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<div class="chapter" title="4.Memcheck: a memory error detector">
<div class="titlepage"><div><div><h2 class="title">
<a name="mc-manual"></a>4.Memcheck: a memory error detector</h2></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.overview">4.1. Overview</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.errormsgs">4.2. Explanation of error messages from Memcheck</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.badrw">4.2.1. Illegal read / Illegal write errors</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.uninitvals">4.2.2. Use of uninitialised values</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.bad-syscall-args">4.2.3. Use of uninitialised or unaddressable values in system
       calls</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.badfrees">4.2.4. Illegal frees</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.rudefn">4.2.5. When a heap block is freed with an inappropriate deallocation
function</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.overlap">4.2.6. Overlapping source and destination blocks</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.leaks">4.2.7. Memory leak detection</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.options">4.3. Memcheck Command-Line Options</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.suppfiles">4.4. Writing suppression files</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.machine">4.5. Details of Memcheck's checking machinery</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.value">4.5.1. Valid-value (V) bits</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.vaddress">4.5.2. Valid-address (A) bits</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.together">4.5.3. Putting it all together</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.monitor-commands">4.6. Memcheck Monitor Commands</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.clientreqs">4.7. Client Requests</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.mempools">4.8. Memory Pools: describing and working with custom allocators</a></span></dt>
<dt><span class="sect1"><a href="mc-manual.html#mc-manual.mpiwrap">4.9. Debugging MPI Parallel Programs with Valgrind</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.build">4.9.1. Building and installing the wrappers</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.gettingstarted">4.9.2. Getting started</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.controlling">4.9.3. Controlling the wrapper library</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.limitations.functions">4.9.4. Functions</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.limitations.types">4.9.5. Types</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.writingwrappers">4.9.6. Writing new wrappers</a></span></dt>
<dt><span class="sect2"><a href="mc-manual.html#mc-manual.mpiwrap.whattoexpect">4.9.7. What to expect when using the wrappers</a></span></dt>
</dl></dd>
</dl>
</div>
<p>To use this tool, you may specify <code class="option">--tool=memcheck</code>
on the Valgrind command line.  You don't have to, though, since Memcheck
is the default tool.</p>
<div class="sect1" title="4.1.Overview">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.overview"></a>4.1.Overview</h2></div></div></div>
<p>Memcheck is a memory error detector.  It can detect the following
problems that are common in C and C++ programs.</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>Accessing memory you shouldn't, e.g. overrunning and underrunning
    heap blocks, overrunning the top of the stack, and accessing memory after
    it has been freed.</p></li>
<li class="listitem"><p>Using undefined values, i.e. values that have not been initialised,
    or that have been derived from other undefined values.</p></li>
<li class="listitem"><p>Incorrect freeing of heap memory, such as double-freeing heap
    blocks, or mismatched use of
    <code class="function">malloc</code>/<code class="computeroutput">new</code>/<code class="computeroutput">new[]</code>
    versus
    <code class="function">free</code>/<code class="computeroutput">delete</code>/<code class="computeroutput">delete[]</code></p></li>
<li class="listitem"><p>Overlapping <code class="computeroutput">src</code> and
    <code class="computeroutput">dst</code> pointers in
    <code class="computeroutput">memcpy</code> and related
    functions.</p></li>
<li class="listitem"><p>Memory leaks.</p></li>
</ul></div>
<p>Problems like these can be difficult to find by other means,
often remaining undetected for long periods, then causing occasional,
difficult-to-diagnose crashes.</p>
</div>
<div class="sect1" title="4.2.Explanation of error messages from Memcheck">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.errormsgs"></a>4.2.Explanation of error messages from Memcheck</h2></div></div></div>
<p>Memcheck issues a range of error messages.  This section presents a
quick summary of what error messages mean.  The precise behaviour of the
error-checking machinery is described in <a class="xref" href="mc-manual.html#mc-manual.machine" title="4.5.Details of Memcheck's checking machinery">Details of Memcheck's checking machinery</a>.</p>
<div class="sect2" title="4.2.1.Illegal read / Illegal write errors">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.badrw"></a>4.2.1.Illegal read / Illegal write errors</h3></div></div></div>
<p>For example:</p>
<pre class="programlisting">
Invalid read of size 4
   at 0x40F6BBCC: (within /usr/lib/libpng.so.2.1.0.9)
   by 0x40F6B804: (within /usr/lib/libpng.so.2.1.0.9)
   by 0x40B07FF4: read_png_image(QImageIO *) (kernel/qpngio.cpp:326)
   by 0x40AC751B: QImageIO::read() (kernel/qimage.cpp:3621)
 Address 0xBFFFF0E0 is not stack'd, malloc'd or free'd
</pre>
<p>This happens when your program reads or writes memory at a place
which Memcheck reckons it shouldn't.  In this example, the program did a
4-byte read at address 0xBFFFF0E0, somewhere within the system-supplied
library libpng.so.2.1.0.9, which was called from somewhere else in the
same library, called from line 326 of <code class="filename">qpngio.cpp</code>,
and so on.</p>
<p>Memcheck tries to establish what the illegal address might relate
to, since that's often useful.  So, if it points into a block of memory
which has already been freed, you'll be informed of this, and also where
the block was freed.  Likewise, if it should turn out to be just off
the end of a heap block, a common result of off-by-one-errors in
array subscripting, you'll be informed of this fact, and also where the
block was allocated.  If you use the <code class="option"><a class="xref" href="manual-core.html#opt.read-var-info">--read-var-info</a></code> option Memcheck will run more slowly
but may give a more detailed description of any illegal address.</p>
<p>In this example, Memcheck can't identify the address.  Actually
the address is on the stack, but, for some reason, this is not a valid
stack address -- it is below the stack pointer and that isn't allowed.
In this particular case it's probably caused by GCC generating invalid
code, a known bug in some ancient versions of GCC.</p>
<p>Note that Memcheck only tells you that your program is about to
access memory at an illegal address.  It can't stop the access from
happening.  So, if your program makes an access which normally would
result in a segmentation fault, you program will still suffer the same
fate -- but you will get a message from Memcheck immediately prior to
this.  In this particular example, reading junk on the stack is
non-fatal, and the program stays alive.</p>
</div>
<div class="sect2" title="4.2.2.Use of uninitialised values">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.uninitvals"></a>4.2.2.Use of uninitialised values</h3></div></div></div>
<p>For example:</p>
<pre class="programlisting">
Conditional jump or move depends on uninitialised value(s)
   at 0x402DFA94: _IO_vfprintf (_itoa.h:49)
   by 0x402E8476: _IO_printf (printf.c:36)
   by 0x8048472: main (tests/manuel1.c:8)
</pre>
<p>An uninitialised-value use error is reported when your program
uses a value which hasn't been initialised -- in other words, is
undefined.  Here, the undefined value is used somewhere inside the
<code class="function">printf</code> machinery of the C library.  This error was
reported when running the following small program:</p>
<pre class="programlisting">
int main()
{
  int x;
  printf ("x = %d\n", x);
}</pre>
<p>It is important to understand that your program can copy around
junk (uninitialised) data as much as it likes.  Memcheck observes this
and keeps track of the data, but does not complain.  A complaint is
issued only when your program attempts to make use of uninitialised
data in a way that might affect your program's externally-visible behaviour.
In this example, <code class="varname">x</code> is uninitialised.  Memcheck observes
the value being passed to <code class="function">_IO_printf</code> and thence to
<code class="function">_IO_vfprintf</code>, but makes no comment.  However,
<code class="function">_IO_vfprintf</code> has to examine the value of
<code class="varname">x</code> so it can turn it into the corresponding ASCII string,
and it is at this point that Memcheck complains.</p>
<p>Sources of uninitialised data tend to be:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>Local variables in procedures which have not been initialised,
    as in the example above.</p></li>
<li class="listitem"><p>The contents of heap blocks (allocated with
    <code class="function">malloc</code>, <code class="function">new</code>, or a similar
    function) before you (or a constructor) write something there.
    </p></li>
</ul></div>
<p>To see information on the sources of uninitialised data in your
program, use the <code class="option">--track-origins=yes</code> option.  This
makes Memcheck run more slowly, but can make it much easier to track down
the root causes of uninitialised value errors.</p>
</div>
<div class="sect2" title="4.2.3.Use of uninitialised or unaddressable values in system calls">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.bad-syscall-args"></a>4.2.3.Use of uninitialised or unaddressable values in system
       calls</h3></div></div></div>
<p>Memcheck checks all parameters to system calls:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>It checks all the direct parameters themselves, whether they are
    initialised.</p></li>
<li class="listitem"><p>Also, if a system call needs to read from a buffer provided by
    your program, Memcheck checks that the entire buffer is addressable
    and its contents are initialised.</p></li>
<li class="listitem"><p>Also, if the system call needs to write to a user-supplied
    buffer, Memcheck checks that the buffer is addressable.</p></li>
</ul></div>
<p>
</p>
<p>After the system call, Memcheck updates its tracked information to
precisely reflect any changes in memory state caused by the system
call.</p>
<p>Here's an example of two system calls with invalid parameters:</p>
<pre class="programlisting">
  #include &lt;stdlib.h&gt;
  #include &lt;unistd.h&gt;
  int main( void )
  {
    char* arr  = malloc(10);
    int*  arr2 = malloc(sizeof(int));
    write( 1 /* stdout */, arr, 10 );
    exit(arr2[0]);
  }
</pre>
<p>You get these complaints ...</p>
<pre class="programlisting">
  Syscall param write(buf) points to uninitialised byte(s)
     at 0x25A48723: __write_nocancel (in /lib/tls/libc-2.3.3.so)
     by 0x259AFAD3: __libc_start_main (in /lib/tls/libc-2.3.3.so)
     by 0x8048348: (within /auto/homes/njn25/grind/head4/a.out)
   Address 0x25AB8028 is 0 bytes inside a block of size 10 alloc'd
     at 0x259852B0: malloc (vg_replace_malloc.c:130)
     by 0x80483F1: main (a.c:5)

  Syscall param exit(error_code) contains uninitialised byte(s)
     at 0x25A21B44: __GI__exit (in /lib/tls/libc-2.3.3.so)
     by 0x8048426: main (a.c:8)
</pre>
<p>... because the program has (a) written uninitialised junk
from the heap block to the standard output, and (b) passed an
uninitialised value to <code class="function">exit</code>.  Note that the first
error refers to the memory pointed to by
<code class="computeroutput">buf</code> (not
<code class="computeroutput">buf</code> itself), but the second error
refers directly to <code class="computeroutput">exit</code>'s argument
<code class="computeroutput">arr2[0]</code>.</p>
</div>
<div class="sect2" title="4.2.4.Illegal frees">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.badfrees"></a>4.2.4.Illegal frees</h3></div></div></div>
<p>For example:</p>
<pre class="programlisting">
Invalid free()
   at 0x4004FFDF: free (vg_clientmalloc.c:577)
   by 0x80484C7: main (tests/doublefree.c:10)
 Address 0x3807F7B4 is 0 bytes inside a block of size 177 free'd
   at 0x4004FFDF: free (vg_clientmalloc.c:577)
   by 0x80484C7: main (tests/doublefree.c:10)
</pre>
<p>Memcheck keeps track of the blocks allocated by your program
with <code class="function">malloc</code>/<code class="computeroutput">new</code>,
so it can know exactly whether or not the argument to
<code class="function">free</code>/<code class="computeroutput">delete</code> is
legitimate or not.  Here, this test program has freed the same block
twice.  As with the illegal read/write errors, Memcheck attempts to
make sense of the address freed.  If, as here, the address is one
which has previously been freed, you wil be told that -- making
duplicate frees of the same block easy to spot.  You will also get this
message if you try to free a pointer that doesn't point to the start of a
heap block.</p>
</div>
<div class="sect2" title="4.2.5.When a heap block is freed with an inappropriate deallocation function">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.rudefn"></a>4.2.5.When a heap block is freed with an inappropriate deallocation
function</h3></div></div></div>
<p>In the following example, a block allocated with
<code class="function">new[]</code> has wrongly been deallocated with
<code class="function">free</code>:</p>
<pre class="programlisting">
Mismatched free() / delete / delete []
   at 0x40043249: free (vg_clientfuncs.c:171)
   by 0x4102BB4E: QGArray::~QGArray(void) (tools/qgarray.cpp:149)
   by 0x4C261C41: PptDoc::~PptDoc(void) (include/qmemarray.h:60)
   by 0x4C261F0E: PptXml::~PptXml(void) (pptxml.cc:44)
 Address 0x4BB292A8 is 0 bytes inside a block of size 64 alloc'd
   at 0x4004318C: operator new[](unsigned int) (vg_clientfuncs.c:152)
   by 0x4C21BC15: KLaola::readSBStream(int) const (klaola.cc:314)
   by 0x4C21C155: KLaola::stream(KLaola::OLENode const *) (klaola.cc:416)
   by 0x4C21788F: OLEFilter::convert(QCString const &amp;) (olefilter.cc:272)
</pre>
<p>In <code class="literal">C++</code> it's important to deallocate memory in a
way compatible with how it was allocated.  The deal is:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>If allocated with
    <code class="function">malloc</code>,
    <code class="function">calloc</code>,
    <code class="function">realloc</code>,
    <code class="function">valloc</code> or
    <code class="function">memalign</code>, you must
    deallocate with <code class="function">free</code>.</p></li>
<li class="listitem"><p>If allocated with <code class="function">new</code>, you must deallocate
   with <code class="function">delete</code>.</p></li>
<li class="listitem"><p>If allocated with <code class="function">new[]</code>, you must
    deallocate with <code class="function">delete[]</code>.</p></li>
</ul></div>
<p>The worst thing is that on Linux apparently it doesn't matter if
you do mix these up, but the same program may then crash on a
different platform, Solaris for example.  So it's best to fix it
properly.  According to the KDE folks "it's amazing how many C++
programmers don't know this".</p>
<p>The reason behind the requirement is as follows.  In some C++
implementations, <code class="function">delete[]</code> must be used for
objects allocated by <code class="function">new[]</code> because the compiler
stores the size of the array and the pointer-to-member to the
destructor of the array's content just before the pointer actually
returned.  <code class="function">delete</code> doesn't account for this and will get
confused, possibly corrupting the heap.</p>
</div>
<div class="sect2" title="4.2.6.Overlapping source and destination blocks">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.overlap"></a>4.2.6.Overlapping source and destination blocks</h3></div></div></div>
<p>The following C library functions copy some data from one
memory block to another (or something similar):
<code class="function">memcpy</code>,
<code class="function">strcpy</code>,
<code class="function">strncpy</code>,
<code class="function">strcat</code>,
<code class="function">strncat</code>. 
The blocks pointed to by their <code class="computeroutput">src</code> and
<code class="computeroutput">dst</code> pointers aren't allowed to overlap.
The POSIX standards have wording along the lines "If copying takes place
between objects that overlap, the behavior is undefined." Therefore,
Memcheck checks for this.
</p>
<p>For example:</p>
<pre class="programlisting">
==27492== Source and destination overlap in memcpy(0xbffff294, 0xbffff280, 21)
==27492==    at 0x40026CDC: memcpy (mc_replace_strmem.c:71)
==27492==    by 0x804865A: main (overlap.c:40)
</pre>
<p>You don't want the two blocks to overlap because one of them could
get partially overwritten by the copying.</p>
<p>You might think that Memcheck is being overly pedantic reporting
this in the case where <code class="computeroutput">dst</code> is less than
<code class="computeroutput">src</code>.  For example, the obvious way to
implement <code class="function">memcpy</code> is by copying from the first
byte to the last.  However, the optimisation guides of some
architectures recommend copying from the last byte down to the first.
Also, some implementations of <code class="function">memcpy</code> zero
<code class="computeroutput">dst</code> before copying, because zeroing the
destination's cache line(s) can improve performance.</p>
<p>The moral of the story is: if you want to write truly portable
code, don't make any assumptions about the language
implementation.</p>
</div>
<div class="sect2" title="4.2.7.Memory leak detection">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.leaks"></a>4.2.7.Memory leak detection</h3></div></div></div>
<p>Memcheck keeps track of all heap blocks issued in response to
calls to
<code class="function">malloc</code>/<code class="function">new</code> et al.
So when the program exits, it knows which blocks have not been freed.
</p>
<p>If <code class="option">--leak-check</code> is set appropriately, for each
remaining block, Memcheck determines if the block is reachable from pointers
within the root-set.  The root-set consists of (a) general purpose registers
of all threads, and (b) initialised, aligned, pointer-sized data words in
accessible client memory, including stacks.</p>
<p>There are two ways a block can be reached.  The first is with a
"start-pointer", i.e. a pointer to the start of the block.  The second is with
an "interior-pointer", i.e. a pointer to the middle of the block.  There are
three ways we know of that an interior-pointer can occur:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>The pointer might have originally been a start-pointer and have been
    moved along deliberately (or not deliberately) by the program.  In
    particular, this can happen if your program uses tagged pointers, i.e.
    if it uses the bottom one, two or three bits of a pointer, which are
    normally always zero due to alignment, in order to store extra
    information.</p></li>
<li class="listitem"><p>It might be a random junk value in memory, entirely unrelated, just
    a coincidence.</p></li>
<li class="listitem"><p>It might be a pointer to an array of C++ objects (which possess
    destructors) allocated with <code class="computeroutput">new[]</code>.  In
    this case, some compilers store a "magic cookie" containing the array
    length at the start of the allocated block, and return a pointer to just
    past that magic cookie, i.e. an interior-pointer.
    See <a class="ulink" href="http://theory.uwinnipeg.ca/gnu/gcc/gxxint_14.html" target="_top">this
    page</a> for more information.</p></li>
</ul></div>
<p>With that in mind, consider the nine possible cases described by the
following figure.</p>
<pre class="programlisting">
     Pointer chain            AAA Category    BBB Category
     -------------            ------------    ------------
(1)  RRR ------------&gt; BBB                    DR
(2)  RRR ---&gt; AAA ---&gt; BBB    DR              IR
(3)  RRR               BBB                    DL
(4)  RRR      AAA ---&gt; BBB    DL              IL
(5)  RRR ------?-----&gt; BBB                    (y)DR, (n)DL
(6)  RRR ---&gt; AAA -?-&gt; BBB    DR              (y)IR, (n)DL
(7)  RRR -?-&gt; AAA ---&gt; BBB    (y)DR, (n)DL    (y)IR, (n)IL
(8)  RRR -?-&gt; AAA -?-&gt; BBB    (y)DR, (n)DL    (y,y)IR, (n,y)IL, (_,n)DL
(9)  RRR      AAA -?-&gt; BBB    DL              (y)IL, (n)DL

Pointer chain legend:
- RRR: a root set node or DR block
- AAA, BBB: heap blocks
- ---&gt;: a start-pointer
- -?-&gt;: an interior-pointer

Category legend:
- DR: Directly reachable
- IR: Indirectly reachable
- DL: Directly lost
- IL: Indirectly lost
- (y)XY: it's XY if the interior-pointer is a real pointer
- (n)XY: it's XY if the interior-pointer is not a real pointer
- (_)XY: it's XY in either case
</pre>
<p>Every possible case can be reduced to one of the above nine.  Memcheck
merges some of these cases in its output, resulting in the following four
categories.</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>"Still reachable". This covers cases 1 and 2 (for the BBB blocks)
    above.  A start-pointer or chain of start-pointers to the block is
    found.  Since the block is still pointed at, the programmer could, at
    least in principle, have freed it before program exit.  Because these
    are very common and arguably not a problem, Memcheck won't report such
    blocks individually unless <code class="option">--show-reachable=yes</code> is
    specified.</p></li>
<li class="listitem"><p>"Definitely lost".  This covers case 3 (for the BBB blocks) above.
    This means that no pointer to the block can be found.  The block is
    classified as "lost", because the programmer could not possibly have
    freed it at program exit, since no pointer to it exists.  This is likely
    a symptom of having lost the pointer at some earlier point in the
    program.  Such cases should be fixed by the programmer.</p></li>
<li class="listitem"><p>"Indirectly lost".  This covers cases 4 and 9 (for the BBB blocks)
    above.  This means that the block is lost, not because there are no
    pointers to it, but rather because all the blocks that point to it are
    themselves lost.  For example, if you have a binary tree and the root
    node is lost, all its children nodes will be indirectly lost.  Because
    the problem will disappear if the definitely lost block that caused the
    indirect leak is fixed, Memcheck won't report such blocks individually
    unless <code class="option">--show-reachable=yes</code> is specified.</p></li>
<li class="listitem"><p>"Possibly lost".  This covers cases 5--8 (for the BBB blocks)
    above.  This means that a chain of one or more pointers to the block has
    been found, but at least one of the pointers is an interior-pointer.
    This could just be a random value in memory that happens to point into a
    block, and so you shouldn't consider this ok unless you know you have
    interior-pointers.</p></li>
</ul></div>
<p>(Note: This mapping of the nine possible cases onto four categories is
not necessarily the best way that leaks could be reported;  in particular,
interior-pointers are treated inconsistently.  It is possible the
categorisation may be improved in the future.)</p>
<p>Furthermore, if suppressions exists for a block, it will be reported
as "suppressed" no matter what which of the above four categories it belongs
to.</p>
<p>The following is an example leak summary.</p>
<pre class="programlisting">
LEAK SUMMARY:
   definitely lost: 48 bytes in 3 blocks.
   indirectly lost: 32 bytes in 2 blocks.
     possibly lost: 96 bytes in 6 blocks.
   still reachable: 64 bytes in 4 blocks.
        suppressed: 0 bytes in 0 blocks.
</pre>
<p>If <code class="option">--leak-check=full</code> is specified,
Memcheck will give details for each definitely lost or possibly lost block,
including where it was allocated.  (Actually, it merges results for all
blocks that have the same category and sufficiently similar stack traces
into a single "loss record".  The
<code class="option">--leak-resolution</code> lets you control the
meaning of "sufficiently similar".)  It cannot tell you when or how or why
the pointer to a leaked block was lost; you have to work that out for
yourself.  In general, you should attempt to ensure your programs do not
have any definitely lost or possibly lost blocks at exit.</p>
<p>For example:</p>
<pre class="programlisting">
8 bytes in 1 blocks are definitely lost in loss record 1 of 14
   at 0x........: malloc (vg_replace_malloc.c:...)
   by 0x........: mk (leak-tree.c:11)
   by 0x........: main (leak-tree.c:39)

88 (8 direct, 80 indirect) bytes in 1 blocks are definitely lost in loss record 13 of 14
   at 0x........: malloc (vg_replace_malloc.c:...)
   by 0x........: mk (leak-tree.c:11)
   by 0x........: main (leak-tree.c:25)
</pre>
<p>The first message describes a simple case of a single 8 byte block
that has been definitely lost.  The second case mentions another 8 byte
block that has been definitely lost;  the difference is that a further 80
bytes in other blocks are indirectly lost because of this lost block.
The loss records are not presented in any notable order, so the loss record
numbers aren't particularly meaningful.</p>
<p>If you specify <code class="option">--show-reachable=yes</code>,
reachable and indirectly lost blocks will also be shown, as the following
two examples show.</p>
<pre class="programlisting">
64 bytes in 4 blocks are still reachable in loss record 2 of 4
   at 0x........: malloc (vg_replace_malloc.c:177)
   by 0x........: mk (leak-cases.c:52)
   by 0x........: main (leak-cases.c:74)

32 bytes in 2 blocks are indirectly lost in loss record 1 of 4
   at 0x........: malloc (vg_replace_malloc.c:177)
   by 0x........: mk (leak-cases.c:52)
   by 0x........: main (leak-cases.c:80)
</pre>
<p>Because there are different kinds of leaks with different severities, an
interesting question is this: which leaks should be counted as true "errors"
and which should not?  The answer to this question affects the numbers printed
in the <code class="computeroutput">ERROR SUMMARY</code> line, and also the effect
of the <code class="option">--error-exitcode</code> option.  Memcheck uses the following
criteria:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>First, a leak is only counted as a true "error" if
    <code class="option">--leak-check=full</code> is specified.  In other words, an
    unprinted leak is not considered a true "error".  If this were not the
    case, it would be possible to get a high error count but not have any
    errors printed, which would be confusing.</p></li>
<li class="listitem"><p>After that, definitely lost and possibly lost blocks are counted as
    true "errors".  Indirectly lost and still reachable blocks are not counted
    as true "errors", even if <code class="option">--show-reachable=yes</code> is
    specified and they are printed;  this is because such blocks don't need
    direct fixing by the programmer.
    </p></li>
</ul></div>
</div>
</div>
<div class="sect1" title="4.3.Memcheck Command-Line Options">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.options"></a>4.3.Memcheck Command-Line Options</h2></div></div></div>
<div class="variablelist">
<a name="mc.opts.list"></a><dl>
<dt>
<a name="opt.leak-check"></a><span class="term">
      <code class="option">--leak-check=&lt;no|summary|yes|full&gt; [default: summary] </code>
    </span>
</dt>
<dd><p>When enabled, search for memory leaks when the client
      program finishes.  If set to <code class="varname">summary</code>, it says how
      many leaks occurred.  If set to <code class="varname">full</code> or
      <code class="varname">yes</code>, it also gives details of each individual
      leak.</p></dd>
<dt>
<a name="opt.show-possibly-lost"></a><span class="term">
      <code class="option">--show-possibly-lost=&lt;yes|no&gt; [default: yes] </code>
    </span>
</dt>
<dd><p>When disabled, the memory leak detector will not show "possibly lost" blocks.  
      </p></dd>
<dt>
<a name="opt.leak-resolution"></a><span class="term">
      <code class="option">--leak-resolution=&lt;low|med|high&gt; [default: high] </code>
    </span>
</dt>
<dd>
<p>When doing leak checking, determines how willing
      Memcheck is to consider different backtraces to
      be the same for the purposes of merging multiple leaks into a single
      leak report.  When set to <code class="varname">low</code>, only the first
      two entries need match.  When <code class="varname">med</code>, four entries
      have to match.  When <code class="varname">high</code>, all entries need to
      match.</p>
<p>For hardcore leak debugging, you probably want to use
      <code class="option">--leak-resolution=high</code> together with
      <code class="option">--num-callers=40</code> or some such large number.
      </p>
<p>Note that the <code class="option">--leak-resolution</code> setting
      does not affect Memcheck's ability to find
      leaks.  It only changes how the results are presented.</p>
</dd>
<dt>
<a name="opt.show-reachable"></a><span class="term">
      <code class="option">--show-reachable=&lt;yes|no&gt; [default: no] </code>
    </span>
</dt>
<dd><p>When disabled, the memory leak detector only shows "definitely
      lost" and "possibly lost" blocks.  When enabled, the leak detector also
      shows "reachable" and "indirectly lost" blocks.  (In other words, it
      shows all blocks, except suppressed ones, so
      <code class="option">--show-all</code> would be a better name for
      it.)</p></dd>
<dt>
<a name="opt.undef-value-errors"></a><span class="term">
      <code class="option">--undef-value-errors=&lt;yes|no&gt; [default: yes] </code>
    </span>
</dt>
<dd><p>Controls whether Memcheck reports
      uses of undefined value errors.  Set this to
      <code class="varname">no</code> if you don't want to see undefined value
      errors.  It also has the side effect of speeding up
      Memcheck somewhat.
      </p></dd>
<dt>
<a name="opt.track-origins"></a><span class="term">
      <code class="option">--track-origins=&lt;yes|no&gt; [default: no] </code>
    </span>
</dt>
<dd>
<p>Controls whether Memcheck tracks
        the origin of uninitialised values.  By default, it does not,
        which means that although it can tell you that an
        uninitialised value is being used in a dangerous way, it
        cannot tell you where the uninitialised value came from.  This
        often makes it difficult to track down the root problem.
        </p>
<p>When set
        to <code class="varname">yes</code>, Memcheck keeps
        track of the origins of all uninitialised values.  Then, when
        an uninitialised value error is
        reported, Memcheck will try to show the
        origin of the value.  An origin can be one of the following
        four places: a heap block, a stack allocation, a client
        request, or miscellaneous other sources (eg, a call
        to <code class="varname">brk</code>).
        </p>
<p>For uninitialised values originating from a heap
        block, Memcheck shows where the block was
        allocated.  For uninitialised values originating from a stack
        allocation, Memcheck can tell you which
        function allocated the value, but no more than that -- typically
        it shows you the source location of the opening brace of the
        function.  So you should carefully check that all of the
        function's local variables are initialised properly.
        </p>
<p>Performance overhead: origin tracking is expensive.  It
        halves Memcheck's speed and increases
        memory use by a minimum of 100MB, and possibly more.
        Nevertheless it can drastically reduce the effort required to
        identify the root cause of uninitialised value errors, and so
        is often a programmer productivity win, despite running
        more slowly.
        </p>
<p>Accuracy: Memcheck tracks origins
        quite accurately.  To avoid very large space and time
        overheads, some approximations are made.  It is possible,
        although unlikely, that Memcheck will report an incorrect origin, or
        not be able to identify any origin.
        </p>
<p>Note that the combination
        <code class="option">--track-origins=yes</code>
        and <code class="option">--undef-value-errors=no</code> is
        nonsensical.  Memcheck checks for and
        rejects this combination at startup.
        </p>
</dd>
<dt>
<a name="opt.partial-loads-ok"></a><span class="term">
      <code class="option">--partial-loads-ok=&lt;yes|no&gt; [default: no] </code>
    </span>
</dt>
<dd>
<p>Controls how Memcheck handles word-sized,
      word-aligned loads from addresses for which some bytes are
      addressable and others are not.  When <code class="varname">yes</code>, such
      loads do not produce an address error.  Instead, loaded bytes
      originating from illegal addresses are marked as uninitialised, and
      those corresponding to legal addresses are handled in the normal
      way.</p>
<p>When <code class="varname">no</code>, loads from partially invalid
      addresses are treated the same as loads from completely invalid
      addresses: an illegal-address error is issued, and the resulting
      bytes are marked as initialised.</p>
<p>Note that code that behaves in this way is in violation of
      the the ISO C/C++ standards, and should be considered broken.  If
      at all possible, such code should be fixed.  This option should be
      used only as a last resort.</p>
</dd>
<dt>
<a name="opt.freelist-vol"></a><span class="term">
      <code class="option">--freelist-vol=&lt;number&gt; [default: 20000000] </code>
    </span>
</dt>
<dd>
<p>When the client program releases memory using
      <code class="function">free</code> (in <code class="literal">C</code>) or
      <code class="computeroutput">delete</code>
      (<code class="literal">C++</code>), that memory is not immediately made
      available for re-allocation.  Instead, it is marked inaccessible
      and placed in a queue of freed blocks.  The purpose is to defer as
      long as possible the point at which freed-up memory comes back
      into circulation.  This increases the chance that
      Memcheck will be able to detect invalid
      accesses to blocks for some significant period of time after they
      have been freed.</p>
<p>This option specifies the maximum total size, in bytes, of the
      blocks in the queue.  The default value is twenty million bytes.
      Increasing this increases the total amount of memory used by
      Memcheck but may detect invalid uses of freed
      blocks which would otherwise go undetected.</p>
</dd>
<dt>
<a name="opt.freelist-big-blocks"></a><span class="term">
      <code class="option">--freelist-big-blocks=&lt;number&gt; [default: 1000000] </code>
    </span>
</dt>
<dd>
<p>When making blocks from the queue of freed blocks available
      for re-allocation, Memcheck will in priority re-circulate the blocks
      with a size greater or equal to <code class="option">--freelist-big-blocks</code>.
      This ensures that freeing big blocks (in particular freeing blocks bigger than
      <code class="option">--freelist-vol</code>) does not immediately lead to a re-circulation
      of all (or a lot of) the small blocks in the free list. In other words,
      this option increases the likelihood to discover dangling pointers
      for the "small" blocks, even when big blocks are freed.</p>
<p>Setting a value of 0 means that all the blocks are re-circulated
      in a FIFO order. </p>
</dd>
<dt>
<a name="opt.workaround-gcc296-bugs"></a><span class="term">
      <code class="option">--workaround-gcc296-bugs=&lt;yes|no&gt; [default: no] </code>
    </span>
</dt>
<dd>
<p>When enabled, assume that reads and writes some small
      distance below the stack pointer are due to bugs in GCC 2.96, and
      does not report them.  The "small distance" is 256 bytes by
      default.  Note that GCC 2.96 is the default compiler on some ancient
      Linux distributions (RedHat 7.X) and so you may need to use this
      option.  Do not use it if you do not have to, as it can cause real
      errors to be overlooked.  A better alternative is to use a more
      recent GCC in which this bug is fixed.</p>
<p>You may also need to use this option when working with
      GCC 3.X or 4.X on 32-bit PowerPC Linux.  This is because
      GCC generates code which occasionally accesses below the
      stack pointer, particularly for floating-point to/from integer
      conversions.  This is in violation of the 32-bit PowerPC ELF
      specification, which makes no provision for locations below the
      stack pointer to be accessible.</p>
</dd>
<dt>
<a name="opt.ignore-ranges"></a><span class="term">
      <code class="option">--ignore-ranges=0xPP-0xQQ[,0xRR-0xSS] </code>
    </span>
</dt>
<dd><p>Any ranges listed in this option (and multiple ranges can be
    specified, separated by commas) will be ignored by Memcheck's
    addressability checking.</p></dd>
<dt>
<a name="opt.malloc-fill"></a><span class="term">
      <code class="option">--malloc-fill=&lt;hexnumber&gt; </code>
    </span>
</dt>
<dd><p>Fills blocks allocated
      by <code class="computeroutput">malloc</code>,
         <code class="computeroutput">new</code>, etc, but not
      by <code class="computeroutput">calloc</code>, with the specified
      byte.  This can be useful when trying to shake out obscure
      memory corruption problems.  The allocated area is still
      regarded by Memcheck as undefined -- this option only affects its
      contents.
      </p></dd>
<dt>
<a name="opt.free-fill"></a><span class="term">
      <code class="option">--free-fill=&lt;hexnumber&gt; </code>
    </span>
</dt>
<dd><p>Fills blocks freed
      by <code class="computeroutput">free</code>,
         <code class="computeroutput">delete</code>, etc, with the
      specified byte value.  This can be useful when trying to shake out
      obscure memory corruption problems.  The freed area is still
      regarded by Memcheck as not valid for access -- this option only
      affects its contents.
      </p></dd>
</dl>
</div>
</div>
<div class="sect1" title="4.4.Writing suppression files">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.suppfiles"></a>4.4.Writing suppression files</h2></div></div></div>
<p>The basic suppression format is described in 
<a class="xref" href="manual-core.html#manual-core.suppress" title="2.5.Suppressing errors">Suppressing errors</a>.</p>
<p>The suppression-type (second) line should have the form:</p>
<pre class="programlisting">
Memcheck:suppression_type</pre>
<p>The Memcheck suppression types are as follows:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="varname">Value1</code>, 
    <code class="varname">Value2</code>,
    <code class="varname">Value4</code>,
    <code class="varname">Value8</code>,
    <code class="varname">Value16</code>,
    meaning an uninitialised-value error when
    using a value of 1, 2, 4, 8 or 16 bytes.</p></li>
<li class="listitem"><p><code class="varname">Cond</code> (or its old
    name, <code class="varname">Value0</code>), meaning use
    of an uninitialised CPU condition code.</p></li>
<li class="listitem"><p><code class="varname">Addr1</code>,
    <code class="varname">Addr2</code>, 
    <code class="varname">Addr4</code>,
    <code class="varname">Addr8</code>,
    <code class="varname">Addr16</code>, 
    meaning an invalid address during a
    memory access of 1, 2, 4, 8 or 16 bytes respectively.</p></li>
<li class="listitem"><p><code class="varname">Jump</code>, meaning an
    jump to an unaddressable location error.</p></li>
<li class="listitem"><p><code class="varname">Param</code>, meaning an
    invalid system call parameter error.</p></li>
<li class="listitem"><p><code class="varname">Free</code>, meaning an
    invalid or mismatching free.</p></li>
<li class="listitem"><p><code class="varname">Overlap</code>, meaning a
    <code class="computeroutput">src</code> /
    <code class="computeroutput">dst</code> overlap in
    <code class="function">memcpy</code> or a similar function.</p></li>
<li class="listitem"><p><code class="varname">Leak</code>, meaning
    a memory leak.</p></li>
</ul></div>
<p><code class="computeroutput">Param</code> errors have an extra
information line at this point, which is the name of the offending
system call parameter.  No other error kinds have this extra
line.</p>
<p>The first line of the calling context: for <code class="varname">ValueN</code>
and <code class="varname">AddrN</code> errors, it is either the name of the function
in which the error occurred, or, failing that, the full path of the
<code class="filename">.so</code> file
or executable containing the error location.  For <code class="varname">Free</code> errors, is the name
of the function doing the freeing (eg, <code class="function">free</code>,
<code class="function">__builtin_vec_delete</code>, etc).  For
<code class="varname">Overlap</code> errors, is the name of the function with the
overlapping arguments (eg.  <code class="function">memcpy</code>,
<code class="function">strcpy</code>, etc).</p>
<p>Lastly, there's the rest of the calling context.</p>
</div>
<div class="sect1" title="4.5.Details of Memcheck's checking machinery">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.machine"></a>4.5.Details of Memcheck's checking machinery</h2></div></div></div>
<p>Read this section if you want to know, in detail, exactly
what and how Memcheck is checking.</p>
<div class="sect2" title="4.5.1.Valid-value (V) bits">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.value"></a>4.5.1.Valid-value (V) bits</h3></div></div></div>
<p>It is simplest to think of Memcheck implementing a synthetic CPU
which is identical to a real CPU, except for one crucial detail.  Every
bit (literally) of data processed, stored and handled by the real CPU
has, in the synthetic CPU, an associated "valid-value" bit, which says
whether or not the accompanying bit has a legitimate value.  In the
discussions which follow, this bit is referred to as the V (valid-value)
bit.</p>
<p>Each byte in the system therefore has a 8 V bits which follow it
wherever it goes.  For example, when the CPU loads a word-size item (4
bytes) from memory, it also loads the corresponding 32 V bits from a
bitmap which stores the V bits for the process' entire address space.
If the CPU should later write the whole or some part of that value to
memory at a different address, the relevant V bits will be stored back
in the V-bit bitmap.</p>
<p>In short, each bit in the system has (conceptually) an associated V
bit, which follows it around everywhere, even inside the CPU.  Yes, all the
CPU's registers (integer, floating point, vector and condition registers)
have their own V bit vectors.  For this to work, Memcheck uses a great deal
of compression to represent the V bits compactly.</p>
<p>Copying values around does not cause Memcheck to check for, or
report on, errors.  However, when a value is used in a way which might
conceivably affect your program's externally-visible behaviour,
the associated V bits are immediately checked.  If any of these indicate
that the value is undefined (even partially), an error is reported.</p>
<p>Here's an (admittedly nonsensical) example:</p>
<pre class="programlisting">
int i, j;
int a[10], b[10];
for ( i = 0; i &lt; 10; i++ ) {
  j = a[i];
  b[i] = j;
}</pre>
<p>Memcheck emits no complaints about this, since it merely copies
uninitialised values from <code class="varname">a[]</code> into
<code class="varname">b[]</code>, and doesn't use them in a way which could
affect the behaviour of the program.  However, if
the loop is changed to:</p>
<pre class="programlisting">
for ( i = 0; i &lt; 10; i++ ) {
  j += a[i];
}
if ( j == 77 ) 
  printf("hello there\n");
</pre>
<p>then Memcheck will complain, at the
<code class="computeroutput">if</code>, that the condition depends on
uninitialised values.  Note that it <span class="command"><strong>doesn't</strong></span> complain
at the <code class="varname">j += a[i];</code>, since at that point the
undefinedness is not "observable".  It's only when a decision has to be
made as to whether or not to do the <code class="function">printf</code> -- an
observable action of your program -- that Memcheck complains.</p>
<p>Most low level operations, such as adds, cause Memcheck to use the
V bits for the operands to calculate the V bits for the result.  Even if
the result is partially or wholly undefined, it does not
complain.</p>
<p>Checks on definedness only occur in three places: when a value is
used to generate a memory address, when control flow decision needs to
be made, and when a system call is detected, Memcheck checks definedness
of parameters as required.</p>
<p>If a check should detect undefinedness, an error message is
issued.  The resulting value is subsequently regarded as well-defined.
To do otherwise would give long chains of error messages.  In other
words, once Memcheck reports an undefined value error, it tries to
avoid reporting further errors derived from that same undefined
value.</p>
<p>This sounds overcomplicated.  Why not just check all reads from
memory, and complain if an undefined value is loaded into a CPU
register?  Well, that doesn't work well, because perfectly legitimate C
programs routinely copy uninitialised values around in memory, and we
don't want endless complaints about that.  Here's the canonical example.
Consider a struct like this:</p>
<pre class="programlisting">
struct S { int x; char c; };
struct S s1, s2;
s1.x = 42;
s1.c = 'z';
s2 = s1;
</pre>
<p>The question to ask is: how large is <code class="varname">struct S</code>,
in bytes?  An <code class="varname">int</code> is 4 bytes and a
<code class="varname">char</code> one byte, so perhaps a <code class="varname">struct
S</code> occupies 5 bytes?  Wrong.  All non-toy compilers we know
of will round the size of <code class="varname">struct S</code> up to a whole
number of words, in this case 8 bytes.  Not doing this forces compilers
to generate truly appalling code for accessing arrays of
<code class="varname">struct S</code>'s on some architectures.</p>
<p>So <code class="varname">s1</code> occupies 8 bytes, yet only 5 of them will
be initialised.  For the assignment <code class="varname">s2 = s1</code>, GCC
generates code to copy all 8 bytes wholesale into <code class="varname">s2</code>
without regard for their meaning.  If Memcheck simply checked values as
they came out of memory, it would yelp every time a structure assignment
like this happened.  So the more complicated behaviour described above
is necessary.  This allows GCC to copy
<code class="varname">s1</code> into <code class="varname">s2</code> any way it likes, and a
warning will only be emitted if the uninitialised values are later
used.</p>
</div>
<div class="sect2" title="4.5.2.Valid-address (A) bits">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.vaddress"></a>4.5.2.Valid-address (A) bits</h3></div></div></div>
<p>Notice that the previous subsection describes how the validity of
values is established and maintained without having to say whether the
program does or does not have the right to access any particular memory
location.  We now consider the latter question.</p>
<p>As described above, every bit in memory or in the CPU has an
associated valid-value (V) bit.  In addition, all bytes in memory, but
not in the CPU, have an associated valid-address (A) bit.  This
indicates whether or not the program can legitimately read or write that
location.  It does not give any indication of the validity of the data
at that location -- that's the job of the V bits -- only whether or not
the location may be accessed.</p>
<p>Every time your program reads or writes memory, Memcheck checks
the A bits associated with the address.  If any of them indicate an
invalid address, an error is emitted.  Note that the reads and writes
themselves do not change the A bits, only consult them.</p>
<p>So how do the A bits get set/cleared?  Like this:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>When the program starts, all the global data areas are
    marked as accessible.</p></li>
<li class="listitem"><p>When the program does
    <code class="function">malloc</code>/<code class="computeroutput">new</code>,
    the A bits for exactly the area allocated, and not a byte more,
    are marked as accessible.  Upon freeing the area the A bits are
    changed to indicate inaccessibility.</p></li>
<li class="listitem"><p>When the stack pointer register (<code class="literal">SP</code>) moves
    up or down, A bits are set.  The rule is that the area from
    <code class="literal">SP</code> up to the base of the stack is marked as
    accessible, and below <code class="literal">SP</code> is inaccessible.  (If
    that sounds illogical, bear in mind that the stack grows down, not
    up, on almost all Unix systems, including GNU/Linux.)  Tracking
    <code class="literal">SP</code> like this has the useful side-effect that the
    section of stack used by a function for local variables etc is
    automatically marked accessible on function entry and inaccessible
    on exit.</p></li>
<li class="listitem"><p>When doing system calls, A bits are changed appropriately.
    For example, <code class="literal">mmap</code>
    magically makes files appear in the process'
    address space, so the A bits must be updated if <code class="literal">mmap</code>
    succeeds.</p></li>
<li class="listitem"><p>Optionally, your program can tell Memcheck about such changes
    explicitly, using the client request mechanism described
    above.</p></li>
</ul></div>
</div>
<div class="sect2" title="4.5.3.Putting it all together">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.together"></a>4.5.3.Putting it all together</h3></div></div></div>
<p>Memcheck's checking machinery can be summarised as
follows:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>Each byte in memory has 8 associated V (valid-value) bits,
    saying whether or not the byte has a defined value, and a single A
    (valid-address) bit, saying whether or not the program currently has
    the right to read/write that address.  As mentioned above, heavy
    use of compression means the overhead is typically around 25%.</p></li>
<li class="listitem"><p>When memory is read or written, the relevant A bits are
    consulted.  If they indicate an invalid address, Memcheck emits an
    Invalid read or Invalid write error.</p></li>
<li class="listitem"><p>When memory is read into the CPU's registers, the relevant V
    bits are fetched from memory and stored in the simulated CPU.  They
    are not consulted.</p></li>
<li class="listitem"><p>When a register is written out to memory, the V bits for that
    register are written back to memory too.</p></li>
<li class="listitem"><p>When values in CPU registers are used to generate a memory
    address, or to determine the outcome of a conditional branch, the V
    bits for those values are checked, and an error emitted if any of
    them are undefined.</p></li>
<li class="listitem"><p>When values in CPU registers are used for any other purpose,
    Memcheck computes the V bits for the result, but does not check
    them.</p></li>
<li class="listitem"><p>Once the V bits for a value in the CPU have been checked, they
    are then set to indicate validity.  This avoids long chains of
    errors.</p></li>
<li class="listitem">
<p>When values are loaded from memory, Memcheck checks the A bits
    for that location and issues an illegal-address warning if needed.
    In that case, the V bits loaded are forced to indicate Valid,
    despite the location being invalid.</p>
<p>This apparently strange choice reduces the amount of confusing
    information presented to the user.  It avoids the unpleasant
    phenomenon in which memory is read from a place which is both
    unaddressable and contains invalid values, and, as a result, you get
    not only an invalid-address (read/write) error, but also a
    potentially large set of uninitialised-value errors, one for every
    time the value is used.</p>
<p>There is a hazy boundary case to do with multi-byte loads from
    addresses which are partially valid and partially invalid.  See
    details of the option <code class="option">--partial-loads-ok</code> for details.
    </p>
</li>
</ul></div>
<p>Memcheck intercepts calls to <code class="function">malloc</code>,
<code class="function">calloc</code>, <code class="function">realloc</code>,
<code class="function">valloc</code>, <code class="function">memalign</code>,
<code class="function">free</code>, <code class="computeroutput">new</code>,
<code class="computeroutput">new[]</code>,
<code class="computeroutput">delete</code> and
<code class="computeroutput">delete[]</code>.  The behaviour you get
is:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="function">malloc</code>/<code class="function">new</code>/<code class="computeroutput">new[]</code>:
    the returned memory is marked as addressable but not having valid
    values.  This means you have to write to it before you can read
    it.</p></li>
<li class="listitem"><p><code class="function">calloc</code>: returned memory is marked both
    addressable and valid, since <code class="function">calloc</code> clears
    the area to zero.</p></li>
<li class="listitem"><p><code class="function">realloc</code>: if the new size is larger than
    the old, the new section is addressable but invalid, as with
    <code class="function">malloc</code>.  If the new size is smaller, the
    dropped-off section is marked as unaddressable.  You may only pass to
    <code class="function">realloc</code> a pointer previously issued to you by
    <code class="function">malloc</code>/<code class="function">calloc</code>/<code class="function">realloc</code>.</p></li>
<li class="listitem"><p><code class="function">free</code>/<code class="computeroutput">delete</code>/<code class="computeroutput">delete[]</code>:
    you may only pass to these functions a pointer previously issued
    to you by the corresponding allocation function.  Otherwise,
    Memcheck complains.  If the pointer is indeed valid, Memcheck
    marks the entire area it points at as unaddressable, and places
    the block in the freed-blocks-queue.  The aim is to defer as long
    as possible reallocation of this block.  Until that happens, all
    attempts to access it will elicit an invalid-address error, as you
    would hope.</p></li>
</ul></div>
</div>
</div>
<div class="sect1" title="4.6.Memcheck Monitor Commands">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.monitor-commands"></a>4.6.Memcheck Monitor Commands</h2></div></div></div>
<p>The Memcheck tool provides monitor commands handled by Valgrind's
built-in gdbserver (see <a class="xref" href="manual-core-adv.html#manual-core-adv.gdbserver-commandhandling" title="3.2.5.Monitor command handling by the Valgrind gdbserver">Monitor command handling by the Valgrind gdbserver</a>).
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p><code class="varname">get_vbits &lt;addr&gt; [&lt;len&gt;]</code>
    shows the definedness (V) bits for &lt;len&gt; (default 1) bytes
    starting at &lt;addr&gt;.  The definedness of each byte in the
    range is given using two hexadecimal digits.  These hexadecimal
    digits encode the validity of each bit of the corresponding byte,
    using 0 if the bit is defined and 1 if the bit is undefined.
    If a byte is not addressable, its validity bits are replaced
    by <code class="varname">__</code> (a double underscore).
    </p>
<p>
    In the following example, <code class="varname">string10</code> is an array
    of 10 characters, in which the even numbered bytes are
    undefined. In the below example, the byte corresponding
    to <code class="varname">string10[5]</code> is not addressable.
    </p>
<pre class="programlisting">
(gdb) p &amp;string10
$4 = (char (*)[10]) 0x8049e28
(gdb) monitor get_vbits 0x8049e28 10
ff00ff00 ff__ff00 ff00
(gdb) 
</pre>
<p> The command get_vbits cannot be used with registers. To get
    the validity bits of a register, you must start Valgrind with the
    option <code class="option">--vgdb-shadow-registers=yes</code>. The validity
    bits of a register can be obtained by printing the 'shadow 1'
    corresponding register.  In the below x86 example, the register
    eax has all its bits undefined, while the register ebx is fully
    defined.
    </p>
<pre class="programlisting">
(gdb) p /x $eaxs1
$9 = 0xffffffff
(gdb) p /x $ebxs1
$10 = 0x0
(gdb) 
</pre>
</li>
<li class="listitem">
<p><code class="varname">make_memory
    [noaccess|undefined|defined|Definedifaddressable] &lt;addr&gt;
    [&lt;len&gt;]</code> marks the range of &lt;len&gt; (default 1)
    bytes at &lt;addr&gt; as having the given status. Parameter
    <code class="varname">noaccess</code> marks the range as non-accessible, so
    Memcheck will report an error on any access to it.
    <code class="varname">undefined</code> or <code class="varname">defined</code> mark
    the area as accessible, but Memcheck regards the bytes in it
    respectively as having undefined or defined values.
    <code class="varname">Definedifaddressable</code> marks as defined, bytes in
    the range which are already addressible, but makes no change to
    the status of bytes in the range which are not addressible. Note
    that the first letter of <code class="varname">Definedifaddressable</code>
    is an uppercase D to avoid confusion with <code class="varname">defined</code>.
    </p>
<p>
    In the following example, the first byte of the
    <code class="varname">string10</code> is marked as defined:
    </p>
<pre class="programlisting">
(gdb) monitor make_memory defined 0x8049e28  1
(gdb) monitor get_vbits 0x8049e28 10
0000ff00 ff00ff00 ff00
(gdb) 
</pre>
</li>
<li class="listitem">
<p><code class="varname">check_memory [addressable|defined] &lt;addr&gt;
    [&lt;len&gt;]</code> checks that the range of &lt;len&gt;
    (default 1) bytes at &lt;addr&gt; has the specified accessibility.
    It then outputs a description of &lt;addr&gt;. In the following
    example, a detailed description is available because the
    option <code class="option">--read-var-info=yes</code> was given Valgrind at
    startup:
    </p>
<pre class="programlisting">
(gdb) monitor check_memory defined 0x8049e28  1
Address 0x8049E28 len 1 defined
==14698==  Location 0x8049e28 is 0 bytes inside string10[0],
==14698==  declared at prog.c:10, in frame #0 of thread 1
(gdb) 
</pre>
</li>
<li class="listitem">
<p><code class="varname">leak_check [full*|summary]
                              [reachable|possibleleak*|definiteleak]
                              [increased*|changed|any]
          </code>
    performs a leak check. The <code class="varname">*</code> in the arguments
    indicates the default value. </p>
<p> If the first argument is <code class="varname">summary</code>, only a
    summary of the leak search is given; otherwise a full leak report
    is produced.  A full leak report gives detailed information for
    each leak: the stack trace where the leaked blocks were allocated,
    the number of blocks leaked and their total size.  When a full
    report is requested, the next two arguments further specify what
    kind of leaks to report.  A leak's details are shown if they match
    both the second and third argument.
    </p>
<p>The second argument controls what kind of blocks are shown for
    a <code class="varname">full</code> leak search.  The
    value <code class="varname">definiteleak</code> specifies that only
    definitely leaked blocks should be shown.  The
    value <code class="varname">possibleleak</code> will also show possibly
    leaked blocks (those for which only an interior pointer was
    found).  The value
    <code class="varname">reachable</code> will show all block categories
    (reachable, possibly leaked, definitely leaked).
    </p>
<p>The third argument controls what kinds of changes are shown
    for a <code class="varname">full</code> leak search. The
    value <code class="varname">increased</code> specifies that only block
    allocation stacks with an increased number of leaked bytes or
    blocks since the previous leak check should be shown.  The
    value <code class="varname">changed</code> specifies that allocation stacks
    with any change since the previous leak check should be shown.
    The value <code class="varname">any</code> specifies that all leak entries
    should be shown, regardless of any increase or decrease.  When
    If <code class="varname">increased</code> or <code class="varname">changed</code> are
    specified, the leak report entries will show the delta relative to
    the previous leak report.
    </p>
<p>The following example shows usage of the 
    <code class="varname">leak_check monitor</code> command on
    the <code class="varname">memcheck/tests/leak-cases.c</code> regression
    test. The first command outputs one entry having an increase in
    the leaked bytes.  The second command is the same as the first
    command, but uses the abbreviated forms accepted by GDB and the
    Valgrind gdbserver. It only outputs the summary information, as
    there was no increase since the previous leak search.</p>
<pre class="programlisting">
(gdb) monitor leak_check full possibleleak increased
==14729== 16 (+16) bytes in 1 (+1) blocks are possibly lost in loss record 13 of 16
==14729==    at 0x4006E9E: malloc (vg_replace_malloc.c:236)
==14729==    by 0x80484D5: mk (leak-cases.c:52)
==14729==    by 0x804855F: f (leak-cases.c:81)
==14729==    by 0x80488F5: main (leak-cases.c:107)
==14729== 
==14729== LEAK SUMMARY:
==14729==    definitely lost: 32 (+0) bytes in 2 (+0) blocks
==14729==    indirectly lost: 16 (+0) bytes in 1 (+0) blocks
==14729==      possibly lost: 32 (+16) bytes in 2 (+1) blocks
==14729==    still reachable: 96 (+16) bytes in 6 (+1) blocks
==14729==         suppressed: 0 (+0) bytes in 0 (+0) blocks
==14729== Reachable blocks (those to which a pointer was found) are not shown.
==14729== To see them, add 'reachable any' args to leak_check
==14729== 
(gdb) mo l
==14729== LEAK SUMMARY:
==14729==    definitely lost: 32 (+0) bytes in 2 (+0) blocks
==14729==    indirectly lost: 16 (+0) bytes in 1 (+0) blocks
==14729==      possibly lost: 32 (+0) bytes in 2 (+0) blocks
==14729==    still reachable: 96 (+0) bytes in 6 (+0) blocks
==14729==         suppressed: 0 (+0) bytes in 0 (+0) blocks
==14729== Reachable blocks (those to which a pointer was found) are not shown.
==14729== To see them, add 'reachable any' args to leak_check
==14729== 
(gdb) 
</pre>
<p>Note that when using Valgrind's gdbserver, it is not
    necessary to rerun
    with <code class="option">--leak-check=full</code>
    <code class="option">--show-reachable=yes</code> to see the reachable
    blocks. You can obtain the same information without rerunning by
    using the GDB command <code class="computeroutput">monitor leak_check full
    reachable any</code> (or, using
    abbreviation: <code class="computeroutput">mo l f r a</code>).
    </p>
</li>
</ul></div>
</div>
<div class="sect1" title="4.7.Client Requests">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.clientreqs"></a>4.7.Client Requests</h2></div></div></div>
<p>The following client requests are defined in
<code class="filename">memcheck.h</code>.
See <code class="filename">memcheck.h</code> for exact details of their
arguments.</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="varname">VALGRIND_MAKE_MEM_NOACCESS</code>,
    <code class="varname">VALGRIND_MAKE_MEM_UNDEFINED</code> and
    <code class="varname">VALGRIND_MAKE_MEM_DEFINED</code>.
    These mark address ranges as completely inaccessible,
    accessible but containing undefined data, and accessible and
    containing defined data, respectively.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_MAKE_MEM_DEFINED_IF_ADDRESSABLE</code>.
    This is just like <code class="varname">VALGRIND_MAKE_MEM_DEFINED</code> but only
    affects those bytes that are already addressable.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_CHECK_MEM_IS_ADDRESSABLE</code> and
    <code class="varname">VALGRIND_CHECK_MEM_IS_DEFINED</code>: check immediately
    whether or not the given address range has the relevant property,
    and if not, print an error message.  Also, for the convenience of
    the client, returns zero if the relevant property holds; otherwise,
    the returned value is the address of the first byte for which the
    property is not true.  Always returns 0 when not run on
    Valgrind.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_CHECK_VALUE_IS_DEFINED</code>: a quick and easy
    way to find out whether Valgrind thinks a particular value
    (lvalue, to be precise) is addressable and defined.  Prints an error
    message if not.  It has no return value.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_DO_LEAK_CHECK</code>: does a full memory leak
    check (like <code class="option">--leak-check=full</code>) right now.
    This is useful for incrementally checking for leaks between arbitrary
    places in the program's execution.  It has no return value.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_DO_ADDED_LEAK_CHECK</code>: same as
   <code class="varname"> VALGRIND_DO_LEAK_CHECK</code> but only shows the
    entries for which there was an increase in leaked bytes or leaked
    number of blocks since the previous leak search.  It has no return
    value.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_DO_CHANGED_LEAK_CHECK</code>: same as
    <code class="varname">VALGRIND_DO_LEAK_CHECK</code> but only shows the
    entries for which there was an increase or decrease in leaked
    bytes or leaked number of blocks since the previous leak search. It
    has no return value.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_DO_QUICK_LEAK_CHECK</code>: like
    <code class="varname">VALGRIND_DO_LEAK_CHECK</code>, except it produces only a leak
    summary (like <code class="option">--leak-check=summary</code>).
    It has no return value.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_COUNT_LEAKS</code>: fills in the four
    arguments with the number of bytes of memory found by the previous
    leak check to be leaked (i.e. the sum of direct leaks and indirect leaks),
    dubious, reachable and suppressed.  This is useful in test harness code,
    after calling <code class="varname">VALGRIND_DO_LEAK_CHECK</code> or
    <code class="varname">VALGRIND_DO_QUICK_LEAK_CHECK</code>.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_COUNT_LEAK_BLOCKS</code>: identical to
    <code class="varname">VALGRIND_COUNT_LEAKS</code> except that it returns the
    number of blocks rather than the number of bytes in each
    category.</p></li>
<li class="listitem"><p><code class="varname">VALGRIND_GET_VBITS</code> and
    <code class="varname">VALGRIND_SET_VBITS</code>: allow you to get and set the
    V (validity) bits for an address range.  You should probably only
    set V bits that you have got with
    <code class="varname">VALGRIND_GET_VBITS</code>.  Only for those who really
    know what they are doing.</p></li>
<li class="listitem">
<p><code class="varname">VALGRIND_CREATE_BLOCK</code> and 
    <code class="varname">VALGRIND_DISCARD</code>.  <code class="varname">VALGRIND_CREATE_BLOCK</code>
    takes an address, a number of bytes and a character string.  The
    specified address range is then associated with that string.  When
    Memcheck reports an invalid access to an address in the range, it
    will describe it in terms of this block rather than in terms of
    any other block it knows about.  Note that the use of this macro
    does not actually change the state of memory in any way -- it
    merely gives a name for the range.
    </p>
<p>At some point you may want Memcheck to stop reporting errors
    in terms of the block named
    by <code class="varname">VALGRIND_CREATE_BLOCK</code>.  To make this
    possible, <code class="varname">VALGRIND_CREATE_BLOCK</code> returns a
    "block handle", which is a C <code class="varname">int</code> value.  You
    can pass this block handle to <code class="varname">VALGRIND_DISCARD</code>.
    After doing so, Valgrind will no longer relate addressing errors
    in the specified range to the block.  Passing invalid handles to
    <code class="varname">VALGRIND_DISCARD</code> is harmless.
   </p>
</li>
</ul></div>
</div>
<div class="sect1" title="4.8.Memory Pools: describing and working with custom allocators">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.mempools"></a>4.8.Memory Pools: describing and working with custom allocators</h2></div></div></div>
<p>Some programs use custom memory allocators, often for performance
reasons.  Left to itself, Memcheck is unable to understand the
behaviour of custom allocation schemes as well as it understands the
standard allocators, and so may miss errors and leaks in your program.  What
this section describes is a way to give Memcheck enough of a description of
your custom allocator that it can make at least some sense of what is
happening.</p>
<p>There are many different sorts of custom allocator, so Memcheck
attempts to reason about them using a loose, abstract model.  We
use the following terminology when describing custom allocation
systems:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>Custom allocation involves a set of independent "memory pools".
    </p></li>
<li class="listitem"><p>Memcheck's notion of a a memory pool consists of a single "anchor
    address" and a set of non-overlapping "chunks" associated with the
    anchor address.</p></li>
<li class="listitem"><p>Typically a pool's anchor address is the address of a 
    book-keeping "header" structure.</p></li>
<li class="listitem"><p>Typically the pool's chunks are drawn from a contiguous
    "superblock" acquired through the system
    <code class="function">malloc</code> or
    <code class="function">mmap</code>.</p></li>
</ul></div>
<p>Keep in mind that the last two points above say "typically": the
Valgrind mempool client request API is intentionally vague about the
exact structure of a mempool. There is no specific mention made of
headers or superblocks. Nevertheless, the following picture may help
elucidate the intention of the terms in the API:</p>
<pre class="programlisting">
   "pool"
   (anchor address)
   |
   v
   +--------+---+
   | header | o |
   +--------+-|-+
              |
              v                  superblock
              +------+---+--------------+---+------------------+
              |      |rzB|  allocation  |rzB|                  |
              +------+---+--------------+---+------------------+
                         ^              ^
                         |              |
                       "addr"     "addr"+"size"
</pre>
<p>
Note that the header and the superblock may be contiguous or
discontiguous, and there may be multiple superblocks associated with a
single header; such variations are opaque to Memcheck. The API
only requires that your allocation scheme can present sensible values
of "pool", "addr" and "size".</p>
<p>
Typically, before making client requests related to mempools, a client
program will have allocated such a header and superblock for their
mempool, and marked the superblock NOACCESS using the
<code class="varname">VALGRIND_MAKE_MEM_NOACCESS</code> client request.</p>
<p>
When dealing with mempools, the goal is to maintain a particular
invariant condition: that Memcheck believes the unallocated portions
of the pool's superblock (including redzones) are NOACCESS. To
maintain this invariant, the client program must ensure that the
superblock starts out in that state; Memcheck cannot make it so, since
Memcheck never explicitly learns about the superblock of a pool, only
the allocated chunks within the pool.</p>
<p>
Once the header and superblock for a pool are established and properly
marked, there are a number of client requests programs can use to
inform Memcheck about changes to the state of a mempool:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p>
    <code class="varname">VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed)</code>:
    This request registers the address <code class="varname">pool</code> as the anchor
    address for a memory pool. It also provides a size
    <code class="varname">rzB</code>, specifying how large the redzones placed around
    chunks allocated from the pool should be. Finally, it provides an
    <code class="varname">is_zeroed</code> argument that specifies whether the pool's
    chunks are zeroed (more precisely: defined) when allocated.
    </p>
<p>
    Upon completion of this request, no chunks are associated with the
    pool.  The request simply tells Memcheck that the pool exists, so that
    subsequent calls can refer to it as a pool.
    </p>
</li>
<li class="listitem"><p><code class="varname">VALGRIND_DESTROY_MEMPOOL(pool)</code>:
    This request tells Memcheck that a pool is being torn down. Memcheck
    then removes all records of chunks associated with the pool, as well
    as its record of the pool's existence. While destroying its records of
    a mempool, Memcheck resets the redzones of any live chunks in the pool
    to NOACCESS.
    </p></li>
<li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_ALLOC(pool, addr, size)</code>:
    This request informs Memcheck that a <code class="varname">size</code>-byte chunk
    has been allocated at <code class="varname">addr</code>, and associates the chunk with the
    specified
    <code class="varname">pool</code>. If the pool was created with nonzero
    <code class="varname">rzB</code> redzones, Memcheck will mark the
    <code class="varname">rzB</code> bytes before and after the chunk as NOACCESS. If
    the pool was created with the <code class="varname">is_zeroed</code> argument set,
    Memcheck will mark the chunk as DEFINED, otherwise Memcheck will mark
    the chunk as UNDEFINED.
    </p></li>
<li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_FREE(pool, addr)</code>:
    This request informs Memcheck that the chunk at <code class="varname">addr</code>
    should no longer be considered allocated. Memcheck will mark the chunk
    associated with <code class="varname">addr</code> as NOACCESS, and delete its
    record of the chunk's existence.
    </p></li>
<li class="listitem">
<p><code class="varname">VALGRIND_MEMPOOL_TRIM(pool, addr, size)</code>:
    This request trims the chunks associated with <code class="varname">pool</code>.
    The request only operates on chunks associated with
    <code class="varname">pool</code>. Trimming is formally defined as:</p>
<div class="itemizedlist"><ul class="itemizedlist" type="circle">
<li class="listitem"><p> All chunks entirely inside the range
        <code class="varname">addr..(addr+size-1)</code> are preserved.</p></li>
<li class="listitem"><p>All chunks entirely outside the range
        <code class="varname">addr..(addr+size-1)</code> are discarded, as though
        <code class="varname">VALGRIND_MEMPOOL_FREE</code> was called on them. </p></li>
<li class="listitem"><p>All other chunks must intersect with the range 
        <code class="varname">addr..(addr+size-1)</code>; areas outside the
        intersection are marked as NOACCESS, as though they had been
        independently freed with
        <code class="varname">VALGRIND_MEMPOOL_FREE</code>.</p></li>
</ul></div>
<p>This is a somewhat rare request, but can be useful in 
    implementing the type of mass-free operations common in custom 
    LIFO allocators.</p>
</li>
<li class="listitem">
<p><code class="varname">VALGRIND_MOVE_MEMPOOL(poolA, poolB)</code>: This
    request informs Memcheck that the pool previously anchored at
    address <code class="varname">poolA</code> has moved to anchor address
    <code class="varname">poolB</code>.  This is a rare request, typically only needed
    if you <code class="function">realloc</code> the header of a mempool.</p>
<p>No memory-status bits are altered by this request.</p>
</li>
<li class="listitem">
<p>
    <code class="varname">VALGRIND_MEMPOOL_CHANGE(pool, addrA, addrB,
    size)</code>: This request informs Memcheck that the chunk
    previously allocated at address <code class="varname">addrA</code> within
    <code class="varname">pool</code> has been moved and/or resized, and should be
    changed to cover the region <code class="varname">addrB..(addrB+size-1)</code>. This
    is a rare request, typically only needed if you
    <code class="function">realloc</code> a superblock or wish to extend a chunk
    without changing its memory-status bits.
    </p>
<p>No memory-status bits are altered by this request.
    </p>
</li>
<li class="listitem"><p><code class="varname">VALGRIND_MEMPOOL_EXISTS(pool)</code>:
    This request informs the caller whether or not Memcheck is currently 
    tracking a mempool at anchor address <code class="varname">pool</code>. It
    evaluates to 1 when there is a mempool associated with that address, 0
    otherwise. This is a rare request, only useful in circumstances when
    client code might have lost track of the set of active mempools.
    </p></li>
</ul></div>
</div>
<div class="sect1" title="4.9.Debugging MPI Parallel Programs with Valgrind">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="mc-manual.mpiwrap"></a>4.9.Debugging MPI Parallel Programs with Valgrind</h2></div></div></div>
<p>Memcheck supports debugging of distributed-memory applications
which use the MPI message passing standard.  This support consists of a
library of wrapper functions for the
<code class="computeroutput">PMPI_*</code> interface.  When incorporated
into the application's address space, either by direct linking or by
<code class="computeroutput">LD_PRELOAD</code>, the wrappers intercept
calls to <code class="computeroutput">PMPI_Send</code>,
<code class="computeroutput">PMPI_Recv</code>, etc.  They then
use client requests to inform Memcheck of memory state changes caused
by the function being wrapped.  This reduces the number of false
positives that Memcheck otherwise typically reports for MPI
applications.</p>
<p>The wrappers also take the opportunity to carefully check
size and definedness of buffers passed as arguments to MPI functions, hence
detecting errors such as passing undefined data to
<code class="computeroutput">PMPI_Send</code>, or receiving data into a
buffer which is too small.</p>
<p>Unlike most of the rest of Valgrind, the wrapper library is subject to a
BSD-style license, so you can link it into any code base you like.
See the top of <code class="computeroutput">mpi/libmpiwrap.c</code>
for license details.</p>
<div class="sect2" title="4.9.1.Building and installing the wrappers">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.build"></a>4.9.1.Building and installing the wrappers</h3></div></div></div>
<p> The wrapper library will be built automatically if possible.
Valgrind's configure script will look for a suitable
<code class="computeroutput">mpicc</code> to build it with.  This must be
the same <code class="computeroutput">mpicc</code> you use to build the
MPI application you want to debug.  By default, Valgrind tries
<code class="computeroutput">mpicc</code>, but you can specify a
different one by using the configure-time option
<code class="option">--with-mpicc</code>.  Currently the
wrappers are only buildable with
<code class="computeroutput">mpicc</code>s which are based on GNU
GCC or Intel's C++ Compiler.</p>
<p>Check that the configure script prints a line like this:</p>
<pre class="programlisting">
checking for usable MPI2-compliant mpicc and mpi.h... yes, mpicc
</pre>
<p>If it says <code class="computeroutput">... no</code>, your
<code class="computeroutput">mpicc</code> has failed to compile and link
a test MPI2 program.</p>
<p>If the configure test succeeds, continue in the usual way with
<code class="computeroutput">make</code> and <code class="computeroutput">make
install</code>.  The final install tree should then contain
<code class="computeroutput">libmpiwrap-&lt;platform&gt;.so</code>.
</p>
<p>Compile up a test MPI program (eg, MPI hello-world) and try
this:</p>
<pre class="programlisting">
LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-&lt;platform&gt;.so   \
           mpirun [args] $prefix/bin/valgrind ./hello
</pre>
<p>You should see something similar to the following</p>
<pre class="programlisting">
valgrind MPI wrappers 31901: Active for pid 31901
valgrind MPI wrappers 31901: Try MPIWRAP_DEBUG=help for possible options
</pre>
<p>repeated for every process in the group.  If you do not see
these, there is an build/installation problem of some kind.</p>
<p> The MPI functions to be wrapped are assumed to be in an ELF
shared object with soname matching
<code class="computeroutput">libmpi.so*</code>.  This is known to be
correct at least for Open MPI and Quadrics MPI, and can easily be
changed if required.</p>
</div>
<div class="sect2" title="4.9.2.Getting started">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.gettingstarted"></a>4.9.2.Getting started</h3></div></div></div>
<p>Compile your MPI application as usual, taking care to link it
using the same <code class="computeroutput">mpicc</code> that your
Valgrind build was configured with.</p>
<p>
Use the following basic scheme to run your application on Valgrind with
the wrappers engaged:</p>
<pre class="programlisting">
MPIWRAP_DEBUG=[wrapper-args]                                  \
   LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-&lt;platform&gt;.so   \
   mpirun [mpirun-args]                                       \
   $prefix/bin/valgrind [valgrind-args]                       \
   [application] [app-args]
</pre>
<p>As an alternative to
<code class="computeroutput">LD_PRELOAD</code>ing
<code class="computeroutput">libmpiwrap-&lt;platform&gt;.so</code>, you can
simply link it to your application if desired.  This should not disturb
native behaviour of your application in any way.</p>
</div>
<div class="sect2" title="4.9.3.Controlling the wrapper library">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.controlling"></a>4.9.3.Controlling the wrapper library</h3></div></div></div>
<p>Environment variable
<code class="computeroutput">MPIWRAP_DEBUG</code> is consulted at
startup.  The default behaviour is to print a starting banner</p>
<pre class="programlisting">
valgrind MPI wrappers 16386: Active for pid 16386
valgrind MPI wrappers 16386: Try MPIWRAP_DEBUG=help for possible options
</pre>
<p> and then be relatively quiet.</p>
<p>You can give a list of comma-separated options in
<code class="computeroutput">MPIWRAP_DEBUG</code>.  These are</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="computeroutput">verbose</code>:
    show entries/exits of all wrappers.  Also show extra
    debugging info, such as the status of outstanding 
    <code class="computeroutput">MPI_Request</code>s resulting
    from uncompleted <code class="computeroutput">MPI_Irecv</code>s.</p></li>
<li class="listitem"><p><code class="computeroutput">quiet</code>: 
    opposite of <code class="computeroutput">verbose</code>, only print 
    anything when the wrappers want
    to report a detected programming error, or in case of catastrophic
    failure of the wrappers.</p></li>
<li class="listitem"><p><code class="computeroutput">warn</code>: 
    by default, functions which lack proper wrappers
    are not commented on, just silently
    ignored.  This causes a warning to be printed for each unwrapped
    function used, up to a maximum of three warnings per function.</p></li>
<li class="listitem"><p><code class="computeroutput">strict</code>: 
    print an error message and abort the program if 
    a function lacking a wrapper is used.</p></li>
</ul></div>
<p> If you want to use Valgrind's XML output facility
(<code class="option">--xml=yes</code>), you should pass
<code class="computeroutput">quiet</code> in
<code class="computeroutput">MPIWRAP_DEBUG</code> so as to get rid of any
extraneous printing from the wrappers.</p>
</div>
<div class="sect2" title="4.9.4.Functions">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.limitations.functions"></a>4.9.4.Functions</h3></div></div></div>
<p>All MPI2 functions except
<code class="computeroutput">MPI_Wtick</code>,
<code class="computeroutput">MPI_Wtime</code> and
<code class="computeroutput">MPI_Pcontrol</code> have wrappers.  The
first two are not wrapped because they return a 
<code class="computeroutput">double</code>, which Valgrind's
function-wrap mechanism cannot handle (but it could easily be
extended to do so).  <code class="computeroutput">MPI_Pcontrol</code> cannot be
wrapped as it has variable arity: 
<code class="computeroutput">int MPI_Pcontrol(const int level, ...)</code></p>
<p>Most functions are wrapped with a default wrapper which does
nothing except complain or abort if it is called, depending on
settings in <code class="computeroutput">MPIWRAP_DEBUG</code> listed
above.  The following functions have "real", do-something-useful
wrappers:</p>
<pre class="programlisting">
PMPI_Send PMPI_Bsend PMPI_Ssend PMPI_Rsend

PMPI_Recv PMPI_Get_count

PMPI_Isend PMPI_Ibsend PMPI_Issend PMPI_Irsend

PMPI_Irecv
PMPI_Wait PMPI_Waitall
PMPI_Test PMPI_Testall

PMPI_Iprobe PMPI_Probe

PMPI_Cancel

PMPI_Sendrecv

PMPI_Type_commit PMPI_Type_free

PMPI_Pack PMPI_Unpack

PMPI_Bcast PMPI_Gather PMPI_Scatter PMPI_Alltoall
PMPI_Reduce PMPI_Allreduce PMPI_Op_create

PMPI_Comm_create PMPI_Comm_dup PMPI_Comm_free PMPI_Comm_rank PMPI_Comm_size

PMPI_Error_string
PMPI_Init PMPI_Initialized PMPI_Finalize
</pre>
<p> A few functions such as
<code class="computeroutput">PMPI_Address</code> are listed as
<code class="computeroutput">HAS_NO_WRAPPER</code>.  They have no wrapper
at all as there is nothing worth checking, and giving a no-op wrapper
would reduce performance for no reason.</p>
<p> Note that the wrapper library itself can itself generate large
numbers of calls to the MPI implementation, especially when walking
complex types.  The most common functions called are
<code class="computeroutput">PMPI_Extent</code>,
<code class="computeroutput">PMPI_Type_get_envelope</code>,
<code class="computeroutput">PMPI_Type_get_contents</code>, and
<code class="computeroutput">PMPI_Type_free</code>.  </p>
</div>
<div class="sect2" title="4.9.5.Types">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.limitations.types"></a>4.9.5.Types</h3></div></div></div>
<p> MPI-1.1 structured types are supported, and walked exactly.
The currently supported combiners are
<code class="computeroutput">MPI_COMBINER_NAMED</code>,
<code class="computeroutput">MPI_COMBINER_CONTIGUOUS</code>,
<code class="computeroutput">MPI_COMBINER_VECTOR</code>,
<code class="computeroutput">MPI_COMBINER_HVECTOR</code>
<code class="computeroutput">MPI_COMBINER_INDEXED</code>,
<code class="computeroutput">MPI_COMBINER_HINDEXED</code> and
<code class="computeroutput">MPI_COMBINER_STRUCT</code>.  This should
cover all MPI-1.1 types.  The mechanism (function
<code class="computeroutput">walk_type</code>) should extend easily to
cover MPI2 combiners.</p>
<p>MPI defines some named structured types
(<code class="computeroutput">MPI_FLOAT_INT</code>,
<code class="computeroutput">MPI_DOUBLE_INT</code>,
<code class="computeroutput">MPI_LONG_INT</code>,
<code class="computeroutput">MPI_2INT</code>,
<code class="computeroutput">MPI_SHORT_INT</code>,
<code class="computeroutput">MPI_LONG_DOUBLE_INT</code>) which are pairs
of some basic type and a C <code class="computeroutput">int</code>.
Unfortunately the MPI specification makes it impossible to look inside
these types and see where the fields are.  Therefore these wrappers
assume the types are laid out as <code class="computeroutput">struct { float val;
int loc; }</code> (for
<code class="computeroutput">MPI_FLOAT_INT</code>), etc, and act
accordingly.  This appears to be correct at least for Open MPI 1.0.2
and for Quadrics MPI.</p>
<p>If <code class="computeroutput">strict</code> is an option specified 
in <code class="computeroutput">MPIWRAP_DEBUG</code>, the application
will abort if an unhandled type is encountered.  Otherwise, the 
application will print a warning message and continue.</p>
<p>Some effort is made to mark/check memory ranges corresponding to
arrays of values in a single pass.  This is important for performance
since asking Valgrind to mark/check any range, no matter how small,
carries quite a large constant cost.  This optimisation is applied to
arrays of primitive types (<code class="computeroutput">double</code>,
<code class="computeroutput">float</code>,
<code class="computeroutput">int</code>,
<code class="computeroutput">long</code>, <code class="computeroutput">long
long</code>, <code class="computeroutput">short</code>,
<code class="computeroutput">char</code>, and <code class="computeroutput">long
double</code> on platforms where <code class="computeroutput">sizeof(long
double) == 8</code>).  For arrays of all other types, the
wrappers handle each element individually and so there can be a very
large performance cost.</p>
</div>
<div class="sect2" title="4.9.6.Writing new wrappers">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.writingwrappers"></a>4.9.6.Writing new wrappers</h3></div></div></div>
<p>
For the most part the wrappers are straightforward.  The only
significant complexity arises with nonblocking receives.</p>
<p>The issue is that <code class="computeroutput">MPI_Irecv</code>
states the recv buffer and returns immediately, giving a handle
(<code class="computeroutput">MPI_Request</code>) for the transaction.
Later the user will have to poll for completion with
<code class="computeroutput">MPI_Wait</code> etc, and when the
transaction completes successfully, the wrappers have to paint the
recv buffer.  But the recv buffer details are not presented to
<code class="computeroutput">MPI_Wait</code> -- only the handle is.  The
library therefore maintains a shadow table which associates
uncompleted <code class="computeroutput">MPI_Request</code>s with the
corresponding buffer address/count/type.  When an operation completes,
the table is searched for the associated address/count/type info, and
memory is marked accordingly.</p>
<p>Access to the table is guarded by a (POSIX pthreads) lock, so as
to make the library thread-safe.</p>
<p>The table is allocated with
<code class="computeroutput">malloc</code> and never
<code class="computeroutput">free</code>d, so it will show up in leak
checks.</p>
<p>Writing new wrappers should be fairly easy.  The source file is
<code class="computeroutput">mpi/libmpiwrap.c</code>.  If possible,
find an existing wrapper for a function of similar behaviour to the
one you want to wrap, and use it as a starting point.  The wrappers
are organised in sections in the same order as the MPI 1.1 spec, to
aid navigation.  When adding a wrapper, remember to comment out the
definition of the default wrapper in the long list of defaults at the
bottom of the file (do not remove it, just comment it out).</p>
</div>
<div class="sect2" title="4.9.7.What to expect when using the wrappers">
<div class="titlepage"><div><div><h3 class="title">
<a name="mc-manual.mpiwrap.whattoexpect"></a>4.9.7.What to expect when using the wrappers</h3></div></div></div>
<p>The wrappers should reduce Memcheck's false-error rate on MPI
applications.  Because the wrapping is done at the MPI interface,
there will still potentially be a large number of errors reported in
the MPI implementation below the interface.  The best you can do is
try to suppress them.</p>
<p>You may also find that the input-side (buffer
length/definedness) checks find errors in your MPI use, for example
passing too short a buffer to
<code class="computeroutput">MPI_Recv</code>.</p>
<p>Functions which are not wrapped may increase the false
error rate.  A possible approach is to run with
<code class="computeroutput">MPI_DEBUG</code> containing
<code class="computeroutput">warn</code>.  This will show you functions
which lack proper wrappers but which are nevertheless used.  You can
then write wrappers for them.
</p>
<p>A known source of potential false errors are the
<code class="computeroutput">PMPI_Reduce</code> family of functions, when
using a custom (user-defined) reduction function.  In a reduction
operation, each node notionally sends data to a "central point" which
uses the specified reduction function to merge the data items into a
single item.  Hence, in general, data is passed between nodes and fed
to the reduction function, but the wrapper library cannot mark the
transferred data as initialised before it is handed to the reduction
function, because all that happens "inside" the
<code class="computeroutput">PMPI_Reduce</code> call.  As a result you
may see false positives reported in your reduction function.</p>
</div>
</div>
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