a new stack.  Use this if you're using a user-level thread
    package and are noticing crashes in stack trace recording or
    spurious errors from Valgrind about uninitialized memory
    reads.</para>

    <para><command>Warning:</command> Unfortunately, this client request is
    unreliable and best avoided.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_STACK_DEREGISTER(id)</computeroutput>:</command></term>
   <listitem>
    <para>Deregisters a previously registered stack.  Informs
    Valgrind that previously registered memory range with stack id
    <computeroutput>id</computeroutput> is no longer a stack.</para>

    <para><command>Warning:</command> Unfortunately, this client request is
    unreliable and best avoided.</para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><command><computeroutput>VALGRIND_STACK_CHANGE(id, start, end)</computeroutput>:</command></term>
   <listitem>
    <para>Changes a previously registered stack.  Informs
    Valgrind that the previously registered stack with stack id
    <computeroutput>id</computeroutput> has changed its start and end
    values.  Use this if your user-level thread package implements
    stack growth.</para>

    <para><command>Warning:</command> Unfortunately, this client request is
    unreliable and best avoided.</para>
   </listitem>
  </varlistentry>

 </variablelist>

</sect1>






<!-- Referenced from both the manual and manpage -->
<sect1 id="&vg-gdbserver-id;"
       xreflabel="&vg-gdbserver-label;">
<title>Debugging your program using Valgrind gdbserver and GDB</title>

<para>A program running under Valgrind is not executed directly by the CPU.
Instead it runs on a synthetic CPU provided by Valgrind.  This is why a debugger
cannot natively debug your program when it runs on Valgrind.
</para>
<para>
This section describes how GDB can interact with the
Valgrind gdbserver to provide a fully debuggable program under
Valgrind. Used in this way, GDB also provides an interactive usage of
Valgrind core or tool functionalities, including incremental leak search
under Memcheck and on-demand Massif snapshot production.
</para>

<sect2 id="manual-core-adv.gdbserver-simple"
       xreflabel="gdbserver simple example">
<title>Quick Start: debugging in 3 steps</title>

<para>The simplest way to get started is to run Valgrind with the
flag <option>--vgdb-error=0</option>.  Then follow the on-screen
directions, which give you the precise commands needed to start GDB
and connect it to your program.</para>

<para>Otherwise, here's a slightly more verbose overview.</para>

<para>If you want to debug a program with GDB when using the Memcheck
tool, start Valgrind like this:
<screen><![CDATA[
valgrind --vgdb=yes --vgdb-error=0 prog
]]></screen></para>

<para>In another shell, start GDB:
<screen><![CDATA[
gdb prog
]]></screen></para>

<para>Then give the following command to GDB:
<screen><![CDATA[
(gdb) target remote | vgdb
]]></screen></para>

<para>You can now debug your program e.g. by inserting a breakpoint
and then using the GDB <computeroutput>continue</computeroutput>
command.</para>

<para>This quick start information is enough for basic usage of the
Valgrind gdbserver.  The sections below describe more advanced
functionality provided by the combination of Valgrind and GDB. Note
that the command line flag <option>--vgdb=yes</option> can be omitted,
as this is the default value.
</para>

</sect2>

<sect2 id="manual-core-adv.gdbserver-concept"
       xreflabel="gdbserver">
<title>Valgrind gdbserver overall organisation</title>
<para>The GNU GDB debugger is typically used to debug a process
running on the same machine.  In this mode, GDB uses system calls to
control and query the program being debugged.  This works well, but
only allows GDB to debug a program running on the same computer.
</para>

<para>GDB can also debug processes running on a different computer.
To achieve this, GDB defines a protocol (that is, a set of query and
reply packets) that facilitates fetching the value of memory or
registers, setting breakpoints, etc.  A gdbserver is an implementation
of this "GDB remote debugging" protocol.  To debug a process running
on a remote computer, a gdbserver (sometimes called a GDB stub)
must run at the remote computer side.
</para>

<para>The Valgrind core provides a built-in gdbserver implementation,
which is activated using <option>--vgdb=yes</option>
or <option>--vgdb=full</option>.  This gdbserver allows the process
running on Valgrind's synthetic CPU to be debugged remotely.
GDB sends protocol query packets (such as "get register contents") to
the Valgrind embedded gdbserver.  The gdbserver executes the queries
(for example, it will get the register values of the synthetic CPU)
and gives the results back to GDB.
</para>

<para>GDB can use various kinds of channels (TCP/IP, serial line, etc)
to communicate with the gdbserver.  In the case of Valgrind's
gdbserver, communication is done via a pipe and a small helper program
called <xref linkend="&vg-vgdb-id;"/>, which acts as an
intermediary.  If no GDB is in use, vgdb can also be
used to send monitor commands to the Valgrind gdbserver from a shell
command line.
</para>

</sect2>

<sect2 id="manual-core-adv.gdbserver-gdb"
       xreflabel="Connecting GDB to a Valgrind gdbserver">
<title>Connecting GDB to a Valgrind gdbserver</title>
<para>To debug a program "<filename>prog</filename>" running under
Valgrind, you must ensure that the Valgrind gdbserver is activated by
specifying either <option>--vgdb=yes</option>
or <option>--vgdb=full</option>.  A secondary command line option,
<option>--vgdb-error=number</option>, can be used to tell the gdbserver
only to become active once the specified number of errors have been
shown.  A value of zero will therefore cause
the gdbserver to become active at startup, which allows you to
insert breakpoints before starting the run.  For example:
<screen><![CDATA[
valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog
]]></screen></para>

<para>The Valgrind gdbserver is invoked at startup
and indicates it is waiting for a connection from a GDB:</para>

<programlisting><![CDATA[
==2418== Memcheck, a memory error detector
==2418== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2418== Using Valgrind-3.14.0.GIT and LibVEX; rerun with -h for copyright info
==2418== Command: ./prog
==2418== 
==2418== (action at startup) vgdb me ... 
]]></programlisting>


<para>GDB (in another shell) can then be connected to the Valgrind gdbserver.
For this, GDB must be started on the program <filename>prog</filename>:
<screen><![CDATA[
gdb ./prog
]]></screen></para>


<para>You then indicate to GDB that you want to debug a remote target:
<screen><![CDATA[
(gdb) target remote | vgdb
]]></screen>
GDB then starts a vgdb relay application to communicate with the 
Valgrind embedded gdbserver:</para>

<programlisting><![CDATA[
(gdb) target remote | vgdb
Remote debugging using | vgdb
relaying data between gdb and process 2418
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2418]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb) 
]]></programlisting>

<para>Note that vgdb is provided as part of the Valgrind
distribution.  You do not need to install it separately.</para>

<para>If vgdb detects that there are multiple Valgrind gdbservers that
can be connected to, it will list all such servers and their PIDs, and
then exit.  You can then reissue the GDB "target" command, but
specifying the PID of the process you want to debug:
</para>

<programlisting><![CDATA[
(gdb) target remote | vgdb
Remote debugging using | vgdb
no --pid= arg given and multiple valgrind pids found:
use --pid=2479 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2481 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2483 for valgrind --vgdb=yes --vgdb-error=0 ./another_prog 
Remote communication error: Resource temporarily unavailable.
(gdb)  target remote | vgdb --pid=2479
Remote debugging using | vgdb --pid=2479
relaying data between gdb and process 2479
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2479]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb) 
]]></programlisting>

<para>If you want to use the <option>--multi</option> mode which makes vgdb start in extended-remote mode, set the following in GDB:
<screen><![CDATA[
# gdb prog
(gdb) set remote exec-file prog
(gdb) set sysroot /
(gdb) target extended-remote | vgdb --multi --vargs -q
(gdb) start
Temporary breakpoint 1 at 0x24e0
Starting program: prog
relaying data between gdb and process 2999348

Temporary breakpoint 1, 0x000000000010a4a0 in main ()
(gdb)
]]></screen>
</para>

<para>Note that in <option>--multi</option> mode you don't have to
start valgrind separately. vgdb will start valgrind for
you. vgdb <option>--multi</option> mode is experimental and currently
has some limitations like not being able to see program stdin and
stdout. Also you have to explicitly set the remote exec-file and
sysroot to tell GDB the "remote" and local files are the same.</para>

<para>Once GDB is connected to the Valgrind gdbserver, it can be used
in the same way as if you were debugging the program natively:</para>
 <itemizedlist>
  <listitem>
    <para>Breakpoints can be inserted or deleted.</para>
  </listitem>
  <listitem>
    <para>Variables and register values can be examined or modified.
    </para>
  </listitem>
  <listitem>
    <para>Signal handling can be configured (printing, ignoring).
    </para>
  </listitem>
  <listitem>
    <para>Execution can be controlled (continue, step, next, stepi, etc).
    </para>
  </listitem>
  <listitem>
    <para>Program execution can be interrupted using Control-C.</para>
  </listitem>
 </itemizedlist>

<para>And so on.  Refer to the GDB user manual for a complete
description of GDB's functionality.
</para>

</sect2>

<sect2 id="manual-core-adv.gdbserver-gdb-android"
       xreflabel="Connecting to an Android gdbserver">
<title>Connecting to an Android gdbserver</title>
<para> When developping applications for Android, you will typically use
a development system (on which the Android NDK is installed) to compile your
application. An Android target system or emulator will be used to run
the application.
In this setup, Valgrind and vgdb will run on the Android system,
while GDB will run on the development system. GDB will connect
to the vgdb running on the Android system using the Android NDK
'adb forward' application.
</para>
<para> Example: on the Android system, execute the following:
    <screen><![CDATA[
valgrind --vgdb-error=0 --vgdb=yes prog
# and then in another shell, run:
vgdb --port=1234
]]></screen>
</para>

<para> On the development system, execute the following commands:
<screen><![CDATA[
adb forward tcp:1234 tcp:1234
gdb prog
(gdb) target remote :1234
]]></screen>
GDB will use a local tcp/ip connection to connect to the Android adb forwarder.
Adb will establish a relay connection between the host system and the Android
target system.  Be sure to use the GDB delivered in the
Android NDK system (typically, arm-linux-androideabi-gdb), as the host
GDB is probably not able to debug Android arm applications.
Note that the local port nr (used by GDB) must not necessarily be equal
to the port number used by vgdb: adb can forward tcp/ip between different
port numbers.
</para>

<para>In the current release, the GDB server is not enabled by default
for Android, due to problems in establishing a suitable directory in
which Valgrind can create the necessary FIFOs (named pipes) for
communication purposes.  You can stil try to use the GDB server, but
you will need to explicitly enable it using the flag 
<computeroutput>--vgdb=yes</computeroutput> or
<computeroutput>--vgdb=full</computeroutput>.
</para>

<para>Additionally, you
will need to select a temporary directory which is (a) writable
by Valgrind, and (b) supports FIFOs.  This is the main difficult
point.  Often, <computeroutput>/sdcard</computeroutput> satisfies
requirement (a), but fails for (b) because it is a VFAT file system
and VFAT does not support pipes.  Possibilities you could try are
<computeroutput>/data/local</computeroutput>,
<computeroutput>/data/local/Inst</computeroutput> (if you
installed Valgrind there), or
<computeroutput>/data/data/name.of.my.app</computeroutput>, if you
are running a specific application and it has its own directory of 
that form.  This last possibility may have the highest probability
of success.</para>

<para>You can specify the temporary directory to use either via
the <computeroutput>--with-tmpdir=</computeroutput> configure time
flag, or by setting environment variable TMPDIR when running Valgrind
(on the Android device, not on the Android NDK development host).
Another alternative is to specify the directory for the FIFOs using
the <computeroutput>--vgdb-prefix=</computeroutput> Valgrind command
line option.
</para>

<para>We hope to have a better story for temporary directory handling
on Android in the future.  The difficulty is that, unlike in standard
Unixes, there is no single temporary file directory that reliably
works across all devices and scenarios.
</para>

</sect2>

<sect2 id="manual-core-adv.gdbserver-commandhandling"
       xreflabel="Monitor command handling by the Valgrind gdbserver">
<title>Monitor command handling by the Valgrind gdbserver</title>

<para>The Valgrind gdbserver provides additional Valgrind-specific
functionality via "monitor commands".  Such monitor commands can be
sent from the GDB command line or from the shell command line or
requested by the client program using the VALGRIND_MONITOR_COMMAND
client request.  See
<xref linkend="&vg-monitor-id;"/> for the
list of the Valgrind core monitor commands available regardless of the
Valgrind tool selected.
</para>

<para>The following tools provide tool-specific monitor commands:
  <itemizedlist>
    <listitem>
      <para><xref linkend="mc-manual.monitor-commands"/></para>
    </listitem>
    <listitem>
      <para><xref linkend="cl-manual.monitor-commands"/></para>
    </listitem>
    <listitem>
      <para><xref linkend="ms-manual.monitor-commands"/></para>
    </listitem>
    <listitem>
      <para><xref linkend="hg-manual.monitor-commands"/></para>
    </listitem>
  </itemizedlist>
</para>

<para>An example of a tool specific monitor command is the Memcheck monitor
command <computeroutput>leak_check full
reachable any</computeroutput>.  This requests a full reporting of the
allocated memory blocks.  To have this leak check executed, use the GDB
command:
<screen><![CDATA[
(gdb) monitor leak_check full reachable any
]]></screen>
</para>

<para>GDB sends as a single string all what follows 'monitor' to the Valgrind
gdbserver.  The Valgrind gdbserver parses the string and will execute the
monitor command itself, if it recognises it to be a Valgrind core monitor
command.  If it is not recognised as such, it is assumed to be tool-specific and
is handed to the tool for execution.  For example:  
</para>
<programlisting><![CDATA[
(gdb) monitor leak_check full reachable any
==2418== 100 bytes in 1 blocks are still reachable in loss record 1 of 1
==2418==    at 0x4006E9E: malloc (vg_replace_malloc.c:236)
==2418==    by 0x804884F: main (prog.c:88)
==2418== 
==2418== LEAK SUMMARY:
==2418==    definitely lost: 0 bytes in 0 blocks
==2418==    indirectly lost: 0 bytes in 0 blocks
==2418==      possibly lost: 0 bytes in 0 blocks
==2418==    still reachable: 100 bytes in 1 blocks
==2418==         suppressed: 0 bytes in 0 blocks
==2418== 
(gdb) 
]]></programlisting>

<para>Similarly to GDB, the Valgrind gdbserver will accept abbreviated monitor
command names and arguments, as long as the given abbreviation is unambiguous.
For example, the above
<computeroutput>leak_check</computeroutput>
command can also be typed as:
<screen><![CDATA[
(gdb) mo l f r a
]]></screen>

The letters <computeroutput>mo</computeroutput> are recognised by GDB as being
an abbreviation for <computeroutput>monitor</computeroutput>.  So GDB sends the
string <computeroutput>l f r a</computeroutput> to the Valgrind
gdbserver.  The letters provided in this string are unambiguous for the
Valgrind gdbserver.  This therefore gives the same output as the
unabbreviated command and arguments.  If the provided abbreviation is
ambiguous, the Valgrind gdbserver will report the list of commands (or
argument values) that can match:
<programlisting><![CDATA[
(gdb) mo v. n
v. can match v.set v.info v.wait v.kill v.translate v.do
(gdb) mo v.i n
n_errs_found 0 n_errs_shown 0 (vgdb-error 0)
(gdb) 
]]></programlisting>
</para>

<para>Instead of sending a monitor command from GDB, you can also send
these from a shell command line.  For example, the following command
lines, when given in a shell, will cause the same leak search to be executed
by the process 3145:
<screen><![CDATA[
vgdb --pid=3145 leak_check full reachable any
vgdb --pid=3145 l f r a
]]></screen></para>

<para>Note that the Valgrind gdbserver automatically continues the
execution of the program after a standalone invocation of
vgdb.  Monitor commands sent from GDB do not cause the program to
continue: the program execution is controlled explicitly using GDB
  commands such as "continue" or "next".</para>

<para>Many monitor commands (e.g. v.info location, memcheck who_points_at, ...) require an address
  argument and an optional length: <varname>&lt;addr&gt; [&lt;len&gt;]</varname>.
  The arguments can also be provided by using a 'C array like syntax' by providing the address
  followed by the length between square brackets.</para>
<para>For example, the following two monitor commands provide the same information:</para>
<programlisting><![CDATA[
(gdb) mo xb 0x804a2f0 10
...
(gdb) mo xb 0x804a2f0[10]
...
]]></programlisting>

</sect2>

<sect2 id="manual-core-adv.gdbserver-gdbmonitorfrontend"
       xreflabel="GDB front end commands for Valgrind gdbserver monitor commands">
<title>GDB front end commands for Valgrind gdbserver monitor commands</title>

<para>As explained in
  <xref linkend="manual-core-adv.gdbserver-commandhandling"/>, valgrind monitor
  commands consist in strings that are not interpreted by GDB.  GDB has no
  knowledge of these valgrind monitor commands.  The GDB 'command line
  interface' infrastructure however provides interesting functionalities to help
  typing commands such as auto-completion, command specific help, searching for
  a command or command help matching a regexp, ...
</para>

<para>To have a better integration of the valgrind monitor commands in the GDB
  command line interface, Valgrind provides python code defining a GDB front end
  command for each valgrind monitor command.  Similarly, for each tool specific
  monitor command, the python code provides a matching GDB front end command.
</para>

<para>Like other GDB commands, the GDB front end Valgrind monitor commands are
  hierarchically structured starting from 5 "top" GDB commands. Subcommands are
  defined below these "top" commands.  To ease typing, shorter aliases are also
  provided.

  <itemizedlist>
    <listitem>
      <para><computeroutput>valgrind</computeroutput> (aliased
      by <computeroutput>vg</computeroutput>
      and <computeroutput>v</computeroutput>) is the top GDB command providing
      front end commands to the Valgrind general monitor commands.</para>
    </listitem>
    <listitem>
      <para><computeroutput>memcheck</computeroutput> (aliased
        by <computeroutput>mc</computeroutput>) is the top GDB command
        providing the front end commands corresponding to the memcheck specific
        monitor commands.</para>
    </listitem>
    <listitem>
      <para><computeroutput>callgrind</computeroutput> (aliased
        by <computeroutput>cg</computeroutput>) is the top GDB command
        providing the front end commands corresponding to the callgrind specific
        monitor commands.</para>
    </listitem>
    <listitem>
      <para><computeroutput>massif</computeroutput> (aliased
        by <computeroutput>ms</computeroutput>) is the top GDB command
        providing the front end commands corresponding to the massif specific
        monitor commands.</para>
    </listitem>
    <listitem>
      <para><computeroutput>helgrind</computeroutput> (aliased
        by <computeroutput>hg</computeroutput>) is the top GDB command
        providing the front end commands corresponding to the helgrind specific
        monitor commands.</para>
    </listitem>
  </itemizedlist>
</para>

<para>The usage of a GDB front end command is compatible with a direct usage of
  the Valgrind monitor command.  The below example shows a direct usage of the
  Memcheck monitor command <computeroutput>xb</computeroutput> to examine the
  definedness status of the some_mem array and equivalent usages based on the GDB
  front end commands.

<programlisting><![CDATA[
(gdb) list
1	int main()
2	{
3	  char some_mem[5];
4	  return 0;
5	}
(gdb) p &some_mem
$2 = (char (*)[5]) 0x1ffefffe5b
(gdb) p sizeof(some_mem)
$3 = 5
(gdb) monitor xb 0x1ffefffe5b 5
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00
(gdb) memcheck xb 0x1ffefffe5b 5
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00
(gdb) mc xb &some_mem sizeof(some_mem)
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00
(gdb) 
]]></programlisting>

It is worth noting down that the third command uses the
alias <computeroutput>mc</computeroutput>.  This command also shows a
significant advantage of using the GDB front end commands: as GDB "understands"
the structure of these front end commands, where relevant, these front end
commands will evaluate their arguments. In the case of
the <computeroutput>xb</computeroutput> command, the
GDB <computeroutput>xb</computeroutput> command evaluates its second argument
(which must be an address expression) and its optional second argument (which
must be an integer expression).
</para>

<para>GDB will auto-load the python code defining the Valgrind front end
  commands as soon as GDB detects that the executable being debugged is running
  under valgrind.  This detection is based on observing that the Valgrind
  process has loaded a specific Valgrind shared library.  The loading of this
  library is done by the dynamic loader very early on in the execution of the
  process.  If GDB is used to connect to a Valgrind process that has not yet
  started its execution (such as when Valgrind was started with the
  option <computeroutput>--vgdb-stop-at=startup</computeroutput>
  or <computeroutput>--vgdb-error=0</computeroutput>), then the GDB front end
  commands will not yet be auto-loaded.  To have the GDB front end commands
  auto-loaded, you can put a breakpoint e.g. in main and use the GDB
  command <computeroutput>continue</computeroutput>. Alternatively, you can add
  in your .gdbinit a line that loads the python code at GDB startup such as:
<programlisting><![CDATA[
source /path/to/valgrind/python/code/valgrind-monitor.py
]]></programlisting>
</para>
<para>
The exact path to use in the source command depends on your Valgrind
installation.  The output of the shell command <computeroutput>vgdb
--help</computeroutput> contains the absolute path name for the python file you
can source in your .gdbinit to define the GDB valgrind front end monitor
commands.
</para>
</sect2>

<sect2 id="manual-core-adv.gdbserver-threads"
       xreflabel="Valgrind gdbserver thread information">
<title>Valgrind gdbserver thread information</title>

<para>Valgrind's gdbserver enriches the output of the
GDB <computeroutput>info threads</computeroutput> command
with Valgrind-specific information.
The operating system's thread number is followed
by Valgrind's internal index for that thread ("tid") and by
the Valgrind scheduler thread state:</para>

<programlisting><![CDATA[
(gdb) info threads
  4 Thread 6239 (tid 4 VgTs_Yielding)  0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
* 3 Thread 6238 (tid 3 VgTs_Runnable)  make_error (s=0x8048b76 "called from London") at prog.c:20
  2 Thread 6237 (tid 2 VgTs_WaitSys)  0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
  1 Thread 6234 (tid 1 VgTs_Yielding)  main (argc=1, argv=0xbedcc274) at prog.c:105
(gdb) 
]]></programlisting>

</sect2>

<sect2 id="manual-core-adv.gdbserver-shadowregisters"
       xreflabel="Examining and modifying Valgrind shadow registers">
<title>Examining and modifying Valgrind shadow registers</title>

<para> When the option <option>--vgdb-shadow-registers=yes</option> is
given, the Valgrind gdbserver will let GDB examine and/or modify
Valgrind's shadow registers.  GDB version 7.1 or later is needed for this
to work. For x86 and amd64, GDB version 7.2 or later is needed.</para>

<para>For each CPU register, the Valgrind core maintains two
shadow register sets.  These shadow registers can be accessed from
GDB by giving a postfix <computeroutput>s1</computeroutput>
or <computeroutput>s2</computeroutput> for respectively the first
and second shadow register.  For example, the x86 register
<computeroutput>eax</computeroutput> and its two shadows
can be examined using the following commands:</para>

<programlisting><![CDATA[
(gdb) p $eax
$1 = 0
(gdb) p $eaxs1
$2 = 0
(gdb) p $eaxs2
$3 = 0
(gdb) 
]]></programlisting>

<para>Float shadow registers are shown by GDB as unsigned integer
values instead of float values, as it is expected that these
shadow values are mostly used for memcheck validity bits. </para>

<para>Intel/amd64 AVX registers <computeroutput>ymm0</computeroutput>
to <computeroutput>ymm15</computeroutput> have also their shadow
registers. However, GDB presents the shadow values using two
"half" registers. For example, the half shadow registers for 
<computeroutput>ymm9</computeroutput> are
<computeroutput>xmm9s1</computeroutput> (lower half for set 1),
<computeroutput>ymm9hs1</computeroutput> (upper half for set 1),
<computeroutput>xmm9s2</computeroutput> (lower half for set 2),
<computeroutput>ymm9hs2</computeroutput> (upper half for set 2).
Note the inconsistent notation for the names of the half registers:
the lower part starts with an <computeroutput>x</computeroutput>,
the upper part starts with an <computeroutput>y</computeroutput>
and has an <computeroutput>h</computeroutput> before the shadow postfix.
</para>
<para>The special presentation of the AVX shadow registers is due to
the fact that GDB independently retrieves the lower and upper half of
the <computeroutput>ymm</computeroutput> registers.  GDB does not
however know that the shadow half registers have to be shown combined.
</para>
</sect2>


<sect2 id="manual-core-adv.gdbserver-limitations"
       xreflabel="Limitations of the Valgrind gdbserver">
<title>Limitations of the Valgrind gdbserver</title>

<para>Debugging with the Valgrind gdbserver is very similar to native
debugging.  Valgrind's gdbserver implementation is quite
complete, and so provides most of the GDB debugging functionality.  There
are however some limitations and peculiarities:</para>
 <itemizedlist>
   <listitem>
     <para>Precision of "stop-at" commands.</para>
     <para>
       GDB commands such as "step", "next", "stepi", breakpoints
       and watchpoints, will stop the execution of the process.  With
       the option <option>--vgdb=yes</option>, the process might not
       stop at the exact requested instruction. Instead, it might
       continue execution of the current basic block and stop at one
       of the following basic blocks. This is linked to the fact that
       Valgrind gdbserver has to instrument a block to allow stopping
       at the exact instruction requested.  Currently,
       re-instrumentation of the block currently being executed is not
       supported. So, if the action requested by GDB (e.g. single
       stepping or inserting a breakpoint) implies re-instrumentation
       of the current block, the GDB action may not be executed
       precisely.
     </para>
     <para>
       This limitation applies when the basic block
       currently being executed has not yet been instrumented for debugging.
       This typically happens when the gdbserver is activated due to the
       tool reporting an error or to a watchpoint.  If the gdbserver
       block has been activated following a breakpoint, or if a
       breakpoint has been inserted in the block before its execution,
       then the block has already been instrumented for debugging.
     </para>
     <para>
       If you use the option <option>--vgdb=full</option>, then GDB
       "stop-at" commands will be obeyed precisely.  The
       downside is that this requires each instruction to be
       instrumented with an additional call to a gdbserver helper
       function, which gives considerable overhead (+500% for memcheck)
       compared to  <option>--vgdb=no</option>.
       Option <option>--vgdb=yes</option> has neglectible overhead compared
       to <option>--vgdb=no</option>.
     </para>
   </listitem>

   <listitem>
     <para>Processor registers and flags values.</para>
     <para>When Valgrind gdbserver stops on an error, on a breakpoint
     or when single stepping, registers and flags values might not be always
     up to date due to the optimisations done by the Valgrind core.
     The default value 
     <option>--vex-iropt-register-updates=unwindregs-at-mem-access</option>
     ensures that the registers needed to make a stack trace (typically
     PC/SP/FP) are up to date at each memory access (i.e. memory exception
     points).
     Disabling some optimisations using the following values will increase
     the precision of registers and flags values (a typical performance 
     impact for memcheck is given for each option).
       <itemizedlist>
         <listitem><para>
           <option>--vex-iropt-register-updates=allregs-at-mem-access</option> (+10%)
           ensures that all registers and flags are up to date at each memory
           access.
         </para></listitem>
         <listitem><para>
           <option>--vex-iropt-register-updates=allregs-at-each-insn</option> (+25%)
           ensures that all registers and flags are up to date at each instruction.
         </para></listitem>
       </itemizedlist>
       Note that <option>--vgdb=full</option> (+500%, see above
       Precision of "stop-at" commands) automatically
       activates <option>--vex-iropt-register-updates=allregs-at-each-insn</option>.
     </para>
   </listitem>

   <listitem>
     <para>Hardware watchpoint support by the Valgrind
     gdbserver.</para>

     <para> The Valgrind gdbserver can simulate hardware watchpoints
     if the selected tool provides support for it.  Currently,
     only Memcheck provides hardware watchpoint simulation.  The
     hardware watchpoint simulation provided by Memcheck is much
     faster that GDB software watchpoints, which are implemented by
     GDB checking the value of the watched zone(s) after each
     instruction.  Hardware watchpoint simulation also provides read
     watchpoints.  The hardware watchpoint simulation by Memcheck has
     some limitations compared to real hardware
     watchpoints. However, the number and length of simulated
     watchpoints are not limited.
     </para>
     <para>Typically, the number of (real) hardware watchpoints is
     limited.  For example, the x86 architecture supports a maximum of
     4 hardware watchpoints, each watchpoint watching 1, 2, 4 or 8
     bytes. The Valgrind gdbserver does not have any limitation on the
     number of simulated hardware watchpoints. It also has no
     limitation on the length of the memory zone being
     watched.  Using GDB version 7.4 or later allow full use of the
     flexibility of the Valgrind gdbserver's simulated hardware watchpoints.
     Previous GDB versions do not understand that Valgrind gdbserver
     watchpoints have no length limit.
     </para>
     <para>Memcheck implements hardware watchpoint simulation by
     marking the watched address ranges as being unaddressable.  When
     a hardware watchpoint is removed, the range is marked as
     addressable and defined.  Hardware watchpoint simulation of
     addressable-but-undefined memory zones works properly, but has
     the undesirable side effect of marking the zone as defined when
     the watchpoint is removed.
     </para>
     <para>Write watchpoints might not be reported at the 
     exact instruction that writes the monitored area,
     unless option <option>--vgdb=full</option> is given.  Read watchpoints
     will always be reported at the exact instruction reading the
     watched memory.
     </para>
     <para>It is better to avoid using hardware watchpoint of not
     addressable (yet) memory: in such a case, GDB will fall back to
     extremely slow software watchpoints.  Also, if you do not quit GDB
     between two debugging sessions, the hardware watchpoints of the
     previous sessions will be re-inserted as software watchpoints if
     the watched memory zone is not addressable at program startup.
     </para>
   </listitem>

   <listitem>
     <para>Stepping inside shared libraries on ARM.</para>
     <para>For unknown reasons, stepping inside shared
     libraries on ARM may fail.  A workaround is to use the
     <computeroutput>ldd</computeroutput> command
     to find the list of shared libraries and their loading address
     and inform GDB of the loading address using the GDB command
     "add-symbol-file". Example:
     <programlisting><![CDATA[
(gdb) shell ldd ./prog
	libc.so.6 => /lib/libc.so.6 (0x4002c000)
	/lib/ld-linux.so.3 (0x40000000)
(gdb) add-symbol-file /lib/libc.so.6 0x4002c000
add symbol table from file "/lib/libc.so.6" at
	.text_addr = 0x4002c000
(y or n) y
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.
(gdb) 
]]></programlisting>
     </para>
   </listitem>

   <listitem>
     <para>GDB version needed for ARM and PPC32/64.</para>
     <para>You must use a GDB version which is able to read XML
     target description sent by a gdbserver.  This is the standard setup
     if GDB was configured and built with the "expat"
     library.  If your GDB was not configured with XML support, it
     will report an error message when using the "target"
     command.  Debugging will not work because GDB will then not be
     able to fetch the registers from the Valgrind gdbserver.
     For ARM programs using the Thumb instruction set, you must use
     a GDB version of 7.1 or later, as earlier versions have problems
     with next/step/breakpoints in Thumb code.
     </para>
   </listitem>

   <listitem>
     <para>Stack unwinding on PPC32/PPC64. </para>
     <para>On PPC32/PPC64, stack unwinding for leaf functions
     (functions that do not call any other functions) works properly
     only when you give the option
     <option>--vex-iropt-register-updates=allregs-at-mem-access</option>
     or <option>--vex-iropt-register-updates=allregs-at-each-insn</option>.
     You must also pass this option in order to get a precise stack when
     a signal is trapped by GDB.
     </para>
   </listitem>

   <listitem>
     <para>Breakpoints encountered multiple times.</para>
     <para>Some instructions (e.g. x86 "rep movsb")
     are translated by Valgrind using a loop.  If a breakpoint is placed
     on such an instruction, the breakpoint will be encountered
     multiple times -- once for each step of the "implicit" loop
     implementing the instruction.
     </para>
   </listitem>

   <listitem>
     <para>Execution of Inferior function calls by the Valgrind
     gdbserver.</para>

     <para>GDB allows the user to "call" functions inside the process
     being debugged.  Such calls are named "inferior calls" in the GDB
     terminology.  A typical use of an inferior call is to execute
     a function that prints a human-readable version of a complex data
     structure.  To make an inferior call, use the GDB "print" command
     followed by the function to call and its arguments.  As an
     example, the following GDB command causes an inferior call to the
     libc "printf" function to be executed by the process
     being debugged:
     </para>
     <programlisting><![CDATA[
(gdb) p printf("process being debugged has pid %d\n", getpid())
$5 = 36
(gdb) 
]]></programlisting>

     <para>The Valgrind gdbserver supports inferior function calls.
     Whilst an inferior call is running, the Valgrind tool will report
     errors as usual.  If you do not want to have such errors stop the
     execution of the inferior call, you can
     use <computeroutput>v.set vgdb-error</computeroutput> to set a
     big value before the call, then manually reset it to its original
     value when the call is complete.</para>

     <para>To execute inferior calls, GDB changes registers such as
     the program counter, and then continues the execution of the
     program. In a multithreaded program, all threads are continued,
     not just the thread instructed to make the inferior call.  If
     another thread reports an error or encounters a breakpoint, the
     evaluation of the inferior call is abandoned.</para>

     <para>Note that inferior function calls are a powerful GDB
     feature, but should be used with caution. For example, if
     the program being debugged is stopped inside the function "printf",
     forcing a recursive call to printf via an inferior call will
     very probably create problems.  The Valgrind tool might also add
     another level of complexity to inferior calls, e.g. by reporting
     tool errors during the Inferior call or due to the
     instrumentation done.
     </para>

   </listitem>

   <listitem>
     <para>Connecting to or interrupting a Valgrind process blocked in
     a system call.</para>

     <para>Connecting to or interrupting a Valgrind process blocked in
     a system call requires the "ptrace" system call to be usable.
     This may be disabled in your kernel for security reasons.
     </para>

     <para>When running your program, Valgrind's scheduler
     periodically checks whether there is any work to be handled by
     the gdbserver.  Unfortunately this check is only done if at least
     one thread of the process is runnable.  If all the threads of the
     process are blocked in a system call, then the checks do not
     happen, and the Valgrind scheduler will not invoke the gdbserver.
     In such a case, the vgdb relay application will "force" the
     gdbserver to be invoked, without the intervention of the Valgrind
     scheduler.
     </para>

     <para>Such forced invocation of the Valgrind gdbserver is
     implemented by vgdb using ptrace system calls.  On a properly
     implemented kernel, the ptrace calls done by vgdb will not
     influence the behaviour of the program running under Valgrind.
     If however they do, giving the
     option <option>--max-invoke-ms=0</option> to the vgdb relay
     application will disable the usage of ptrace calls.  The
     consequence of disabling ptrace usage in vgdb is that a Valgrind
     process blocked in a system call cannot be woken up or
     interrupted from GDB until it executes enough basic blocks to let
     the Valgrind scheduler's normal checking take effect.
     </para>

     <para>When ptrace is disabled in vgdb, you can increase the
     responsiveness of the Valgrind gdbserver to commands or
     interrupts by giving a lower value to the
     option <option>--vgdb-poll</option>.  If your application is
     blocked in system calls most of the time, using a very low value
     for <option>--vgdb-poll</option> will cause a the gdbserver to be
     invoked sooner.  The gdbserver polling done by Valgrind's
     scheduler is very efficient, so the increased polling frequency
     should not cause significant performance degradation.
     </para>

     <para>When ptrace is disabled in vgdb, a query packet sent by GDB
     may take significant time to be handled by the Valgrind
     gdbserver.  In such cases, GDB might encounter a protocol
     timeout.  To avoid this,
     you can increase the value of the timeout by using the GDB
     command "set remotetimeout".
     </para>

     <para>Ubuntu versions 10.10 and later may restrict the scope of
     ptrace to the children of the process calling ptrace.  As the
     Valgrind process is not a child of vgdb, such restricted scoping
     causes the ptrace calls to fail.  To avoid that, Valgrind will
     automatically allow all processes belonging to the same userid to
     "ptrace" a Valgrind process, by using PR_SET_PTRACER.</para>

     <para>Unblocking processes blocked in system calls is not
     currently implemented on Mac OS X and Android.  So you cannot
     connect to or interrupt a process blocked in a system call on Mac
     OS X or Android.
     </para>

     <para>Unblocking processes blocked in system calls is implemented
     via agent thread on Solaris. This is quite a different approach
     than using ptrace on Linux, but leads to equivalent result - Valgrind
     gdbserver is invoked. Note that agent thread is a Solaris OS
     feature and cannot be disabled.
     </para>
   </listitem>

   <listitem>
     <para>Changing register values.</para>
     <para>The Valgrind gdbserver will only modify the values of the
     thread's registers when the thread is in status Runnable or
     Yielding.  In other states (typically, WaitSys), attempts to
     change register values will fail.  Amongst other things, this
     means that inferior calls are not executed for a thread which is
     in a system call, since the Valgrind gdbserver does not implement
     system call restart.
     </para>
   </listitem>

   <listitem>
     <para>Unsupported GDB functionality.</para>
     <para>GDB provides a lot of debugging functionality and not all
     of it is supported.  Specifically, the following are not
     supported: reversible debugging and tracepoints.
     </para>
   </listitem>

   <listitem>
     <para>Unknown limitations or problems.</para>
     <para>The combination of GDB, Valgrind and the Valgrind gdbserver
     probably has unknown other limitations and problems.  If you
     encounter strange or unexpected behaviour, feel free to report a
     bug.  But first please verify that the limitation or problem is
     not inherent to GDB or the GDB remote protocol.  You may be able
     to do so by checking the behaviour when using standard gdbserver
     part of the GDB package.
     </para>
   </listitem>

 </itemizedlist>

</sect2>

<!-- Referenced from both the manual and manpage -->
<sect2 id="&vg-vgdb-id;"
       xreflabel="&vg-vgdb-label;">
<title>vgdb command line options</title>
<para> Usage: <computeroutput>vgdb [OPTION]... [[-c] COMMAND]...</computeroutput></para>

<para> vgdb ("Valgrind to GDB") is a small program that is used as an
intermediary between Valgrind and GDB or a shell.
It has three usage modes:
</para>
<!-- start of xi:include in the manpage -->
<orderedlist id="vgdb.desc.modes">
  <listitem id="manual-core-adv.vgdb-standalone" xreflabel="vgdb standalone">
    <para>As a standalone utility, it is used from a shell command
    line to send monitor commands to a process running under
    Valgrind. For this usage, the vgdb OPTION(s) must be followed by
    the monitor command to send. To send more than one command,
    separate them with the <option>-c</option> option.
    </para>
  </listitem>

  <listitem id="manual-core-adv.vgdb-relay" xreflabel="vgdb relay">
    <para>In combination with GDB "target remote |" command, it is
    used as the relay application between GDB and the Valgrind
    gdbserver.  For this usage, only OPTION(s) can be given, but no
    COMMAND can be given.
    </para>
  </listitem>

  <listitem id="manual-core-adv.vgdb-multi" xreflabel="vgdb multi">
      <para>In the <option>--multi</option> mode, vgdb uses the extended
      remote protocol to communicate with GDB.  This allows you to view
      output from both valgrind and GDB in the GDB session. This is
      accomplished via the "target extended-remote | vgdb --multi". In
      this mode you no longer need to start valgrind yourself. vgdb will
      start up valgrind when gdb tells it to run a new program. For this
      usage, the vgdb OPTIONS(s) can also include <option>--valgrind</option>
      and <option>--vargs</option> to describe how valgrind should be
      started.
      </para>
  </listitem>

</orderedlist>
<!-- end of xi:include in the manpage -->

<para><computeroutput>vgdb</computeroutput> accepts the following
options:</para>
<!-- start of xi:include in the manpage -->
<variablelist id="vgdb.opts.list">
  <varlistentry>
    <term><option>--pid=&lt;number&gt;</option></term>
    <listitem><para>Specifies the PID of
    the process to which vgdb must connect to.  This option is useful
    in case more than one Valgrind gdbserver can be connected to.  If
    the <option>--pid</option> argument is not given and multiple
    Valgrind gdbserver processes are running, vgdb will report the
    list of such processes and then exit.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--vgdb-prefix</option></term>
    <listitem><para>Must be given to both
    Valgrind and vgdb if you want to change the default prefix for the
    FIFOs (named pipes) used for communication between the Valgrind
    gdbserver and vgdb.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--wait=&lt;number&gt;</option></term>
    <listitem><para>Instructs vgdb to
    search for available Valgrind gdbservers for the specified number
    of seconds.  This makes it possible start a vgdb process 
    before starting the Valgrind gdbserver with which you intend the
    vgdb to communicate.  This option is useful when used in
    conjunction with a <option>--vgdb-prefix</option> that is
    unique to the process you want to wait for.
    Also, if you use the <option>--wait</option> argument in the GDB
    "target remote" command, you must set the GDB remotetimeout to a
    value bigger than the --wait argument value.  See option
    <option>--max-invoke-ms</option> (just below)
    for an example of setting the remotetimeout value.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--max-invoke-ms=&lt;number&gt;</option></term>
    <listitem><para>Gives the
    number of milliseconds after which vgdb will force the invocation
    of gdbserver embedded in Valgrind.  The default value is 100
    milliseconds. A value of 0 disables forced invocation. The forced
    invocation is used when vgdb is connected to a Valgrind gdbserver,
    and the Valgrind process has all its threads blocked in a system
    call.
    </para>

    <para>If you specify a large value, you might need to increase the
    GDB "remotetimeout" value from its default value of 2 seconds.
    You should ensure that the timeout (in seconds) is
    bigger than the <option>--max-invoke-ms</option> value.  For
    example, for <option>--max-invoke-ms=5000</option>, the following
    GDB command is suitable:
    <screen><![CDATA[
    (gdb) set remotetimeout 6
    ]]></screen>
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--cmd-time-out=&lt;number&gt;</option></term>
    <listitem><para>Instructs a
    standalone vgdb to exit if the Valgrind gdbserver it is connected
    to does not process a command in the specified number of seconds.
    The default value is to never time out.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--port=&lt;portnr&gt;</option></term>
    <listitem><para>Instructs vgdb to
    use tcp/ip and listen for GDB on the specified port nr rather than
    to use a pipe to communicate with GDB. Using tcp/ip allows to have
    GDB running on one computer and debugging a Valgrind process
    running on another target computer.
    Example: 
    <screen><![CDATA[
# On the target computer, start your program under valgrind using
valgrind --vgdb-error=0 prog
# and then in another shell, run:
vgdb --port=1234
]]></screen></para>
    <para>On the computer which hosts GDB, execute the command:
    <screen><![CDATA[
gdb prog
(gdb) target remote targetip:1234
]]></screen>
    where targetip is the ip address or hostname of the target computer.
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--vgdb-multi</option></term>
    <listitem><para>Makes vgdb start in extended-remote mode and to wait
    for gdb to tell us what to run.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--valgrind</option></term>
    <listitem><para>The path to valgrind to use, in extended-remote
    mode. If not specified, the system valgrind will be
    launched.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>--vargs</option></term>
    <listitem><para>Options to run valgrind with, in extended-remote
    mode. For example <option>-q</option>. Everything
    following <option>--vargs</option> will be provided as arguments
    to valgrind as is. </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>-c</option></term>
    <listitem><para>To give more than one command to a
    standalone vgdb, separate the commands by an
    option <option>-c</option>. Example:
    <screen><![CDATA[
vgdb v.set log_output -c leak_check any
]]></screen></para></listitem>
  </varlistentry>  

  <varlistentry>
    <term><option>-l</option></term>
    <listitem><para>Instructs a standalone vgdb to report
    the list of the Valgrind gdbserver processes running and then
    exit.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>-T</option></term>
    <listitem><para>Instructs vgdb to add timestamps to vgdb
    information messages.
    </para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>-D</option></term>
    <listitem><para>Instructs a standalone vgdb to show the
    state of the shared memory used by the Valgrind gdbserver.  vgdb
    will exit after having shown the Valgrind gdbserver shared memory
    state.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><option>-d</option></term>
    <listitem><para>Instructs vgdb to produce debugging
    output.  Give multiple <option>-d</option> args to increase the
    verbosity. When giving <option>-d</option> to a relay vgdb, you better
    redirect the standard error (stderr) of vgdb to a file to avoid
    interaction between GDB and vgdb debugging output.</para></listitem>
  </varlistentry>
  
</variablelist>
<!-- end of xi:include in the manpage -->

</sect2>


<!-- Referenced from both the manual and manpage -->
<sect2 id="&vg-monitor-id;"
       xreflabel="&vg-monitor-label;">
<title>Valgrind monitor commands</title>

<para>This section describes the Valgrind monitor commands, available
regardless of the Valgrind tool selected. For the tool specific
commands, refer to <xref linkend="mc-manual.monitor-commands"/>,
<xref linkend="hg-manual.monitor-commands"/>,
<xref linkend="cl-manual.monitor-commands"/> and
<xref linkend="ms-manual.monitor-commands"/>. </para>

<para> The monitor commands can be sent either from a shell command line, by using a
standalone vgdb, or from GDB, by using GDB's "monitor"
command (see <xref linkend="manual-core-adv.gdbserver-commandhandling"/>) or
by GDB's "valgrind" front end commands (see
<xref linkend="manual-core-adv.gdbserver-gdbmonitorfrontend"/>).  
They can also be launched by the client program, using the VALGRIND_MONITOR_COMMAND
client request.
</para>
<para>Whatever the way the monitor command is launched, it will behave the same
  way. However, using the GDB's valgrind front end commands allows to benefit
  from the GDB infrastructure, such as expression evaluation.  When relevant,
  the description of a monitor command below describes the additional
  flexibility provided by the GDB valgrind front end command.  To launch a
  valgrind monitor command via its GDB front end command, instead of prefixing
  the command with "monitor", you must use the GDB <varname>valgrind</varname>
  command (or the shorter aliases <varname>vg</varname>
  or <varname>v</varname>).  In GDB, you can use <varname>help
  valgrind</varname> to get help about the valgrind front end monitor commands
  and you can use <varname>apropos valgrind</varname> to get all the commands
  mentionning the word "valgrind" in their name or on-line help.
</para>

<itemizedlist>
  <listitem>
    <para><varname>help [debug]</varname> instructs Valgrind's gdbserver
    to give the list of all monitor commands of the Valgrind core and
    of the tool. The optional "debug" argument tells to also give help
    for the monitor commands aimed at Valgrind internals debugging.
    </para>
    <para>Note that this monitor command produces the help information as
      provided by valgrind gdbserver.  The GDB help given e.g. by <varname>help
      valgrind</varname> or <varname>help valgrind v.info</varname> provides the
      help of the GDB front end command for the equivalent valgrind gdbserver
      monitor command. This "GDB help" describes the additional flexibility
      provided by the GDB front end command.
      </para>
  </listitem>

  <listitem>
    <para><varname>v.info all_errors [also_suppressed]</varname> shows all errors found
      so far.</para>
    <para> The optional "also_suppressed" argument indicates to also output
      the suppressed errors.</para>
  </listitem>
  <listitem>
    <para><varname>v.info last_error</varname> shows the last error
    found.</para>
  </listitem>

  <listitem>
    <para><varname>v.info location &lt;addr&gt;</varname> outputs
    information about the location &lt;addr&gt;. Possibly, the
    following are described: global variables, local (stack)
    variables, allocated or freed blocks, ...  The information
    produced depends on the tool and on the options given to valgrind.
    Some tools (e.g. memcheck and helgrind) produce more detailed
    information for client heap blocks. For example, these tools show
    the stacktrace where the heap block was allocated. If a tool does
    not replace the malloc/free/... functions, then client heap blocks
    will not be described.  Use the
    option <varname>--read-var-info=yes</varname> to obtain more
    detailed information about global or local (stack) variables.
    </para>
<programlisting><![CDATA[
(gdb) monitor v.info location 0x1130a0
 Location 0x1130a0 is 0 bytes inside global var "mx"
 declared at tc19_shadowmem.c:19
(gdb) mo v.in loc 0x1ffefffe10
 Location 0x1ffefffe10 is 0 bytes inside info.child,
 declared at tc19_shadowmem.c:139, in frame #1 of thread 1
(gdb) 
]]></programlisting>

<para>The GDB valgrind front end command <varname>valgrind v.info location ADDR</varname>
  accepts any address expression for its ADDR argument. In the below examples, mx is
  a global struct and info is a pointer to a structure. Instead of having to print the
  addresses of the structure and printing the pointer variable, you can directly use the
  expressions in the GDB valgrind front end command argument.
  </para>
<programlisting><![CDATA[
(gdb) valgrind v.info location &mx
 Location 0x1130a0 is 0 bytes inside global var "mx"
 declared at tc19_shadowmem.c:19
(gdb) v v.i lo info
 Location 0x1ffefffe10 is 0 bytes inside info.child,
 declared at tc19_shadowmem.c:139, in frame #1 of thread 1
(gdb) 
]]></programlisting>
  </listitem>

  <listitem>
    <para><varname>v.info n_errs_found [msg]</varname> shows the number of
    errors found so far, the nr of errors shown so far and the current
    value of the <option>--vgdb-error</option> argument. The optional
    <computeroutput>msg</computeroutput> (one or more words) is appended.
    Typically, this can be used to insert markers in a process output
    file between several tests executed in sequence by a process
    started only once. This allows to associate the errors reported
    by Valgrind with the specific test that produced these errors.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info open_fds</varname> shows the list of open file
    descriptors and details related to the file descriptor.
    This only works if <option>--track-fds=yes</option> or
    <option>--track-fds=all</option> (to include
    <computeroutput>stdin</computeroutput>,
    <computeroutput>stdout</computeroutput> and
    <computeroutput>stderr</computeroutput>) was given at Valgrind startup.</para>
  </listitem>
  
  <listitem>
    <para><varname>v.clo &lt;clo_option&gt;...</varname> changes one or more
        dynamic command line options. If no clo_option is given, lists the
        dynamically changeable options. See
      <xref linkend="manual-core.dynopts"/>.
      The below shows example of changing the value of <varname>--vgdb-error</varname>
      using directly the valgrind monitor command, using the equivalent GDB valgrind front end command.
      It also shows how a more flexible setting can be done using the GDB eval command.
    </para>
<programlisting><![CDATA[
(gdb) mo v.clo --vgdb-error=10
==2808839== Handling new value --vgdb-error=10 for option --vgdb-error
(gdb) v v.clo --vgdb-error=11
==2808839== Handling new value --vgdb-error=11 for option --vgdb-error
(gdb) set var $nnn = 15
(gdb) eval "v v.clo --vgdb-error=%d", $nnn + 1
==2808839== Handling new value --vgdb-error=16 for option --vgdb-error
(gdb)
]]></programlisting>
  </listitem>

  <listitem>
    <para><varname>v.set {gdb_output | log_output |
    mixed_output}</varname> allows redirection of the Valgrind output
    (e.g. the errors detected by the tool).  The default setting is
    <computeroutput>mixed_output</computeroutput>.</para>
    
    <para>With <computeroutput>mixed_output</computeroutput>, the
    Valgrind output goes to the Valgrind log (typically stderr) while
    the output of the interactive GDB monitor commands (e.g. 
    <computeroutput>v.info last_error</computeroutput>)
    is displayed by GDB.</para>
    
    <para>With <computeroutput>gdb_output</computeroutput>, both the
    Valgrind output and the interactive GDB monitor commands output are
    displayed by GDB.</para>
    
    <para>With <computeroutput>log_output</computeroutput>, both the
    Valgrind output and the interactive GDB monitor commands output go
    to the Valgrind log.</para>
  </listitem>
  
  <listitem>
    <para><varname>v.wait [ms (default 0)]</varname> instructs
    Valgrind gdbserver to sleep "ms" milli-seconds and then
    continue.  When sent from a standalone vgdb, if this is the last
    command, the Valgrind process will continue the execution of the
    guest process. The typical usage of this is to use vgdb to send a
    "no-op" command to a Valgrind gdbserver so as to continue the
    execution of the guest process.
    </para>
    <para>The GDB valgrind front end command <varname>valgrind v.wait
      MS</varname> accepts any integer expression for its MS argument, while the
      monitor command accepts only integer numbers.
    </para>  </listitem>

  <listitem>
    <para><varname>v.kill</varname> requests the gdbserver to kill
    the process. This can be used from a standalone vgdb to properly
    kill a Valgrind process which is currently expecting a vgdb
    connection.</para>
  </listitem>

  <listitem>
    <para><varname>v.set vgdb-error &lt;errornr&gt;</varname>
      dynamically changes the value of the 
      <option>--vgdb-error</option> valgrind command line argument. A
      typical usage of this is to start with
      <option>--vgdb-error=0</option> on the
      command line, then set a few breakpoints, set the vgdb-error value
      to a huge value and continue execution. Note that you can also change
      this value using e.g. <varname>v.clo --vgdb-error=12</varname></para>
    <para>The GDB valgrind front end command <varname>valgrind v.set vgdb-error
      NUM</varname> accepts any integer expression for its ERRORNR argument, while
      the monitor command accepts only integer numbers.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.set merge-recursive-frames &lt;num&gt;</varname>
    dynamically changes the value of the 
      <option>--merge-recursive-frames</option> valgrind command line argument.
     Note that you can also change
    this value using e.g. <varname>v.clo --merge-recursive-frames=5</varname></para>
    <para>The GDB valgrind front end command <varname>valgrind v.set merge-recursive-frames
      NUM</varname> accepts any integer expression for its NUM argument, while
      the monitor command accepts only integer numbers.
    </para>
  </listitem>

  <listitem>
    <para><varname>xtmemory [&lt;filename&gt; default xtmemory.kcg.%p.%n]</varname>
      requests the tool (Memcheck, Massif, Helgrind) to produce an xtree heap memory report. 
      See <xref linkend="&vg-xtree-id;"/> for
      a detailed explanation about execution trees. </para>
  </listitem>

</itemizedlist>

<para>The following Valgrind monitor commands are useful for
investigating the behaviour of Valgrind or its gdbserver in case of
problems or bugs.</para>

<itemizedlist>

  <listitem>
    <para><varname>v.do expensive_sanity_check_general</varname>
    executes various sanity checks. In particular, the sanity of the
    Valgrind heap is verified. This can be useful if you suspect that
    your program and/or Valgrind has a bug corrupting Valgrind data
    structure.  It can also be used when a Valgrind tool
    reports a client error to the connected GDB, in order to verify
    the sanity of Valgrind before continuing the execution.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info gdbserver_status</varname> shows the
    gdbserver status. In case of problems (e.g. of communications),
    this shows the values of some relevant Valgrind gdbserver internal
    variables.  Note that the variables related to breakpoints and
    watchpoints (e.g. the number of breakpoint addresses and the number of
    watchpoints) will be zero, as GDB by default removes all
    watchpoints and breakpoints when execution stops, and re-inserts
    them when resuming the execution of the debugged process.  You can
    change this GDB behaviour by using the GDB command
    <computeroutput>set breakpoint always-inserted on</computeroutput>.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info memory [aspacemgr]</varname> shows the statistics of
    Valgrind's internal heap management. If
    option <option>--profile-heap=yes</option> was given, detailed
    statistics will be output. With the optional argument
    <computeroutput>aspacemgr</computeroutput>. the segment list maintained
    by valgrind address space manager will be output. Note that
    this list of segments is always output on the Valgrind log.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info exectxt</varname> shows information about
    the "executable contexts" (i.e. the stack traces) recorded by
    Valgrind.  For some programs, Valgrind can record a very high
    number of such stack traces, causing a high memory usage.  This
    monitor command shows all the recorded stack traces, followed by
    some statistics. This can be used to analyse the reason for having
    a big number of stack traces. Typically, you will use this command
    if <varname>v.info memory</varname> has shown significant memory
    usage by the "exectxt" arena.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info scheduler</varname> shows various
    information about threads. First, it outputs the host stack trace,
    i.e. the Valgrind code being executed. Then, for each thread, it
    outputs the thread state. For non terminated threads, the state is
    followed by the guest (client) stack trace. Finally, for each
    active thread or for each terminated thread slot not yet re-used,
    it shows the max usage of the valgrind stack.</para>
    <para>Showing the client stack traces allows to compare the stack
    traces produced by the Valgrind unwinder with the stack traces
    produced by GDB+Valgrind gdbserver. Pay attention that GDB and
    Valgrind scheduler status have their own thread numbering
    scheme. To make the link between the GDB thread number and the
    corresponding Valgrind scheduler thread number, use the GDB
    command <computeroutput>info threads</computeroutput>.  The output
    of this command shows the GDB thread number and the valgrind
    'tid'. The 'tid' is the thread number output
    by <computeroutput>v.info scheduler</computeroutput>.  When using
    the callgrind tool, the callgrind monitor command
    <computeroutput>status</computeroutput> outputs internal callgrind
    information about the stack/call graph it maintains.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info stats</varname> shows various valgrind core and
    tool statistics. With this, Valgrind and tool statistics can
    be examined while running, even without option <option>--stats=yes</option>.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info unwind  &lt;addr&gt; [&lt;len&gt;]</varname> shows
    the CFI unwind debug info for the address range [addr, addr+len-1].
    The default value of &lt;len&gt; is 1, giving the unwind information
    for the instruction at &lt;addr&gt;.
    </para>
    <para>The GDB valgrind front end command <varname>valgrind v.info unwind
      ADDR [LEN]</varname> accepts any address expression for its first ADDR
      argument, such as $pc. The second optional argument is any integer
      expression. Note that these 2 arguments must be separated by a space.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.set debuglog &lt;intvalue&gt;</varname> sets the
    Valgrind debug log level to &lt;intvalue&gt;.  This allows to
    dynamically change the log level of Valgrind e.g. when a problem
    is detected.</para>
    <para>The GDB valgrind front end command <varname>valgrind v.set debuglog
      LEVEL</varname> accepts any address expression for its LEVEL argument.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.set hostvisibility [yes*|no]</varname> The value
    "yes" indicates to gdbserver that GDB can look at the Valgrind
    'host' (internal) status/memory. "no" disables this access.
    When hostvisibility is activated, GDB can e.g. look at Valgrind
    global variables. As an example, to examine a Valgrind global
    variable of the memcheck tool on an x86, do the following setup:</para>

<screen><![CDATA[
(gdb) monitor v.set hostvisibility yes
Enabled access to Valgrind memory/status by GDB
If not yet done, tell GDB which valgrind file(s) to use, typically:
add-symbol-file /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux 0x58001000
(gdb) add-symbol-file /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux 0x58001000
add symbol table from file "/home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux" at
	.text_addr = 0x58001000
(y or n) y
Reading symbols from /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux...
(gdb) 
]]></screen>

  <para>After that, variables defined in memcheck-x86-linux can be accessed, e.g.</para>

<screen><![CDATA[
(gdb) p /x vgPlain_threads[1].os_state
$3 = {lwpid = 0x4688, threadgroup = 0x4688, parent = 0x0, 
  valgrind_stack_base = 0x62e78000, valgrind_stack_init_SP = 0x62f79fe0, 
  exitcode = 0x0, fatalsig = 0x0}
(gdb) p vex_control
$5 = {iropt_verbosity = 0, iropt_level = 2, 
  iropt_register_updates = VexRegUpdUnwindregsAtMemAccess, 
  iropt_unroll_thresh = 120, guest_max_insns = 60, guest_chase_thresh = 10}
(gdb) 
]]></screen>
  </listitem>

  <listitem>
    <para><varname>v.translate &lt;address&gt;
    [&lt;traceflags&gt;]</varname> shows the translation of the block
    containing <computeroutput>address</computeroutput> with the given
    trace flags. The <computeroutput>traceflags</computeroutput> value
    bit patterns have similar meaning to Valgrind's
    <option>--trace-flags</option> option.  It can be given
    in hexadecimal (e.g. 0x20) or decimal (e.g. 32) or in binary 1s
    and 0s bit (e.g. 0b00100000). The default value of the traceflags
    is 0b00100000, corresponding to "show after instrumentation". 
    The output of this command always goes to the Valgrind
    log.</para>
    <para>The additional bit flag 0b100000000 (bit 8)
    has no equivalent in the <option>--trace-flags</option> option.
    It enables tracing of the gdbserver specific instrumentation.  Note
    that this bit 8 can only enable the addition of gdbserver
    instrumentation in the trace.  Setting it to 0 will not
    disable the tracing of the gdbserver instrumentation if it is
    active for some other reason, for example because there is a breakpoint at
    this address or because gdbserver is in single stepping
    mode.</para>
    <para>The GDB valgrind front end command <varname>valgrind v.translate ADDR
      [TRACEFLAG]</varname> accepts any address expression for its first ADDR
      argument, such as $pc. The second optional argument is any integer
      expression. Note that these 2 arguments must be separated by a space.
    </para>
  </listitem>

</itemizedlist>

</sect2>

</sect1>





<sect1 id="manual-core-adv.wrapping" xreflabel="Function Wrapping">
<title>Function wrapping</title>

<para>
Valgrind allows calls to some specified functions to be intercepted and
rerouted to a different, user-supplied function.  This can do whatever it
likes, typically examining the arguments, calling onwards to the original,
and possibly examining the result.  Any number of functions may be
wrapped.</para>

<para>
Function wrapping is useful for instrumenting an API in some way.  For
example, Helgrind wraps functions in the POSIX pthreads API so it can know
about thread status changes, and the core is able to wrap
functions in the MPI (message-passing) API so it can know
of memory status changes associated with message arrival/departure.
Such information is usually passed to Valgrind by using client
requests in the wrapper functions, although the exact mechanism may vary.
</para>

<sect2 id="manual-core-adv.wrapping.example" xreflabel="A Simple Example">
<title>A Simple Example</title>

<para>Supposing we want to wrap some function</para>

<programlisting><![CDATA[
int foo ( int x, int y ) { return x + y; }]]></programlisting>

<para>A wrapper is a function of identical type, but with a special name
which identifies it as the wrapper for <computeroutput>foo</computeroutput>.
Wrappers need to include
supporting macros from <filename>valgrind.h</filename>.
Here is a simple wrapper which prints the arguments and return value:</para>

<programlisting><![CDATA[
#include <stdio.h>
#include "valgrind.h"
int I_WRAP_SONAME_FNNAME_ZU(NONE,foo)( int x, int y )
{
   int    result;
   OrigFn fn;
   VALGRIND_GET_ORIG_FN(fn);
   printf("foo's wrapper: args %d %d\n", x, y);
   CALL_FN_W_WW(result, fn, x,y);
   printf("foo's wrapper: result %d\n", result);
   return result;
}
]]></programlisting>

<para>To become active, the wrapper merely needs to be present in a text
section somewhere in the same process' address space as the function
it wraps, and for its ELF symbol name to be visible to Valgrind.  In
practice, this means either compiling to a 
<computeroutput>.o</computeroutput> and linking it in, or
compiling to a <computeroutput>.so</computeroutput> and 
<computeroutput>LD_PRELOAD</computeroutput>ing it in.  The latter is more
convenient in that it doesn't require relinking.</para>

<para>All wrappers have approximately the above form.  There are three
crucial macros:</para>

<para><computeroutput>I_WRAP_SONAME_FNNAME_ZU</computeroutput>: 
this generates the real name of the wrapper.
This is an encoded name which Valgrind notices when reading symbol
table information.  What it says is: I am the wrapper for any function
named <computeroutput>foo</computeroutput> which is found in 
an ELF shared object with an empty
("<computeroutput>NONE</computeroutput>") soname field.  The specification 
mechanism is powerful in
that wildcards are allowed for both sonames and function names.  
The details are discussed below.</para>

<para><computeroutput>VALGRIND_GET_ORIG_FN</computeroutput>: 
once in the wrapper, the first priority is
to get hold of the address of the original (and any other supporting
information needed).  This is stored in a value of opaque 
type <computeroutput>OrigFn</computeroutput>.
The information is acquired using 
<computeroutput>VALGRIND_GET_ORIG_FN</computeroutput>.  It is crucial
to make this macro call before calling any other wrapped function
in the same thread.</para>

<para><computeroutput>CALL_FN_W_WW</computeroutput>: eventually we will
want to call the function being
wrapped.  Calling it directly does not work, since that just gets us
back to the wrapper and leads to an infinite loop.  Instead, the result
lvalue, 
<computeroutput>OrigFn</computeroutput> and arguments are
handed to one of a family of macros of the form 
<computeroutput>CALL_FN_*</computeroutput>.  These
cause Valgrind to call the original and avoid recursion back to the
wrapper.</para>
</sect2>

<sect2 id="manual-core-adv.wrapping.specs" xreflabel="Wrapping Specifications">
<title>Wrapping Specifications</title>

<para>This scheme has the advantage of being self-contained.  A library of
wrappers can be compiled to object code in the normal way, and does
not rely on an external script telling Valgrind which wrappers pertain
to which originals.</para>

<para>Each wrapper has a name which, in the most general case says: I am the
wrapper for any function whose name matches FNPATT and whose ELF
"soname" matches SOPATT.  Both FNPATT and SOPATT may contain wildcards
(asterisks) and other characters (spaces, dots, @, etc) which are not 
generally regarded as valid C identifier names.</para> 

<para>This flexibility is needed to write robust wrappers for POSIX pthread
functions, where typically we are not completely sure of either the
function name or the soname, or alternatively we want to wrap a whole
set of functions at once.</para> 

<para>For example, <computeroutput>pthread_create</computeroutput> 
in GNU libpthread is usually a
versioned symbol - one whose name ends in, eg, 
<computeroutput>@GLIBC_2.3</computeroutput>.  Hence we
are not sure what its real name is.  We also want to cover any soname
of the form <computeroutput>libpthread.so*</computeroutput>.
So the header of the wrapper will be</para>

<programlisting><![CDATA[
int I_WRAP_SONAME_FNNAME_ZZ(libpthreadZdsoZd0,pthreadZucreateZAZa)
  ( ... formals ... )
  { ... body ... }
]]></programlisting>

<para>In order to write unusual characters as valid C function names, a
Z-encoding scheme is used.  Names are written literally, except that
a capital Z acts as an escape character, with the following encoding:</para>

<programlisting><![CDATA[
     Za   encodes    *
     Zp              +
     Zc              :
     Zd              .
     Zu              _
     Zh              -
     Zs              (space)
     ZA              @
     ZZ              Z
     ZL              (       # only in valgrind 3.3.0 and later
     ZR              )       # only in valgrind 3.3.0 and later
]]></programlisting>

<para>Hence <computeroutput>libpthreadZdsoZd0</computeroutput> is an 
encoding of the soname <computeroutput>libpthread.so.0</computeroutput>
and <computeroutput>pthreadZucreateZAZa</computeroutput> is an encoding 
of the function name <computeroutput>pthread_create@*</computeroutput>.
</para>

<para>The macro <computeroutput>I_WRAP_SONAME_FNNAME_ZZ</computeroutput> 
constructs a wrapper name in which
both the soname (first component) and function name (second component)
are Z-encoded.  Encoding the function name can be tiresome and is
often unnecessary, so a second macro,
<computeroutput>I_WRAP_SONAME_FNNAME_ZU</computeroutput>, can be
used instead.  The <computeroutput>_ZU</computeroutput> variant is 
also useful for writing wrappers for
C++ functions, in which the function name is usually already mangled
using some other convention in which Z plays an important role.  Having
to encode a second time quickly becomes confusing.</para>

<para>Since the function name field may contain wildcards, it can be
anything, including just <computeroutput>*</computeroutput>.
The same is true for the soname.
However, some ELF objects - specifically, main executables - do not
have sonames.  Any object lacking a soname is treated as if its soname
was <computeroutput>NONE</computeroutput>, which is why the original 
example above had a name
<computeroutput>I_WRAP_SONAME_FNNAME_ZU(NONE,foo)</computeroutput>.</para>

<para>Note that the soname of an ELF object is not the same as its
file name, although it is often similar.  You can find the soname of
an object <computeroutput>libfoo.so</computeroutput> using the command
<computeroutput>readelf -a libfoo.so | grep soname</computeroutput>.</para>
</sect2>

<sect2 id="manual-core-adv.wrapping.semantics" xreflabel="Wrapping Semantics">
<title>Wrapping Semantics</title>

<para>The ability for a wrapper to replace an infinite family of functions
is powerful but brings complications in situations where ELF objects
appear and disappear (are dlopen'd and dlclose'd) on the fly.
Valgrind tries to maintain sensible behaviour in such situations.</para>

<para>For example, suppose a process has dlopened (an ELF object with
soname) <filename>object1.so</filename>, which contains 
<computeroutput>function1</computeroutput>.  It starts to use
<computeroutput>function1</computeroutput> immediately.</para>

<para>After a while it dlopens <filename>wrappers.so</filename>,
which contains a wrapper
for <computeroutput>function1</computeroutput> in (soname) 
<filename>object1.so</filename>.  All subsequent calls to 
<computeroutput>function1</computeroutput> are rerouted to the wrapper.</para>

<para>If <filename>wrappers.so</filename> is 
later dlclose'd, calls to <computeroutput>function1</computeroutput> are 
naturally routed back to the original.</para>

<para>Alternatively, if <filename>object1.so</filename>
is dlclose'd but <filename>wrappers.so</filename> remains,
then the wrapper exported by <filename>wrappers.so</filename>
becomes inactive, since there
is no way to get to it - there is no original to call any more.  However,
Valgrind remembers that the wrapper is still present.  If 
<filename>object1.so</filename> is
eventually dlopen'd again, the wrapper will become active again.</para>

<para>In short, valgrind inspects all code loading/unloading events to
ensure that the set of currently active wrappers remains consistent.</para>

<para>A second possible problem is that of conflicting wrappers.  It is 
easily possible to load two or more wrappers, both of which claim
to be wrappers for some third function.  In such cases Valgrind will
complain about conflicting wrappers when the second one appears, and
will honour only the first one.</para>
</sect2>

<sect2 id="manual-core-adv.wrapping.debugging" xreflabel="Debugging">
<title>Debugging</title>

<para>Figuring out what's going on given the dynamic nature of wrapping
can be difficult.  The 
<option>--trace-redir=yes</option> option makes 
this possible
by showing the complete state of the redirection subsystem after
every
<function>mmap</function>/<function>munmap</function>
event affecting code (text).</para>

<para>There are two central concepts:</para>

<itemizedlist>

  <listitem><para>A "redirection specification" is a binding of 
  a (soname pattern, fnname pattern) pair to a code address.
  These bindings are created by writing functions with names
  made with the 
  <computeroutput>I_WRAP_SONAME_FNNAME_{ZZ,_ZU}</computeroutput>
  macros.</para></listitem>

  <listitem><para>An "active redirection" is a code-address to 
  code-address binding currently in effect.</para></listitem>

</itemizedlist>

<para>The state of the wrapping-and-redirection subsystem comprises a set of
specifications and a set of active bindings.  The specifications are
acquired/discarded by watching all 
<function>mmap</function>/<function>munmap</function>
events on code (text)
sections.  The active binding set is (conceptually) recomputed from
the specifications, and all known symbol names, following any change
to the specification set.</para>

<para><option>--trace-redir=yes</option> shows the contents 
of both sets following any such event.</para>

<para><option>-v</option> prints a line of text each 
time an active specification is used for the first time.</para>

<para>Hence for maximum debugging effectiveness you will need to use both
options.</para>

<para>One final comment.  The function-wrapping facility is closely
tied to Valgrind's ability to replace (redirect) specified
functions, for example to redirect calls to 
<function>malloc</function> to its
own implementation.  Indeed, a replacement function can be
regarded as a wrapper function which does not call the original.
However, to make the implementation more robust, the two kinds
of interception (wrapping vs replacement) are treated differently.
</para>

<para><option>--trace-redir=yes</option> shows 
specifications and bindings for both
replacement and wrapper functions.  To differentiate the 
two, replacement bindings are printed using 
<computeroutput>R-></computeroutput> whereas 
wraps are printed using <computeroutput>W-></computeroutput>.
</para>
</sect2>


<sect2 id="manual-core-adv.wrapping.limitations-cf" 
       xreflabel="Limitations - control flow">
<title>Limitations - control flow</title>

<para>For the most part, the function wrapping implementation is robust.
The only important caveat is: in a wrapper, get hold of
the <computeroutput>OrigFn</computeroutput> information using 
<computeroutput>VALGRIND_GET_ORIG_FN</computeroutput> before calling any
other wrapped function.  Once you have the 
<computeroutput>OrigFn</computeroutput>, arbitrary
calls between, recursion between, and longjumps out of wrappers
should work correctly.  There is never any interaction between wrapped
functions and merely replaced functions 
(eg <function>malloc</function>), so you can call
<function>malloc</function> etc safely from within wrappers.
</para>

<para>The above comments are true for {x86,amd64,ppc32,arm,mips32,s390}-linux.
On
ppc64-linux function wrapping is more fragile due to the (arguably
poorly designed) ppc64-linux ABI.  This mandates the use of a shadow
stack which tracks entries/exits of both wrapper and replacement
functions.  This gives two limitations: firstly, longjumping out of
wrappers will rapidly lead to disaster, since the shadow stack will
not get correctly cleared.  Secondly, since the shadow stack has
finite size, recursion between wrapper/replacement functions is only
possible to a limited depth, beyond which Valgrind has to abort the
run.  This depth is currently 16 calls.</para>

<para>For all platforms ({x86,amd64,ppc32,ppc64,arm,mips32,s390}-linux)
all the above
comments apply on a per-thread basis.  In other words, wrapping is
thread-safe: each thread must individually observe the above
restrictions, but there is no need for any kind of inter-thread
cooperation.</para>
</sect2>


<sect2 id="manual-core-adv.wrapping.limitations-sigs" 
       xreflabel="Limitations - original function signatures">
<title>Limitations - original function signatures</title>

<para>As shown in the above example, to call the original you must use a
macro of the form <computeroutput>CALL_FN_*</computeroutput>.  
For technical reasons it is impossible
to create a single macro to deal with all argument types and numbers,
so a family of macros covering the most common cases is supplied.  In
what follows, 'W' denotes a machine-word-typed value (a pointer or a
C <computeroutput>long</computeroutput>), 
and 'v' denotes C's <computeroutput>void</computeroutput> type.
The currently available macros are:</para>

<programlisting><![CDATA[
CALL_FN_v_v    -- call an original of type  void fn ( void )
CALL_FN_W_v    -- call an original of type  long fn ( void )

CALL_FN_v_W    -- call an original of type  void fn ( long )
CALL_FN_W_W    -- call an original of type  long fn ( long )

CALL_FN_v_WW   -- call an original of type  void fn ( long, long )
CALL_FN_W_WW   -- call an original of type  long fn ( long, long )

CALL_FN_v_WWW  -- call an original of type  void fn ( long, long, long )
CALL_FN_W_WWW  -- call an original of type  long fn ( long, long, long )

CALL_FN_W_WWWW -- call an original of type  long fn ( long, long, long, long )
CALL_FN_W_5W   -- call an original of type  long fn ( long, long, long, long, long )
CALL_FN_W_6W   -- call an original of type  long fn ( long, long, long, long, long, long )
and so on, up to 
CALL_FN_W_12W
]]></programlisting>

<para>The set of supported types can be expanded as needed.  It is
regrettable that this limitation exists.  Function wrapping has proven
difficult to implement, with a certain apparently unavoidable level of
ickiness.  After several implementation attempts, the present
arrangement appears to be the least-worst tradeoff.  At least it works
reliably in the presence of dynamic linking and dynamic code
loading/unloading.</para>

<para>You should not attempt to wrap a function of one type signature with a
wrapper of a different type signature.  Such trickery will surely lead
to crashes or strange behaviour.  This is not a limitation
of the function wrapping implementation, merely a reflection of the
fact that it gives you sweeping powers to shoot yourself in the foot
if you are not careful.  Imagine the instant havoc you could wreak by
writing a wrapper which matched any function name in any soname - in
effect, one which claimed to be a wrapper for all functions in the
process.</para>
</sect2>

<sect2 id="manual-core-adv.wrapping.examples" xreflabel="Examples">
<title>Examples</title>

<para>In the source tree, 
<filename>memcheck/tests/wrap[1-8].c</filename> provide a series of
examples, ranging from very simple to quite advanced.</para>

<para><filename>mpi/libmpiwrap.c</filename> is an example 
of wrapping a big, complex API (the MPI-2 interface).  This file defines 
almost 300 different wrappers.</para>
</sect2>

</sect1>



</chapter>
