ftmemsim-valgrind/docs/xml/manual-core-adv.xml

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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>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.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>