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<chapter id="mc-manual" xreflabel="Memcheck: a memory error detector">
<title>Memcheck: a memory error detector</title>
<para>To use this tool, you may specify <option>--tool=memcheck</option>
on the Valgrind command line. You don't have to, though, since Memcheck
is the default tool.</para>
<sect1 id="mc-manual.overview" xreflabel="Overview">
<title>Overview</title>
<para>Memcheck is a memory error detector. It can detect the following
problems that are common in C and C++ programs.</para>
<itemizedlist>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>Using undefined values, i.e. values that have not been initialised,
or that have been derived from other undefined values.</para>
</listitem>
<listitem>
<para>Incorrect freeing of heap memory, such as double-freeing heap
blocks, or mismatched use of
<function>malloc</function>/<computeroutput>new</computeroutput>/<computeroutput>new[]</computeroutput>
versus
<function>free</function>/<computeroutput>delete</computeroutput>/<computeroutput>delete[]</computeroutput></para>
</listitem>
<listitem>
<para>Overlapping <computeroutput>src</computeroutput> and
<computeroutput>dst</computeroutput> pointers in
<computeroutput>memcpy</computeroutput> and related
functions.</para>
</listitem>
<listitem>
<para>Memory leaks.</para>
</listitem>
</itemizedlist>
<para>Problems like these can be difficult to find by other means,
often remaining undetected for long periods, then causing occasional,
difficult-to-diagnose crashes.</para>
</sect1>
<sect1 id="mc-manual.errormsgs"
xreflabel="Explanation of error messages from Memcheck">
<title>Explanation of error messages from Memcheck</title>
<para>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 <xref
linkend="mc-manual.machine"/>.</para>
<sect2 id="mc-manual.badrw"
xreflabel="Illegal read / Illegal write errors">
<title>Illegal read / Illegal write errors</title>
<para>For example:</para>
<programlisting><![CDATA[
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
]]></programlisting>
<para>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 <filename>qpngio.cpp</filename>,
and so on.</para>
<para>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 <option><xref
linkend="opt.read-var-info"/></option> option Memcheck will run more slowly
but may give a more detailed description of any illegal address.</para>
<para>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.</para>
<para>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.</para>
</sect2>
<sect2 id="mc-manual.uninitvals"
xreflabel="Use of uninitialised values">
<title>Use of uninitialised values</title>
<para>For example:</para>
<programlisting><![CDATA[
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)
]]></programlisting>
<para>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
<function>printf</function> machinery of the C library. This error was
reported when running the following small program:</para>
<programlisting><![CDATA[
int main()
{
int x;
printf ("x = %d\n", x);
}]]></programlisting>
<para>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, <varname>x</varname> is uninitialised. Memcheck observes
the value being passed to <function>_IO_printf</function> and thence to
<function>_IO_vfprintf</function>, but makes no comment. However,
<function>_IO_vfprintf</function> has to examine the value of
<varname>x</varname> so it can turn it into the corresponding ASCII string,
and it is at this point that Memcheck complains.</para>
<para>Sources of uninitialised data tend to be:</para>
<itemizedlist>
<listitem>
<para>Local variables in procedures which have not been initialised,
as in the example above.</para>
</listitem>
<listitem>
<para>The contents of heap blocks (allocated with
<function>malloc</function>, <function>new</function>, or a similar
function) before you (or a constructor) write something there.
</para>
</listitem>
</itemizedlist>
<para>To see information on the sources of uninitialised data in your
program, use the <option>--track-origins=yes</option> option. This
makes Memcheck run more slowly, but can make it much easier to track down
the root causes of uninitialised value errors.</para>
</sect2>
<sect2 id="mc-manual.bad-syscall-args"
xreflabel="Use of uninitialised or unaddressable values in system
calls">
<title>Use of uninitialised or unaddressable values in system
calls</title>
<para>Memcheck checks all parameters to system calls:
<itemizedlist>
<listitem>
<para>It checks all the direct parameters themselves, whether they are
initialised.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>Also, if the system call needs to write to a user-supplied
buffer, Memcheck checks that the buffer is addressable.</para>
</listitem>
</itemizedlist>
</para>
<para>After the system call, Memcheck updates its tracked information to
precisely reflect any changes in memory state caused by the system
call.</para>
<para>Here's an example of two system calls with invalid parameters:</para>
<programlisting><![CDATA[
#include <stdlib.h>
#include <unistd.h>
int main( void )
{
char* arr = malloc(10);
int* arr2 = malloc(sizeof(int));
write( 1 /* stdout */, arr, 10 );
exit(arr2[0]);
}
]]></programlisting>
<para>You get these complaints ...</para>
<programlisting><![CDATA[
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)
]]></programlisting>
<para>... because the program has (a) written uninitialised junk
from the heap block to the standard output, and (b) passed an
uninitialised value to <function>exit</function>. Note that the first
error refers to the memory pointed to by
<computeroutput>buf</computeroutput> (not
<computeroutput>buf</computeroutput> itself), but the second error
refers directly to <computeroutput>exit</computeroutput>'s argument
<computeroutput>arr2[0]</computeroutput>.</para>
</sect2>
<sect2 id="mc-manual.badfrees" xreflabel="Illegal frees">
<title>Illegal frees</title>
<para>For example:</para>
<programlisting><![CDATA[
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)
]]></programlisting>
<para>Memcheck keeps track of the blocks allocated by your program
with <function>malloc</function>/<computeroutput>new</computeroutput>,
so it can know exactly whether or not the argument to
<function>free</function>/<computeroutput>delete</computeroutput> 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.</para>
</sect2>
<sect2 id="mc-manual.rudefn"
xreflabel="When a heap block is freed with an inappropriate deallocation
function">
<title>When a heap block is freed with an inappropriate deallocation
function</title>
<para>In the following example, a block allocated with
<function>new[]</function> has wrongly been deallocated with
<function>free</function>:</para>
<programlisting><![CDATA[
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 &) (olefilter.cc:272)
]]></programlisting>
<para>In <literal>C++</literal> it's important to deallocate memory in a
way compatible with how it was allocated. The deal is:</para>
<itemizedlist>
<listitem>
<para>If allocated with
<function>malloc</function>,
<function>calloc</function>,
<function>realloc</function>,
<function>valloc</function> or
<function>memalign</function>, you must
deallocate with <function>free</function>.</para>
</listitem>
<listitem>
<para>If allocated with <function>new</function>, you must deallocate
with <function>delete</function>.</para>
</listitem>
<listitem>
<para>If allocated with <function>new[]</function>, you must
deallocate with <function>delete[]</function>.</para>
</listitem>
</itemizedlist>
<para>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".</para>
<para>The reason behind the requirement is as follows. In some C++
implementations, <function>delete[]</function> must be used for
objects allocated by <function>new[]</function> 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. <function>delete</function> doesn't account for this and will get
confused, possibly corrupting the heap.</para>
</sect2>
<sect2 id="mc-manual.overlap"
xreflabel="Overlapping source and destination blocks">
<title>Overlapping source and destination blocks</title>
<para>The following C library functions copy some data from one
memory block to another (or something similar):
<function>memcpy</function>,
<function>strcpy</function>,
<function>strncpy</function>,
<function>strcat</function>,
<function>strncat</function>.
The blocks pointed to by their <computeroutput>src</computeroutput> and
<computeroutput>dst</computeroutput> 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.
</para>
<para>For example:</para>
<programlisting><![CDATA[
==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)
]]></programlisting>
<para>You don't want the two blocks to overlap because one of them could
get partially overwritten by the copying.</para>
<para>You might think that Memcheck is being overly pedantic reporting
this in the case where <computeroutput>dst</computeroutput> is less than
<computeroutput>src</computeroutput>. For example, the obvious way to
implement <function>memcpy</function> 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 <function>memcpy</function> zero
<computeroutput>dst</computeroutput> before copying, because zeroing the
destination's cache line(s) can improve performance.</para>
<para>The moral of the story is: if you want to write truly portable
code, don't make any assumptions about the language
implementation.</para>
</sect2>
<sect2 id="mc-manual.leaks" xreflabel="Memory leak detection">
<title>Memory leak detection</title>
<para>Memcheck keeps track of all heap blocks issued in response to
calls to
<function>malloc</function>/<function>new</function> et al.
So when the program exits, it knows which blocks have not been freed.
</para>
<para>If <option>--leak-check</option> 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.</para>
<para>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:</para>
<itemizedlist>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>It might be a random junk value in memory, entirely unrelated, just
a coincidence.</para>
</listitem>
<listitem>
<para>It might be a pointer to an array of C++ objects (which possess
destructors) allocated with <computeroutput>new[]</computeroutput>. 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 <ulink url="http://theory.uwinnipeg.ca/gnu/gcc/gxxint_14.html">this
page</ulink> for more information.</para>
</listitem>
</itemizedlist>
<para>With that in mind, consider the nine possible cases described by the
following figure.</para>
<programlisting><![CDATA[
Pointer chain AAA Category BBB Category
------------- ------------ ------------
(1) RRR ------------> BBB DR
(2) RRR ---> AAA ---> BBB DR IR
(3) RRR BBB DL
(4) RRR AAA ---> BBB DL IL
(5) RRR ------?-----> BBB (y)DR, (n)DL
(6) RRR ---> AAA -?-> BBB DR (y)IR, (n)DL
(7) RRR -?-> AAA ---> BBB (y)DR, (n)DL (y)IR, (n)IL
(8) RRR -?-> AAA -?-> BBB (y)DR, (n)DL (y,y)IR, (n,y)IL, (_,n)DL
(9) RRR AAA -?-> BBB DL (y)IL, (n)DL
Pointer chain legend:
- RRR: a root set node or DR block
- AAA, BBB: heap blocks
- --->: a start-pointer
- -?->: 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
]]></programlisting>
<para>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.</para>
<itemizedlist>
<listitem>
<para>"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 <option>--show-reachable=yes</option> is
specified.</para>
</listitem>
<listitem>
<para>"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.</para>
</listitem>
<listitem>
<para>"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 <option>--show-reachable=yes</option> is specified.</para>
</listitem>
<listitem>
<para>"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.</para>
</listitem>
</itemizedlist>
<para>(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.)</para>
<para>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.</para>
<para>The following is an example leak summary.</para>
<programlisting><![CDATA[
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.
]]></programlisting>
<para>If <option>--leak-check=full</option> 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
<option>--leak-resolution</option> 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.</para>
<para>For example:</para>
<programlisting><![CDATA[
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)
]]></programlisting>
<para>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.</para>
<para>If you specify <option>--show-reachable=yes</option>,
reachable and indirectly lost blocks will also be shown, as the following
two examples show.</para>
<programlisting><![CDATA[
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)
]]></programlisting>
<para>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 <computeroutput>ERROR SUMMARY</computeroutput> line, and also the effect
of the <option>--error-exitcode</option> option. Memcheck uses the following
criteria:</para>
<itemizedlist>
<listitem>
<para>First, a leak is only counted as a true "error" if
<option>--leak-check=full</option> 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.</para>
</listitem>
<listitem>
<para>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 <option>--show-reachable=yes</option> is
specified and they are printed; this is because such blocks don't need
direct fixing by the programmer.
</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id="mc-manual.options"
xreflabel="Memcheck Command-Line Options">
<title>Memcheck Command-Line Options</title>
<!-- start of xi:include in the manpage -->
<variablelist id="mc.opts.list">
<varlistentry id="opt.leak-check" xreflabel="--leak-check">
<term>
<option><![CDATA[--leak-check=<no|summary|yes|full> [default: summary] ]]></option>
</term>
<listitem>
<para>When enabled, search for memory leaks when the client
program finishes. If set to <varname>summary</varname>, it says how
many leaks occurred. If set to <varname>full</varname> or
<varname>yes</varname>, it also gives details of each individual
leak.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.show-possibly-lost" xreflabel="--show-possibly-lost">
<term>
<option><![CDATA[--show-possibly-lost=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>When disabled, the memory leak detector will not show "possibly lost" blocks.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.leak-resolution" xreflabel="--leak-resolution">
<term>
<option><![CDATA[--leak-resolution=<low|med|high> [default: high] ]]></option>
</term>
<listitem>
<para>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 <varname>low</varname>, only the first
two entries need match. When <varname>med</varname>, four entries
have to match. When <varname>high</varname>, all entries need to
match.</para>
<para>For hardcore leak debugging, you probably want to use
<option>--leak-resolution=high</option> together with
<option>--num-callers=40</option> or some such large number.
</para>
<para>Note that the <option>--leak-resolution</option> setting
does not affect Memcheck's ability to find
leaks. It only changes how the results are presented.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.show-reachable" xreflabel="--show-reachable">
<term>
<option><![CDATA[--show-reachable=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>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
<option>--show-all</option> would be a better name for
it.)</para>
</listitem>
</varlistentry>
<varlistentry id="opt.undef-value-errors" xreflabel="--undef-value-errors">
<term>
<option><![CDATA[--undef-value-errors=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>Controls whether Memcheck reports
uses of undefined value errors. Set this to
<varname>no</varname> if you don't want to see undefined value
errors. It also has the side effect of speeding up
Memcheck somewhat.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.track-origins" xreflabel="--track-origins">
<term>
<option><![CDATA[--track-origins=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>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.
</para>
<para>When set
to <varname>yes</varname>, 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 <varname>brk</varname>).
</para>
<para>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.
</para>
<para>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.
</para>
<para>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.
</para>
<para>Note that the combination
<option>--track-origins=yes</option>
and <option>--undef-value-errors=no</option> is
nonsensical. Memcheck checks for and
rejects this combination at startup.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.partial-loads-ok" xreflabel="--partial-loads-ok">
<term>
<option><![CDATA[--partial-loads-ok=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>Controls how Memcheck handles word-sized,
word-aligned loads from addresses for which some bytes are
addressable and others are not. When <varname>yes</varname>, 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.</para>
<para>When <varname>no</varname>, 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.</para>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.freelist-vol" xreflabel="--freelist-vol">
<term>
<option><![CDATA[--freelist-vol=<number> [default: 20000000] ]]></option>
</term>
<listitem>
<para>When the client program releases memory using
<function>free</function> (in <literal>C</literal>) or
<computeroutput>delete</computeroutput>
(<literal>C++</literal>), 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.</para>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.freelist-big-blocks" xreflabel="--freelist-big-blocks">
<term>
<option><![CDATA[--freelist-big-blocks=<number> [default: 1000000] ]]></option>
</term>
<listitem>
<para>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 <option>--freelist-big-blocks</option>.
This ensures that freeing big blocks (in particular freeing blocks bigger than
<option>--freelist-vol</option>) 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.</para>
<para>Setting a value of 0 means that all the blocks are re-circulated
in a FIFO order. </para>
</listitem>
</varlistentry>
<varlistentry id="opt.workaround-gcc296-bugs" xreflabel="--workaround-gcc296-bugs">
<term>
<option><![CDATA[--workaround-gcc296-bugs=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>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.</para>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.ignore-ranges" xreflabel="--ignore-ranges">
<term>
<option><![CDATA[--ignore-ranges=0xPP-0xQQ[,0xRR-0xSS] ]]></option>
</term>
<listitem>
<para>Any ranges listed in this option (and multiple ranges can be
specified, separated by commas) will be ignored by Memcheck's
addressability checking.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.malloc-fill" xreflabel="--malloc-fill">
<term>
<option><![CDATA[--malloc-fill=<hexnumber> ]]></option>
</term>
<listitem>
<para>Fills blocks allocated
by <computeroutput>malloc</computeroutput>,
<computeroutput>new</computeroutput>, etc, but not
by <computeroutput>calloc</computeroutput>, 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.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.free-fill" xreflabel="--free-fill">
<term>
<option><![CDATA[--free-fill=<hexnumber> ]]></option>
</term>
<listitem>
<para>Fills blocks freed
by <computeroutput>free</computeroutput>,
<computeroutput>delete</computeroutput>, 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.
</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect1>
<sect1 id="mc-manual.suppfiles" xreflabel="Writing suppression files">
<title>Writing suppression files</title>
<para>The basic suppression format is described in
<xref linkend="manual-core.suppress"/>.</para>
<para>The suppression-type (second) line should have the form:</para>
<programlisting><![CDATA[
Memcheck:suppression_type]]></programlisting>
<para>The Memcheck suppression types are as follows:</para>
<itemizedlist>
<listitem>
<para><varname>Value1</varname>,
<varname>Value2</varname>,
<varname>Value4</varname>,
<varname>Value8</varname>,
<varname>Value16</varname>,
meaning an uninitialised-value error when
using a value of 1, 2, 4, 8 or 16 bytes.</para>
</listitem>
<listitem>
<para><varname>Cond</varname> (or its old
name, <varname>Value0</varname>), meaning use
of an uninitialised CPU condition code.</para>
</listitem>
<listitem>
<para><varname>Addr1</varname>,
<varname>Addr2</varname>,
<varname>Addr4</varname>,
<varname>Addr8</varname>,
<varname>Addr16</varname>,
meaning an invalid address during a
memory access of 1, 2, 4, 8 or 16 bytes respectively.</para>
</listitem>
<listitem>
<para><varname>Jump</varname>, meaning an
jump to an unaddressable location error.</para>
</listitem>
<listitem>
<para><varname>Param</varname>, meaning an
invalid system call parameter error.</para>
</listitem>
<listitem>
<para><varname>Free</varname>, meaning an
invalid or mismatching free.</para>
</listitem>
<listitem>
<para><varname>Overlap</varname>, meaning a
<computeroutput>src</computeroutput> /
<computeroutput>dst</computeroutput> overlap in
<function>memcpy</function> or a similar function.</para>
</listitem>
<listitem>
<para><varname>Leak</varname>, meaning
a memory leak.</para>
</listitem>
</itemizedlist>
<para><computeroutput>Param</computeroutput> 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.</para>
<para>The first line of the calling context: for <varname>ValueN</varname>
and <varname>AddrN</varname> errors, it is either the name of the function
in which the error occurred, or, failing that, the full path of the
<filename>.so</filename> file
or executable containing the error location. For <varname>Free</varname> errors, is the name
of the function doing the freeing (eg, <function>free</function>,
<function>__builtin_vec_delete</function>, etc). For
<varname>Overlap</varname> errors, is the name of the function with the
overlapping arguments (eg. <function>memcpy</function>,
<function>strcpy</function>, etc).</para>
<para>Lastly, there's the rest of the calling context.</para>
</sect1>
<sect1 id="mc-manual.machine"
xreflabel="Details of Memcheck's checking machinery">
<title>Details of Memcheck's checking machinery</title>
<para>Read this section if you want to know, in detail, exactly
what and how Memcheck is checking.</para>
<sect2 id="mc-manual.value" xreflabel="Valid-value (V) bit">
<title>Valid-value (V) bits</title>
<para>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.</para>
<para>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.</para>
<para>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.</para>
<para>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.</para>
<para>Here's an (admittedly nonsensical) example:</para>
<programlisting><![CDATA[
int i, j;
int a[10], b[10];
for ( i = 0; i < 10; i++ ) {
j = a[i];
b[i] = j;
}]]></programlisting>
<para>Memcheck emits no complaints about this, since it merely copies
uninitialised values from <varname>a[]</varname> into
<varname>b[]</varname>, and doesn't use them in a way which could
affect the behaviour of the program. However, if
the loop is changed to:</para>
<programlisting><![CDATA[
for ( i = 0; i < 10; i++ ) {
j += a[i];
}
if ( j == 77 )
printf("hello there\n");
]]></programlisting>
<para>then Memcheck will complain, at the
<computeroutput>if</computeroutput>, that the condition depends on
uninitialised values. Note that it <command>doesn't</command> complain
at the <varname>j += a[i];</varname>, 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 <function>printf</function> -- an
observable action of your program -- that Memcheck complains.</para>
<para>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.</para>
<para>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.</para>
<para>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.</para>
<para>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:</para>
<programlisting><![CDATA[
struct S { int x; char c; };
struct S s1, s2;
s1.x = 42;
s1.c = 'z';
s2 = s1;
]]></programlisting>
<para>The question to ask is: how large is <varname>struct S</varname>,
in bytes? An <varname>int</varname> is 4 bytes and a
<varname>char</varname> one byte, so perhaps a <varname>struct
S</varname> occupies 5 bytes? Wrong. All non-toy compilers we know
of will round the size of <varname>struct S</varname> 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
<varname>struct S</varname>'s on some architectures.</para>
<para>So <varname>s1</varname> occupies 8 bytes, yet only 5 of them will
be initialised. For the assignment <varname>s2 = s1</varname>, GCC
generates code to copy all 8 bytes wholesale into <varname>s2</varname>
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
<varname>s1</varname> into <varname>s2</varname> any way it likes, and a
warning will only be emitted if the uninitialised values are later
used.</para>
</sect2>
<sect2 id="mc-manual.vaddress" xreflabel=" Valid-address (A) bits">
<title>Valid-address (A) bits</title>
<para>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.</para>
<para>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.</para>
<para>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.</para>
<para>So how do the A bits get set/cleared? Like this:</para>
<itemizedlist>
<listitem>
<para>When the program starts, all the global data areas are
marked as accessible.</para>
</listitem>
<listitem>
<para>When the program does
<function>malloc</function>/<computeroutput>new</computeroutput>,
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.</para>
</listitem>
<listitem>
<para>When the stack pointer register (<literal>SP</literal>) moves
up or down, A bits are set. The rule is that the area from
<literal>SP</literal> up to the base of the stack is marked as
accessible, and below <literal>SP</literal> 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
<literal>SP</literal> 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.</para>
</listitem>
<listitem>
<para>When doing system calls, A bits are changed appropriately.
For example, <literal>mmap</literal>
magically makes files appear in the process'
address space, so the A bits must be updated if <literal>mmap</literal>
succeeds.</para>
</listitem>
<listitem>
<para>Optionally, your program can tell Memcheck about such changes
explicitly, using the client request mechanism described
above.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="mc-manual.together" xreflabel="Putting it all together">
<title>Putting it all together</title>
<para>Memcheck's checking machinery can be summarised as
follows:</para>
<itemizedlist>
<listitem>
<para>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%.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>When a register is written out to memory, the V bits for that
register are written back to memory too.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>When values in CPU registers are used for any other purpose,
Memcheck computes the V bits for the result, but does not check
them.</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>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.</para>
<para>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.</para>
<para>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 <option>--partial-loads-ok</option> for details.
</para>
</listitem>
</itemizedlist>
<para>Memcheck intercepts calls to <function>malloc</function>,
<function>calloc</function>, <function>realloc</function>,
<function>valloc</function>, <function>memalign</function>,
<function>free</function>, <computeroutput>new</computeroutput>,
<computeroutput>new[]</computeroutput>,
<computeroutput>delete</computeroutput> and
<computeroutput>delete[]</computeroutput>. The behaviour you get
is:</para>
<itemizedlist>
<listitem>
<para><function>malloc</function>/<function>new</function>/<computeroutput>new[]</computeroutput>:
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.</para>
</listitem>
<listitem>
<para><function>calloc</function>: returned memory is marked both
addressable and valid, since <function>calloc</function> clears
the area to zero.</para>
</listitem>
<listitem>
<para><function>realloc</function>: if the new size is larger than
the old, the new section is addressable but invalid, as with
<function>malloc</function>. If the new size is smaller, the
dropped-off section is marked as unaddressable. You may only pass to
<function>realloc</function> a pointer previously issued to you by
<function>malloc</function>/<function>calloc</function>/<function>realloc</function>.</para>
</listitem>
<listitem>
<para><function>free</function>/<computeroutput>delete</computeroutput>/<computeroutput>delete[]</computeroutput>:
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.</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id="mc-manual.monitor-commands" xreflabel="Memcheck Monitor Commands">
<title>Memcheck Monitor Commands</title>
<para>The Memcheck tool provides monitor commands handled by Valgrind's
built-in gdbserver (see <xref linkend="manual-core-adv.gdbserver-commandhandling"/>).
</para>
<itemizedlist>
<listitem>
<para><varname>get_vbits <addr> [<len>]</varname>
shows the definedness (V) bits for <len> (default 1) bytes
starting at <addr>. 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 <varname>__</varname> (a double underscore).
</para>
<para>
In the following example, <varname>string10</varname> is an array
of 10 characters, in which the even numbered bytes are
undefined. In the below example, the byte corresponding
to <varname>string10[5]</varname> is not addressable.
</para>
<programlisting><![CDATA[
(gdb) p &string10
$4 = (char (*)[10]) 0x8049e28
(gdb) monitor get_vbits 0x8049e28 10
ff00ff00 ff__ff00 ff00
(gdb)
]]></programlisting>
<para> The command get_vbits cannot be used with registers. To get
the validity bits of a register, you must start Valgrind with the
option <option>--vgdb-shadow-registers=yes</option>. 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.
</para>
<programlisting><![CDATA[
(gdb) p /x $eaxs1
$9 = 0xffffffff
(gdb) p /x $ebxs1
$10 = 0x0
(gdb)
]]></programlisting>
</listitem>
<listitem>
<para><varname>make_memory
[noaccess|undefined|defined|Definedifaddressable] <addr>
[<len>]</varname> marks the range of <len> (default 1)
bytes at <addr> as having the given status. Parameter
<varname>noaccess</varname> marks the range as non-accessible, so
Memcheck will report an error on any access to it.
<varname>undefined</varname> or <varname>defined</varname> mark
the area as accessible, but Memcheck regards the bytes in it
respectively as having undefined or defined values.
<varname>Definedifaddressable</varname> 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 <varname>Definedifaddressable</varname>
is an uppercase D to avoid confusion with <varname>defined</varname>.
</para>
<para>
In the following example, the first byte of the
<varname>string10</varname> is marked as defined:
</para>
<programlisting><![CDATA[
(gdb) monitor make_memory defined 0x8049e28 1
(gdb) monitor get_vbits 0x8049e28 10
0000ff00 ff00ff00 ff00
(gdb)
]]></programlisting>
</listitem>
<listitem>
<para><varname>check_memory [addressable|defined] <addr>
[<len>]</varname> checks that the range of <len>
(default 1) bytes at <addr> has the specified accessibility.
It then outputs a description of <addr>. In the following
example, a detailed description is available because the
option <option>--read-var-info=yes</option> was given Valgrind at
startup:
</para>
<programlisting><![CDATA[
(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)
]]></programlisting>
</listitem>
<listitem>
<para><varname>leak_check [full*|summary]
[reachable|possibleleak*|definiteleak]
[increased*|changed|any]
</varname>
performs a leak check. The <varname>*</varname> in the arguments
indicates the default value. </para>
<para> If the first argument is <varname>summary</varname>, 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.
</para>
<para>The second argument controls what kind of blocks are shown for
a <varname>full</varname> leak search. The
value <varname>definiteleak</varname> specifies that only
definitely leaked blocks should be shown. The
value <varname>possibleleak</varname> will also show possibly
leaked blocks (those for which only an interior pointer was
found). The value
<varname>reachable</varname> will show all block categories
(reachable, possibly leaked, definitely leaked).
</para>
<para>The third argument controls what kinds of changes are shown
for a <varname>full</varname> leak search. The
value <varname>increased</varname> 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 <varname>changed</varname> specifies that allocation stacks
with any change since the previous leak check should be shown.
The value <varname>any</varname> specifies that all leak entries
should be shown, regardless of any increase or decrease. When
If <varname>increased</varname> or <varname>changed</varname> are
specified, the leak report entries will show the delta relative to
the previous leak report.
</para>
<para>The following example shows usage of the
<varname>leak_check monitor</varname> command on
the <varname>memcheck/tests/leak-cases.c</varname> 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.</para>
<programlisting><![CDATA[
(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)
]]></programlisting>
<para>Note that when using Valgrind's gdbserver, it is not
necessary to rerun
with <option>--leak-check=full</option>
<option>--show-reachable=yes</option> to see the reachable
blocks. You can obtain the same information without rerunning by
using the GDB command <computeroutput>monitor leak_check full
reachable any</computeroutput> (or, using
abbreviation: <computeroutput>mo l f r a</computeroutput>).
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="mc-manual.clientreqs" xreflabel="Client requests">
<title>Client Requests</title>
<para>The following client requests are defined in
<filename>memcheck.h</filename>.
See <filename>memcheck.h</filename> for exact details of their
arguments.</para>
<itemizedlist>
<listitem>
<para><varname>VALGRIND_MAKE_MEM_NOACCESS</varname>,
<varname>VALGRIND_MAKE_MEM_UNDEFINED</varname> and
<varname>VALGRIND_MAKE_MEM_DEFINED</varname>.
These mark address ranges as completely inaccessible,
accessible but containing undefined data, and accessible and
containing defined data, respectively.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MAKE_MEM_DEFINED_IF_ADDRESSABLE</varname>.
This is just like <varname>VALGRIND_MAKE_MEM_DEFINED</varname> but only
affects those bytes that are already addressable.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_CHECK_MEM_IS_ADDRESSABLE</varname> and
<varname>VALGRIND_CHECK_MEM_IS_DEFINED</varname>: 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.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_CHECK_VALUE_IS_DEFINED</varname>: 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.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_DO_LEAK_CHECK</varname>: does a full memory leak
check (like <option>--leak-check=full</option>) right now.
This is useful for incrementally checking for leaks between arbitrary
places in the program's execution. It has no return value.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_DO_ADDED_LEAK_CHECK</varname>: same as
<varname> VALGRIND_DO_LEAK_CHECK</varname> 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.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_DO_CHANGED_LEAK_CHECK</varname>: same as
<varname>VALGRIND_DO_LEAK_CHECK</varname> 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.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_DO_QUICK_LEAK_CHECK</varname>: like
<varname>VALGRIND_DO_LEAK_CHECK</varname>, except it produces only a leak
summary (like <option>--leak-check=summary</option>).
It has no return value.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_COUNT_LEAKS</varname>: 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 <varname>VALGRIND_DO_LEAK_CHECK</varname> or
<varname>VALGRIND_DO_QUICK_LEAK_CHECK</varname>.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_COUNT_LEAK_BLOCKS</varname>: identical to
<varname>VALGRIND_COUNT_LEAKS</varname> except that it returns the
number of blocks rather than the number of bytes in each
category.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_GET_VBITS</varname> and
<varname>VALGRIND_SET_VBITS</varname>: 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
<varname>VALGRIND_GET_VBITS</varname>. Only for those who really
know what they are doing.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_CREATE_BLOCK</varname> and
<varname>VALGRIND_DISCARD</varname>. <varname>VALGRIND_CREATE_BLOCK</varname>
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.
</para>
<para>At some point you may want Memcheck to stop reporting errors
in terms of the block named
by <varname>VALGRIND_CREATE_BLOCK</varname>. To make this
possible, <varname>VALGRIND_CREATE_BLOCK</varname> returns a
"block handle", which is a C <varname>int</varname> value. You
can pass this block handle to <varname>VALGRIND_DISCARD</varname>.
After doing so, Valgrind will no longer relate addressing errors
in the specified range to the block. Passing invalid handles to
<varname>VALGRIND_DISCARD</varname> is harmless.
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="mc-manual.mempools" xreflabel="Memory Pools">
<title>Memory Pools: describing and working with custom allocators</title>
<para>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.</para>
<para>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:</para>
<itemizedlist>
<listitem>
<para>Custom allocation involves a set of independent "memory pools".
</para>
</listitem>
<listitem>
<para>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.</para>
</listitem>
<listitem>
<para>Typically a pool's anchor address is the address of a
book-keeping "header" structure.</para>
</listitem>
<listitem>
<para>Typically the pool's chunks are drawn from a contiguous
"superblock" acquired through the system
<function>malloc</function> or
<function>mmap</function>.</para>
</listitem>
</itemizedlist>
<para>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:</para>
<programlisting><![CDATA[
"pool"
(anchor address)
|
v
+--------+---+
| header | o |
+--------+-|-+
|
v superblock
+------+---+--------------+---+------------------+
| |rzB| allocation |rzB| |
+------+---+--------------+---+------------------+
^ ^
| |
"addr" "addr"+"size"
]]></programlisting>
<para>
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".</para>
<para>
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
<varname>VALGRIND_MAKE_MEM_NOACCESS</varname> client request.</para>
<para>
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.</para>
<para>
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:</para>
<itemizedlist>
<listitem>
<para>
<varname>VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed)</varname>:
This request registers the address <varname>pool</varname> as the anchor
address for a memory pool. It also provides a size
<varname>rzB</varname>, specifying how large the redzones placed around
chunks allocated from the pool should be. Finally, it provides an
<varname>is_zeroed</varname> argument that specifies whether the pool's
chunks are zeroed (more precisely: defined) when allocated.
</para>
<para>
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.
</para>
</listitem>
<listitem>
<para><varname>VALGRIND_DESTROY_MEMPOOL(pool)</varname>:
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.
</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MEMPOOL_ALLOC(pool, addr, size)</varname>:
This request informs Memcheck that a <varname>size</varname>-byte chunk
has been allocated at <varname>addr</varname>, and associates the chunk with the
specified
<varname>pool</varname>. If the pool was created with nonzero
<varname>rzB</varname> redzones, Memcheck will mark the
<varname>rzB</varname> bytes before and after the chunk as NOACCESS. If
the pool was created with the <varname>is_zeroed</varname> argument set,
Memcheck will mark the chunk as DEFINED, otherwise Memcheck will mark
the chunk as UNDEFINED.
</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MEMPOOL_FREE(pool, addr)</varname>:
This request informs Memcheck that the chunk at <varname>addr</varname>
should no longer be considered allocated. Memcheck will mark the chunk
associated with <varname>addr</varname> as NOACCESS, and delete its
record of the chunk's existence.
</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MEMPOOL_TRIM(pool, addr, size)</varname>:
This request trims the chunks associated with <varname>pool</varname>.
The request only operates on chunks associated with
<varname>pool</varname>. Trimming is formally defined as:</para>
<itemizedlist>
<listitem>
<para> All chunks entirely inside the range
<varname>addr..(addr+size-1)</varname> are preserved.</para>
</listitem>
<listitem>
<para>All chunks entirely outside the range
<varname>addr..(addr+size-1)</varname> are discarded, as though
<varname>VALGRIND_MEMPOOL_FREE</varname> was called on them. </para>
</listitem>
<listitem>
<para>All other chunks must intersect with the range
<varname>addr..(addr+size-1)</varname>; areas outside the
intersection are marked as NOACCESS, as though they had been
independently freed with
<varname>VALGRIND_MEMPOOL_FREE</varname>.</para>
</listitem>
</itemizedlist>
<para>This is a somewhat rare request, but can be useful in
implementing the type of mass-free operations common in custom
LIFO allocators.</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MOVE_MEMPOOL(poolA, poolB)</varname>: This
request informs Memcheck that the pool previously anchored at
address <varname>poolA</varname> has moved to anchor address
<varname>poolB</varname>. This is a rare request, typically only needed
if you <function>realloc</function> the header of a mempool.</para>
<para>No memory-status bits are altered by this request.</para>
</listitem>
<listitem>
<para>
<varname>VALGRIND_MEMPOOL_CHANGE(pool, addrA, addrB,
size)</varname>: This request informs Memcheck that the chunk
previously allocated at address <varname>addrA</varname> within
<varname>pool</varname> has been moved and/or resized, and should be
changed to cover the region <varname>addrB..(addrB+size-1)</varname>. This
is a rare request, typically only needed if you
<function>realloc</function> a superblock or wish to extend a chunk
without changing its memory-status bits.
</para>
<para>No memory-status bits are altered by this request.
</para>
</listitem>
<listitem>
<para><varname>VALGRIND_MEMPOOL_EXISTS(pool)</varname>:
This request informs the caller whether or not Memcheck is currently
tracking a mempool at anchor address <varname>pool</varname>. 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.
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="mc-manual.mpiwrap" xreflabel="MPI Wrappers">
<title>Debugging MPI Parallel Programs with Valgrind</title>
<para>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
<computeroutput>PMPI_*</computeroutput> interface. When incorporated
into the application's address space, either by direct linking or by
<computeroutput>LD_PRELOAD</computeroutput>, the wrappers intercept
calls to <computeroutput>PMPI_Send</computeroutput>,
<computeroutput>PMPI_Recv</computeroutput>, 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.</para>
<para>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
<computeroutput>PMPI_Send</computeroutput>, or receiving data into a
buffer which is too small.</para>
<para>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 <computeroutput>mpi/libmpiwrap.c</computeroutput>
for license details.</para>
<sect2 id="mc-manual.mpiwrap.build" xreflabel="Building MPI Wrappers">
<title>Building and installing the wrappers</title>
<para> The wrapper library will be built automatically if possible.
Valgrind's configure script will look for a suitable
<computeroutput>mpicc</computeroutput> to build it with. This must be
the same <computeroutput>mpicc</computeroutput> you use to build the
MPI application you want to debug. By default, Valgrind tries
<computeroutput>mpicc</computeroutput>, but you can specify a
different one by using the configure-time option
<option>--with-mpicc</option>. Currently the
wrappers are only buildable with
<computeroutput>mpicc</computeroutput>s which are based on GNU
GCC or Intel's C++ Compiler.</para>
<para>Check that the configure script prints a line like this:</para>
<programlisting><![CDATA[
checking for usable MPI2-compliant mpicc and mpi.h... yes, mpicc
]]></programlisting>
<para>If it says <computeroutput>... no</computeroutput>, your
<computeroutput>mpicc</computeroutput> has failed to compile and link
a test MPI2 program.</para>
<para>If the configure test succeeds, continue in the usual way with
<computeroutput>make</computeroutput> and <computeroutput>make
install</computeroutput>. The final install tree should then contain
<computeroutput>libmpiwrap-<platform>.so</computeroutput>.
</para>
<para>Compile up a test MPI program (eg, MPI hello-world) and try
this:</para>
<programlisting><![CDATA[
LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \
mpirun [args] $prefix/bin/valgrind ./hello
]]></programlisting>
<para>You should see something similar to the following</para>
<programlisting><![CDATA[
valgrind MPI wrappers 31901: Active for pid 31901
valgrind MPI wrappers 31901: Try MPIWRAP_DEBUG=help for possible options
]]></programlisting>
<para>repeated for every process in the group. If you do not see
these, there is an build/installation problem of some kind.</para>
<para> The MPI functions to be wrapped are assumed to be in an ELF
shared object with soname matching
<computeroutput>libmpi.so*</computeroutput>. This is known to be
correct at least for Open MPI and Quadrics MPI, and can easily be
changed if required.</para>
</sect2>
<sect2 id="mc-manual.mpiwrap.gettingstarted"
xreflabel="Getting started with MPI Wrappers">
<title>Getting started</title>
<para>Compile your MPI application as usual, taking care to link it
using the same <computeroutput>mpicc</computeroutput> that your
Valgrind build was configured with.</para>
<para>
Use the following basic scheme to run your application on Valgrind with
the wrappers engaged:</para>
<programlisting><![CDATA[
MPIWRAP_DEBUG=[wrapper-args] \
LD_PRELOAD=$prefix/lib/valgrind/libmpiwrap-<platform>.so \
mpirun [mpirun-args] \
$prefix/bin/valgrind [valgrind-args] \
[application] [app-args]
]]></programlisting>
<para>As an alternative to
<computeroutput>LD_PRELOAD</computeroutput>ing
<computeroutput>libmpiwrap-<platform>.so</computeroutput>, you can
simply link it to your application if desired. This should not disturb
native behaviour of your application in any way.</para>
</sect2>
<sect2 id="mc-manual.mpiwrap.controlling"
xreflabel="Controlling the MPI Wrappers">
<title>Controlling the wrapper library</title>
<para>Environment variable
<computeroutput>MPIWRAP_DEBUG</computeroutput> is consulted at
startup. The default behaviour is to print a starting banner</para>
<programlisting><![CDATA[
valgrind MPI wrappers 16386: Active for pid 16386
valgrind MPI wrappers 16386: Try MPIWRAP_DEBUG=help for possible options
]]></programlisting>
<para> and then be relatively quiet.</para>
<para>You can give a list of comma-separated options in
<computeroutput>MPIWRAP_DEBUG</computeroutput>. These are</para>
<itemizedlist>
<listitem>
<para><computeroutput>verbose</computeroutput>:
show entries/exits of all wrappers. Also show extra
debugging info, such as the status of outstanding
<computeroutput>MPI_Request</computeroutput>s resulting
from uncompleted <computeroutput>MPI_Irecv</computeroutput>s.</para>
</listitem>
<listitem>
<para><computeroutput>quiet</computeroutput>:
opposite of <computeroutput>verbose</computeroutput>, only print
anything when the wrappers want
to report a detected programming error, or in case of catastrophic
failure of the wrappers.</para>
</listitem>
<listitem>
<para><computeroutput>warn</computeroutput>:
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.</para>
</listitem>
<listitem>
<para><computeroutput>strict</computeroutput>:
print an error message and abort the program if
a function lacking a wrapper is used.</para>
</listitem>
</itemizedlist>
<para> If you want to use Valgrind's XML output facility
(<option>--xml=yes</option>), you should pass
<computeroutput>quiet</computeroutput> in
<computeroutput>MPIWRAP_DEBUG</computeroutput> so as to get rid of any
extraneous printing from the wrappers.</para>
</sect2>
<sect2 id="mc-manual.mpiwrap.limitations.functions"
xreflabel="Functions: Abilities and Limitations">
<title>Functions</title>
<para>All MPI2 functions except
<computeroutput>MPI_Wtick</computeroutput>,
<computeroutput>MPI_Wtime</computeroutput> and
<computeroutput>MPI_Pcontrol</computeroutput> have wrappers. The
first two are not wrapped because they return a
<computeroutput>double</computeroutput>, which Valgrind's
function-wrap mechanism cannot handle (but it could easily be
extended to do so). <computeroutput>MPI_Pcontrol</computeroutput> cannot be
wrapped as it has variable arity:
<computeroutput>int MPI_Pcontrol(const int level, ...)</computeroutput></para>
<para>Most functions are wrapped with a default wrapper which does
nothing except complain or abort if it is called, depending on
settings in <computeroutput>MPIWRAP_DEBUG</computeroutput> listed
above. The following functions have "real", do-something-useful
wrappers:</para>
<programlisting><![CDATA[
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
]]></programlisting>
<para> A few functions such as
<computeroutput>PMPI_Address</computeroutput> are listed as
<computeroutput>HAS_NO_WRAPPER</computeroutput>. They have no wrapper
at all as there is nothing worth checking, and giving a no-op wrapper
would reduce performance for no reason.</para>
<para> 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
<computeroutput>PMPI_Extent</computeroutput>,
<computeroutput>PMPI_Type_get_envelope</computeroutput>,
<computeroutput>PMPI_Type_get_contents</computeroutput>, and
<computeroutput>PMPI_Type_free</computeroutput>. </para>
</sect2>
<sect2 id="mc-manual.mpiwrap.limitations.types"
xreflabel="Types: Abilities and Limitations">
<title>Types</title>
<para> MPI-1.1 structured types are supported, and walked exactly.
The currently supported combiners are
<computeroutput>MPI_COMBINER_NAMED</computeroutput>,
<computeroutput>MPI_COMBINER_CONTIGUOUS</computeroutput>,
<computeroutput>MPI_COMBINER_VECTOR</computeroutput>,
<computeroutput>MPI_COMBINER_HVECTOR</computeroutput>
<computeroutput>MPI_COMBINER_INDEXED</computeroutput>,
<computeroutput>MPI_COMBINER_HINDEXED</computeroutput> and
<computeroutput>MPI_COMBINER_STRUCT</computeroutput>. This should
cover all MPI-1.1 types. The mechanism (function
<computeroutput>walk_type</computeroutput>) should extend easily to
cover MPI2 combiners.</para>
<para>MPI defines some named structured types
(<computeroutput>MPI_FLOAT_INT</computeroutput>,
<computeroutput>MPI_DOUBLE_INT</computeroutput>,
<computeroutput>MPI_LONG_INT</computeroutput>,
<computeroutput>MPI_2INT</computeroutput>,
<computeroutput>MPI_SHORT_INT</computeroutput>,
<computeroutput>MPI_LONG_DOUBLE_INT</computeroutput>) which are pairs
of some basic type and a C <computeroutput>int</computeroutput>.
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 <computeroutput>struct { float val;
int loc; }</computeroutput> (for
<computeroutput>MPI_FLOAT_INT</computeroutput>), etc, and act
accordingly. This appears to be correct at least for Open MPI 1.0.2
and for Quadrics MPI.</para>
<para>If <computeroutput>strict</computeroutput> is an option specified
in <computeroutput>MPIWRAP_DEBUG</computeroutput>, the application
will abort if an unhandled type is encountered. Otherwise, the
application will print a warning message and continue.</para>
<para>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 (<computeroutput>double</computeroutput>,
<computeroutput>float</computeroutput>,
<computeroutput>int</computeroutput>,
<computeroutput>long</computeroutput>, <computeroutput>long
long</computeroutput>, <computeroutput>short</computeroutput>,
<computeroutput>char</computeroutput>, and <computeroutput>long
double</computeroutput> on platforms where <computeroutput>sizeof(long
double) == 8</computeroutput>). For arrays of all other types, the
wrappers handle each element individually and so there can be a very
large performance cost.</para>
</sect2>
<sect2 id="mc-manual.mpiwrap.writingwrappers"
xreflabel="Writing new MPI Wrappers">
<title>Writing new wrappers</title>
<para>
For the most part the wrappers are straightforward. The only
significant complexity arises with nonblocking receives.</para>
<para>The issue is that <computeroutput>MPI_Irecv</computeroutput>
states the recv buffer and returns immediately, giving a handle
(<computeroutput>MPI_Request</computeroutput>) for the transaction.
Later the user will have to poll for completion with
<computeroutput>MPI_Wait</computeroutput> etc, and when the
transaction completes successfully, the wrappers have to paint the
recv buffer. But the recv buffer details are not presented to
<computeroutput>MPI_Wait</computeroutput> -- only the handle is. The
library therefore maintains a shadow table which associates
uncompleted <computeroutput>MPI_Request</computeroutput>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.</para>
<para>Access to the table is guarded by a (POSIX pthreads) lock, so as
to make the library thread-safe.</para>
<para>The table is allocated with
<computeroutput>malloc</computeroutput> and never
<computeroutput>free</computeroutput>d, so it will show up in leak
checks.</para>
<para>Writing new wrappers should be fairly easy. The source file is
<computeroutput>mpi/libmpiwrap.c</computeroutput>. 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).</para>
</sect2>
<sect2 id="mc-manual.mpiwrap.whattoexpect"
xreflabel="What to expect with MPI Wrappers">
<title>What to expect when using the wrappers</title>
<para>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.</para>
<para>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
<computeroutput>MPI_Recv</computeroutput>.</para>
<para>Functions which are not wrapped may increase the false
error rate. A possible approach is to run with
<computeroutput>MPI_DEBUG</computeroutput> containing
<computeroutput>warn</computeroutput>. This will show you functions
which lack proper wrappers but which are nevertheless used. You can
then write wrappers for them.
</para>
<para>A known source of potential false errors are the
<computeroutput>PMPI_Reduce</computeroutput> 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
<computeroutput>PMPI_Reduce</computeroutput> call. As a result you
may see false positives reported in your reduction function.</para>
</sect2>
</sect1>
</chapter>