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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_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 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>


<sect1 id="manual-core-adv.gdbserver" 
       xreflabel="Debugging your program using Valgrind's gdbserver and GDB">
<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 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="manual-core-adv.vgdb"/>, 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
reported.  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) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==2418== Using Valgrind-3.7.0.SVN 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>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 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. Pay attention 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>

</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.  See
<xref linkend="manual-core-adv.valgrind-monitor-commands"/> for the list
of the Valgrind core monitor commands.
</para>

<para>Each tool can also provide tool-specific monitor commands.
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 will send the <computeroutput>leak_check</computeroutput>
command to the Valgrind gdbserver.  The Valgrind gdbserver 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>As with other GDB commands, 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
(gdb) mo v.i n
n_errs_found 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>

</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.</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>

</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 compared to
       <option>--vgdb=no</option>.  Option <option>--vgdb=yes</option>
       has neglectible overhead compared
       to <option>--vgdb=no</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-precise-memory-exns=yes</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, when Valgrind
     gdbserver receives the first packet from a vgdb, it calls
     <computeroutput>prctl(PR_SET_PTRACER, vgdb_pid, 0, 0,
     0)</computeroutput> to ensure vgdb can reliably use ptrace.
     Once <computeroutput>vgdb_pid</computeroutput> has been marked as
     a ptracer, vgdb can then properly force the invocation of
     Valgrind gdbserver when needed.  To ensure the vgdb is set as a
     ptracer before the Valgrind process gets blocked in a system
     call, connect your GDB to the Valgrind gdbserver at startup by
     passing <option>--vgdb-error=0</option> to Valgrind.</para>

     <para>Note that
     this "set ptracer" technique does not solve the problem in the
     case where a standalone vgdb process wants to connect to the
     gdbserver, since the first command to be sent by a standalone
     vgdb must wake up the Valgrind process before Valgrind gdbserver
     will mark vgdb as a 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>

   </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>

<sect2 id="manual-core-adv.vgdb"
       xreflabel="vgdb">
<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.
Therefore, it has two usage modes:
</para>
<orderedlist>
  <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>

</orderedlist>

<para><computeroutput>vgdb</computeroutput> accepts the following
options:</para>
<itemizedlist>
  <listitem>
    <para><option>--pid=&lt;number&gt;</option>: 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>

  <listitem>
    <para><option>--vgdb-prefix</option> 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>

  <listitem>
    <para><option>--wait=&lt;number&gt;</option> 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>

  <listitem>
    <para><option>--max-invoke-ms=&lt;number&gt;</option> 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>

  <listitem>
    <para><option>--cmd-time-out=&lt;number&gt;</option> 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>

  <listitem>
    <para><option>--port=&lt;portnr&gt;</option> 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>
   
  <listitem>
    <para><option>-c</option> 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>  

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

  <listitem>
    <para><option>-D</option> 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>

  <listitem>
    <para><option>-d</option> 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>
  
</itemizedlist>
 
</sect2>


<sect2 id="manual-core-adv.valgrind-monitor-commands" 
       xreflabel="Valgrind monitor commands">
<title>Valgrind monitor commands</title>

<para>The Valgrind monitor commands are available regardless of the
Valgrind tool selected.  They can be sent either from a shell command
line, by using a standalone vgdb, or from GDB, by using GDB's
"monitor" command.</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>
  </listitem>

  <listitem>
    <para><varname>v.info all_errors</varname> shows all errors found
    so far.</para>
  </listitem>
  <listitem>
    <para><varname>v.info last_error</varname> shows the last error
    found.</para>
  </listitem>

  <listitem>
    <para><varname>v.info n_errs_found</varname> shows the number of
    errors found so far and the current value of the 
    <option>--vgdb-error</option>
    argument.</para>
  </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>
  </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> 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.</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.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</varname> shows the statistics of
    Valgrind's internal heap management. If
    option <option>--profile-heap=yes</option> was given, detailed
    statistics will be output.
    </para>
  </listitem>

  <listitem>
    <para><varname>v.info scheduler</varname> shows the state and
    stack trace for all threads, as known by Valgrind.  This 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.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>
  </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>
  </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 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}-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}-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>