docs/xml/manual-core.xml

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<chapter id="manual-core" xreflabel="Valgrind's core">
<title>Using and understanding the Valgrind core</title>

<para>This chapter describes the Valgrind core services, command-line
options and behaviours.  That means it is relevant regardless of what
particular tool you are using.  The information should be sufficient for you
to make effective day-to-day use of Valgrind.  Advanced topics related to
the Valgrind core are described in <xref linkend="manual-core-adv"/>.
</para>

<para>
A point of terminology: most references to "Valgrind" in this chapter
refer to the Valgrind core services.  </para>



<sect1 id="manual-core.whatdoes" 
       xreflabel="What Valgrind does with your program">
<title>What Valgrind does with your program</title>

<para>Valgrind is designed to be as non-intrusive as possible. It works
directly with existing executables. You don't need to recompile, relink,
or otherwise modify the program to be checked.</para>

<para>You invoke Valgrind like this:</para>
<programlisting><![CDATA[
valgrind [valgrind-options] your-prog [your-prog-options]]]></programlisting>

<para>The most important option is <option>--tool</option> which dictates
which Valgrind tool to run.  For example, if want to run the command
<computeroutput>ls -l</computeroutput> using the memory-checking tool
Memcheck, issue this command:</para>

<programlisting><![CDATA[
valgrind --tool=memcheck ls -l]]></programlisting>

<para>However, Memcheck is the default, so if you want to use it you can
omit the <option>--tool</option> option.</para>

<para>Regardless of which tool is in use, Valgrind takes control of your
program before it starts.  Debugging information is read from the
executable and associated libraries, so that error messages and other
outputs can be phrased in terms of source code locations, when
appropriate.</para>

<para>Your program is then run on a synthetic CPU provided by the
Valgrind core.  As new code is executed for the first time, the core
hands the code to the selected tool.  The tool adds its own
instrumentation code to this and hands the result back to the core,
which coordinates the continued execution of this instrumented
code.</para>

<para>The amount of instrumentation code added varies widely between
tools.  At one end of the scale, Memcheck adds code to check every
memory access and every value computed,
making it run 10-50 times slower than natively.
At the other end of the spectrum, the minimal tool, called Nulgrind,
adds no instrumentation at all and causes in total "only" about a 4 times
slowdown.</para>

<para>Valgrind simulates every single instruction your program executes.
Because of this, the active tool checks, or profiles, not only the code
in your application but also in all supporting dynamically-linked libraries,
including the C library, graphical libraries, and so on.</para>

<para>If you're using an error-detection tool, Valgrind may
detect errors in system libraries, for example the GNU C or X11
libraries, which you have to use.  You might not be interested in these
errors, since you probably have no control over that code.  Therefore,
Valgrind allows you to selectively suppress errors, by recording them in
a suppressions file which is read when Valgrind starts up.  The build
mechanism selects default suppressions which give reasonable
behaviour for the OS and libraries detected on your machine.
To make it easier to write suppressions, you can use the
<option>--gen-suppressions=yes</option> option.  This tells Valgrind to
print out a suppression for each reported error, which you can then
copy into a suppressions file.</para>

<para>Different error-checking tools report different kinds of errors.
The suppression mechanism therefore allows you to say which tool or
tool(s) each suppression applies to.</para>

</sect1>


<sect1 id="manual-core.started" xreflabel="Getting started">
<title>Getting started</title>

<para>First off, consider whether it might be beneficial to recompile
your application and supporting libraries with debugging info enabled
(the <option>-g</option> option).  Without debugging info, the best
Valgrind tools will be able to do is guess which function a particular
piece of code belongs to, which makes both error messages and profiling
output nearly useless.  With <option>-g</option>, you'll get
messages which point directly to the relevant source code lines.</para>

<para>Another option you might like to consider, if you are working with
C++, is <option>-fno-inline</option>.  That makes it easier to see the
function-call chain, which can help reduce confusion when navigating
around large C++ apps.  For example, debugging
OpenOffice.org with Memcheck is a bit easier when using this option.  You
don't have to do this, but doing so helps Valgrind produce more accurate
and less confusing error reports.  Chances are you're set up like this
already, if you intended to debug your program with GNU GDB, or some
other debugger.</para>

<para>If you are planning to use Memcheck: On rare
occasions, compiler optimisations (at <option>-O2</option>
and above, and sometimes <option>-O1</option>) have been
observed to generate code which fools Memcheck into wrongly reporting
uninitialised value errors, or missing uninitialised value errors.  We have
looked in detail into fixing this, and unfortunately the result is that
doing so would give a further significant slowdown in what is already a slow
tool.  So the best solution is to turn off optimisation altogether.  Since
this often makes things unmanageably slow, a reasonable compromise is to use
<option>-O</option>.  This gets you the majority of the
benefits of higher optimisation levels whilst keeping relatively small the
chances of false positives or false negatives from Memcheck.  Also, you
should compile your code with <option>-Wall</option> because
it can identify some or all of the problems that Valgrind can miss at the
higher optimisation levels.  (Using <option>-Wall</option>
is also a good idea in general.)  All other tools (as far as we know) are
unaffected by optimisation level, and for profiling tools like Cachegrind it
is better to compile your program at its normal optimisation level.</para>

<para>Valgrind understands both the older "stabs" debugging format, used
by GCC versions prior to 3.1, and the newer DWARF2/3/4 formats
used by GCC
3.1 and later.  We continue to develop our debug-info readers,
although the majority of effort will naturally enough go into the newer
DWARF readers.</para>

<para>When you're ready to roll, run Valgrind as described above.
Note that you should run the real
(machine-code) executable here.  If your application is started by, for
example, a shell or Perl script, you'll need to modify it to invoke
Valgrind on the real executables.  Running such scripts directly under
Valgrind will result in you getting error reports pertaining to
<filename>/bin/sh</filename>,
<filename>/usr/bin/perl</filename>, or whatever interpreter
you're using.  This may not be what you want and can be confusing.  You
can force the issue by giving the option
<option>--trace-children=yes</option>, but confusion is still
likely.</para>

</sect1>


<!-- Referenced from both the manual and manpage -->
<sect1 id="&vg-comment-id;" xreflabel="&vg-comment-label;">
<title>The Commentary</title>

<para>Valgrind tools write a commentary, a stream of text, detailing
error reports and other significant events.  All lines in the commentary
have following form:

<programlisting><![CDATA[
==12345== some-message-from-Valgrind]]></programlisting>
</para>

<para>The <computeroutput>12345</computeroutput> is the process ID.
This scheme makes it easy to distinguish program output from Valgrind
commentary, and also easy to differentiate commentaries from different
processes which have become merged together, for whatever reason.</para>

<para>By default, Valgrind tools write only essential messages to the
commentary, so as to avoid flooding you with information of secondary
importance.  If you want more information about what is happening,
re-run, passing the <option>-v</option> option to Valgrind.  A second
<option>-v</option> gives yet more detail.
</para>

<para>You can direct the commentary to three different places:</para>

<orderedlist>

  <listitem id="manual-core.out2fd" xreflabel="Directing output to fd">
    <para>The default: send it to a file descriptor, which is by default
    2 (stderr).  So, if you give the core no options, it will write
    commentary to the standard error stream.  If you want to send it to
    some other file descriptor, for example number 9, you can specify
    <option>--log-fd=9</option>.</para>

    <para>This is the simplest and most common arrangement, but can
    cause problems when Valgrinding entire trees of processes which
    expect specific file descriptors, particularly stdin/stdout/stderr,
    to be available for their own use.</para>
  </listitem>

  <listitem id="manual-core.out2file" 
            xreflabel="Directing output to file"> <para>A less intrusive
    option is to write the commentary to a file, which you specify by
    <option>--log-file=filename</option>.  There are special format
    specifiers that can be used to use a process ID or an environment
    variable name in the log file name.  These are useful/necessary if your
    program invokes multiple processes (especially for MPI programs).
    See the <link linkend="manual-core.basicopts">basic options section</link>
    for more details.</para>
  </listitem>

  <listitem id="manual-core.out2socket" 
            xreflabel="Directing output to network socket"> <para>The
    least intrusive option is to send the commentary to a network
    socket.  The socket is specified as an IP address and port number
    pair, like this: <option>--log-socket=192.168.0.1:12345</option> if
    you want to send the output to host IP 192.168.0.1 port 12345
    (note: we
    have no idea if 12345 is a port of pre-existing significance).  You
    can also omit the port number:
    <option>--log-socket=192.168.0.1</option>, in which case a default
    port of 1500 is used.  This default is defined by the constant
    <computeroutput>VG_CLO_DEFAULT_LOGPORT</computeroutput> in the
    sources.</para>

    <para>Note, unfortunately, that you have to use an IP address here,
    rather than a hostname.</para>

    <para>Writing to a network socket is pointless if you don't
    have something listening at the other end.  We provide a simple
    listener program,
    <computeroutput>valgrind-listener</computeroutput>, which accepts
    connections on the specified port and copies whatever it is sent to
    stdout.  Probably someone will tell us this is a horrible security
    risk.  It seems likely that people will write more sophisticated
    listeners in the fullness of time.</para>

    <para><computeroutput>valgrind-listener</computeroutput> can accept
    simultaneous connections from up to 50 Valgrinded processes.  In front
    of each line of output it prints the current number of active
    connections in round brackets.</para>

    <para><computeroutput>valgrind-listener</computeroutput> accepts two
    command-line options:</para>
    <!-- start of xi:include in the manpage -->
    <variablelist id="listener.opts.list">
       <varlistentry>
         <term><option>-e --exit-at-zero</option></term>
         <listitem>
           <para>When the number of connected processes falls back to zero,
           exit.  Without this, it will run forever, that is, until you
           send it Control-C.</para>
         </listitem>
       </varlistentry>
       <varlistentry>
        <term><option>portnumber</option></term>
        <listitem>
          <para>Changes the port it listens on from the default (1500).
          The specified port must be in the range 1024 to 65535.
          The same restriction applies to port numbers specified by a
          <option>--log-socket</option> to Valgrind itself.</para>
        </listitem>
      </varlistentry>
    </variablelist>
    <!-- end of xi:include in the manpage -->

    <para>If a Valgrinded process fails to connect to a listener, for
    whatever reason (the listener isn't running, invalid or unreachable
    host or port, etc), Valgrind switches back to writing the commentary
    to stderr.  The same goes for any process which loses an established
    connection to a listener.  In other words, killing the listener
    doesn't kill the processes sending data to it.</para>
  </listitem>

</orderedlist>

<para>Here is an important point about the relationship between the
commentary and profiling output from tools.  The commentary contains a
mix of messages from the Valgrind core and the selected tool.  If the
tool reports errors, it will report them to the commentary.  However, if
the tool does profiling, the profile data will be written to a file of
some kind, depending on the tool, and independent of what
<option>--log-*</option> options are in force.  The commentary is
intended to be a low-bandwidth, human-readable channel.  Profiling data,
on the other hand, is usually voluminous and not meaningful without
further processing, which is why we have chosen this arrangement.</para>

</sect1>


<sect1 id="manual-core.report" xreflabel="Reporting of errors">
<title>Reporting of errors</title>

<para>When an error-checking tool
detects something bad happening in the program, an error
message is written to the commentary.  Here's an example from Memcheck:</para>

<programlisting><![CDATA[
==25832== Invalid read of size 4
==25832==    at 0x8048724: BandMatrix::ReSize(int, int, int) (bogon.cpp:45)
==25832==    by 0x80487AF: main (bogon.cpp:66)
==25832==  Address 0xBFFFF74C is not stack'd, malloc'd or free'd]]></programlisting>

<para>This message says that the program did an illegal 4-byte read of
address 0xBFFFF74C, which, as far as Memcheck can tell, is not a valid
stack address, nor corresponds to any current heap blocks or recently freed
heap blocks.  The read is happening at line 45 of
<filename>bogon.cpp</filename>, called from line 66 of the same file,
etc.  For errors associated with an identified (current or freed) heap block,
for example reading freed memory, Valgrind reports not only the
location where the error happened, but also where the associated heap block
was allocated/freed.</para>

<para>Valgrind remembers all error reports.  When an error is detected,
it is compared against old reports, to see if it is a duplicate.  If so,
the error is noted, but no further commentary is emitted.  This avoids
you being swamped with bazillions of duplicate error reports.</para>

<para>If you want to know how many times each error occurred, run with
the <option>-v</option> option.  When execution finishes, all the
reports are printed out, along with, and sorted by, their occurrence
counts.  This makes it easy to see which errors have occurred most
frequently.</para>

<para>Errors are reported before the associated operation actually
happens.  For example, if you're using Memcheck and your program attempts to
read from address zero, Memcheck will emit a message to this effect, and
your program will then likely die with a segmentation fault.</para>

<para>In general, you should try and fix errors in the order that they
are reported.  Not doing so can be confusing.  For example, a program
which copies uninitialised values to several memory locations, and later
uses them, will generate several error messages, when run on Memcheck.
The first such error message may well give the most direct clue to the
root cause of the problem.</para>

<para>The process of detecting duplicate errors is quite an
expensive one and can become a significant performance overhead
if your program generates huge quantities of errors.  To avoid
serious problems, Valgrind will simply stop collecting
errors after 1,000 different errors have been seen, or 10,000,000 errors
in total have been seen.  In this situation you might as well
stop your program and fix it, because Valgrind won't tell you
anything else useful after this.  Note that the 1,000/10,000,000 limits
apply after suppressed errors are removed.  These limits are
defined in <filename>m_errormgr.c</filename> and can be increased
if necessary.</para>

<para>To avoid this cutoff you can use the
<option>--error-limit=no</option> option.  Then Valgrind will always show
errors, regardless of how many there are.  Use this option carefully,
since it may have a bad effect on performance.</para>

</sect1>


<sect1 id="manual-core.suppress" xreflabel="Suppressing errors">
<title>Suppressing errors</title>

<para>The error-checking tools detect numerous problems in the system
libraries, such as the C library, 
which come pre-installed with your OS.  You can't easily fix
these, but you don't want to see these errors (and yes, there are many!)
So Valgrind reads a list of errors to suppress at startup.  A default
suppression file is created by the
<computeroutput>./configure</computeroutput> script when the system is
built.</para>

<para>You can modify and add to the suppressions file at your leisure,
or, better, write your own.  Multiple suppression files are allowed.
This is useful if part of your project contains errors you can't or
don't want to fix, yet you don't want to continuously be reminded of
them.</para>

<formalpara><title>Note:</title> <para>By far the easiest way to add
suppressions is to use the <option>--gen-suppressions=yes</option> option
described in <xref linkend="manual-core.options"/>.  This generates
suppressions automatically.  For best results,
though, you may want to edit the output
    of  <option>--gen-suppressions=yes</option> by hand, in which
case it would be advisable to read through this section.
</para>
</formalpara>

<para>Each error to be suppressed is described very specifically, to
minimise the possibility that a suppression-directive inadvertently
suppresses a bunch of similar errors which you did want to see.  The
suppression mechanism is designed to allow precise yet flexible
specification of errors to suppress.</para>

<para>If you use the <option>-v</option> option, at the end of execution,
Valgrind prints out one line for each used suppression, giving the number of times
it got used, its name and the filename and line number where the suppression is
defined. Depending on the suppression kind, the filename and line number are optionally
followed by additional information (such as the number of blocks and bytes suppressed
by a memcheck leak suppression). Here's the suppressions used by a
run of <computeroutput>valgrind -v --tool=memcheck ls -l</computeroutput>:</para>

<programlisting><![CDATA[
--1610-- used_suppression:      2 dl-hack3-cond-1 /usr/lib/valgrind/default.supp:1234
--1610-- used_suppression:      2 glibc-2.5.x-on-SUSE-10.2-(PPC)-2a /usr/lib/valgrind/default.supp:1234
]]></programlisting>

<para>Multiple suppressions files are allowed.  By default, Valgrind
uses <filename>$PREFIX/lib/valgrind/default.supp</filename>.  You can
ask to add suppressions from another file, by specifying
<option>--suppressions=/path/to/file.supp</option>.
</para>

<para>If you want to understand more about suppressions, look at an
existing suppressions file whilst reading the following documentation.
The file <filename>glibc-2.3.supp</filename>, in the source
distribution, provides some good examples.</para>

<para>Each suppression has the following components:</para>

<itemizedlist>

  <listitem>
    <para>First line: its name.  This merely gives a handy name to the
    suppression, by which it is referred to in the summary of used
    suppressions printed out when a program finishes.  It's not
    important what the name is; any identifying string will do.</para>
  </listitem>

  <listitem>
    <para>Second line: name of the tool(s) that the suppression is for
    (if more than one, comma-separated), and the name of the suppression
    itself, separated by a colon (n.b.: no spaces are allowed), eg:</para>
<programlisting><![CDATA[
tool_name1,tool_name2:suppression_name]]></programlisting>

    <para>Recall that Valgrind is a modular system, in which
    different instrumentation tools can observe your program whilst it
    is running.  Since different tools detect different kinds of errors,
    it is necessary to say which tool(s) the suppression is meaningful
    to.</para>

    <para>Tools will complain, at startup, if a tool does not understand
    any suppression directed to it.  Tools ignore suppressions which are
    not directed to them.  As a result, it is quite practical to put
    suppressions for all tools into the same suppression file.</para>
  </listitem>

  <listitem>
    <para>Next line: a small number of suppression types have extra
    information after the second line (eg. the <varname>Param</varname>
    suppression for Memcheck)</para>
  </listitem>

  <listitem>
    <para>Remaining lines: This is the calling context for the error --
    the chain of function calls that led to it.  There can be up to 24
    of these lines.</para>

    <para>Locations may be names of either shared objects or
    functions.  They begin
    <computeroutput>obj:</computeroutput> and
    <computeroutput>fun:</computeroutput> respectively.  Function and
    object names to match against may use the wildcard characters
    <computeroutput>*</computeroutput> and
    <computeroutput>?</computeroutput>.</para>

    <para><command>Important note: </command> C++ function names must be
    <command>mangled</command>.  If you are writing suppressions by
    hand, use the <option>--demangle=no</option> option to get the
    mangled names in your error messages.  An example of a mangled
    C++ name is  <computeroutput>_ZN9QListView4showEv</computeroutput>.
    This is the form that the GNU C++ compiler uses internally, and
    the form that must be used in suppression files.  The equivalent
    demangled name, <computeroutput>QListView::show()</computeroutput>,
    is what you see at the C++ source code level.
    </para>

    <para>A location line may also be
    simply "<computeroutput>...</computeroutput>" (three dots).  This is
    a frame-level wildcard, which matches zero or more frames.  Frame
    level wildcards are useful because they make it easy to ignore
    varying numbers of uninteresting frames in between frames of
    interest.  That is often important when writing suppressions which
    are intended to be robust against variations in the amount of
    function inlining done by compilers.</para>
  </listitem>

  <listitem>
    <para>Finally, the entire suppression must be between curly
    braces. Each brace must be the first character on its own
    line.</para>
  </listitem>

 </itemizedlist>

<para>A suppression only suppresses an error when the error matches all
the details in the suppression.  Here's an example:</para>

<programlisting><![CDATA[
{
  __gconv_transform_ascii_internal/__mbrtowc/mbtowc
  Memcheck:Value4
  fun:__gconv_transform_ascii_internal
  fun:__mbr*toc
  fun:mbtowc
}]]></programlisting>


<para>What it means is: for Memcheck only, suppress a
use-of-uninitialised-value error, when the data size is 4, when it
occurs in the function
<computeroutput>__gconv_transform_ascii_internal</computeroutput>, when
that is called from any function of name matching
<computeroutput>__mbr*toc</computeroutput>, when that is called from
<computeroutput>mbtowc</computeroutput>.  It doesn't apply under any
other circumstances.  The string by which this suppression is identified
to the user is
<computeroutput>__gconv_transform_ascii_internal/__mbrtowc/mbtowc</computeroutput>.</para>

<para>(See <xref linkend="mc-manual.suppfiles"/> for more details
on the specifics of Memcheck's suppression kinds.)</para>

<para>Another example, again for the Memcheck tool:</para>

<programlisting><![CDATA[
{
  libX11.so.6.2/libX11.so.6.2/libXaw.so.7.0
  Memcheck:Value4
  obj:/usr/X11R6/lib/libX11.so.6.2
  obj:/usr/X11R6/lib/libX11.so.6.2
  obj:/usr/X11R6/lib/libXaw.so.7.0
}]]></programlisting>

<para>This suppresses any size 4 uninitialised-value error which occurs
anywhere in <filename>libX11.so.6.2</filename>, when called from
anywhere in the same library, when called from anywhere in
<filename>libXaw.so.7.0</filename>.  The inexact specification of
locations is regrettable, but is about all you can hope for, given that
the X11 libraries shipped on the Linux distro on which this example
was made have had their symbol tables removed.</para>

<para>Although the above two examples do not make this clear, you can
freely mix <computeroutput>obj:</computeroutput> and
<computeroutput>fun:</computeroutput> lines in a suppression.</para>

<para>Finally, here's an example using three frame-level wildcards:</para>

<programlisting><![CDATA[
{
   a-contrived-example
   Memcheck:Leak
   fun:malloc
   ...
   fun:ddd
   ...
   fun:ccc
   ...
   fun:main
}
]]></programlisting>
This suppresses Memcheck memory-leak errors, in the case where
the allocation was done by <computeroutput>main</computeroutput>
calling (though any number of intermediaries, including zero)
<computeroutput>ccc</computeroutput>,
calling onwards via
<computeroutput>ddd</computeroutput> and eventually
to <computeroutput>malloc.</computeroutput>.
</sect1>


<sect1 id="manual-core.options" 
       xreflabel="Core Command-line Options">
<title>Core Command-line Options</title>

<para>As mentioned above, Valgrind's core accepts a common set of options.
The tools also accept tool-specific options, which are documented
separately for each tool.</para>

<para>Valgrind's default settings succeed in giving reasonable behaviour
in most cases.  We group the available options by rough categories.</para>

<sect2 id="manual-core.toolopts" xreflabel="Tool-selection Option">
<title>Tool-selection Option</title>

<para id="tool.opts.para">The single most important option.</para>

<variablelist id="tool.opts.list">

  <varlistentry id="tool_name" xreflabel="--tool">
    <term>
      <option><![CDATA[--tool=<toolname> [default: memcheck] ]]></option>
    </term>
    <listitem>
      <para>Run the Valgrind tool called <varname>toolname</varname>,
      e.g. Memcheck, Cachegrind, etc.</para>
    </listitem>
  </varlistentry>

</variablelist>

</sect2>



<sect2 id="manual-core.basicopts" xreflabel="Basic Options">
<title>Basic Options</title>

<!-- start of xi:include in the manpage -->
<para id="basic.opts.para">These options work with all tools.</para>

<variablelist id="basic.opts.list">

  <varlistentry id="opt.help" xreflabel="--help">
    <term><option>-h --help</option></term>
    <listitem>
      <para>Show help for all options, both for the core and for the
      selected tool.  If the option is repeated it is equivalent to giving
      <option>--help-debug</option>.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.help-debug" xreflabel="--help-debug">
    <term><option>--help-debug</option></term>
    <listitem>
      <para>Same as <option>--help</option>, but also lists debugging
      options which usually are only of use to Valgrind's
      developers.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.version" xreflabel="--version">
    <term><option>--version</option></term>
    <listitem>
      <para>Show the version number of the Valgrind core. Tools can have
      their own version numbers. There is a scheme in place to ensure
      that tools only execute when the core version is one they are
      known to work with. This was done to minimise the chances of
      strange problems arising from tool-vs-core version
      incompatibilities.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.quiet" xreflabel="--quiet">
    <term><option>-q</option>, <option>--quiet</option></term>
    <listitem>
      <para>Run silently, and only print error messages. Useful if you
      are running regression tests or have some other automated test
      machinery.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.verbose" xreflabel="--verbose">
    <term><option>-v</option>, <option>--verbose</option></term>
    <listitem>
      <para>Be more verbose. Gives extra information on various aspects
      of your program, such as: the shared objects loaded, the
      suppressions used, the progress of the instrumentation and
      execution engines, and warnings about unusual behaviour. Repeating
      the option increases the verbosity level.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.trace-children" xreflabel="--trace-children">
    <term>
      <option><![CDATA[--trace-children=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will trace into sub-processes
      initiated via the <varname>exec</varname> system call.  This is
      necessary for multi-process programs.
      </para>
      <para>Note that Valgrind does trace into the child of a
      <varname>fork</varname> (it would be difficult not to, since
      <varname>fork</varname> makes an identical copy of a process), so this
      option is arguably badly named.  However, most children of
      <varname>fork</varname> calls immediately call <varname>exec</varname>
      anyway.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.trace-children-skip" xreflabel="--trace-children-skip">
    <term>
      <option><![CDATA[--trace-children-skip=patt1,patt2,... ]]></option>
    </term>
    <listitem>
      <para>This option only has an effect when 
        <option>--trace-children=yes</option> is specified.  It allows
        for some children to be skipped.  The option takes a comma
        separated list of patterns for the names of child executables
        that Valgrind should not trace into.  Patterns may include the
        metacharacters <computeroutput>?</computeroutput>
        and <computeroutput>*</computeroutput>, which have the usual
        meaning.</para>
      <para>
        This can be useful for pruning uninteresting branches from a
        tree of processes being run on Valgrind.  But you should be
        careful when using it.  When Valgrind skips tracing into an
        executable, it doesn't just skip tracing that executable, it
        also skips tracing any of that executable's child processes.
        In other words, the flag doesn't merely cause tracing to stop
        at the specified executables -- it skips tracing of entire
        process subtrees rooted at any of the specified
        executables.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.trace-children-skip-by-arg"
                xreflabel="--trace-children-skip-by-arg">
    <term>
      <option><![CDATA[--trace-children-skip-by-arg=patt1,patt2,... ]]></option>
    </term>
    <listitem>
      <para>This is the same as  
        <option>--trace-children-skip</option>, with one difference:
        the decision as to whether to trace into a child process is
        made by examining the arguments to the child process, rather
        than the name of its executable.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.child-silent-after-fork"
                xreflabel="--child-silent-after-fork">
    <term>
      <option><![CDATA[--child-silent-after-fork=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will not show any debugging or
      logging output for the child process resulting from
      a <varname>fork</varname> call.  This can make the output less
      confusing (although more misleading) when dealing with processes
      that create children.  It is particularly useful in conjunction
      with <varname>--trace-children=</varname>.  Use of this option is also
      strongly recommended if you are requesting XML output
      (<varname>--xml=yes</varname>), since otherwise the XML from child and
      parent may become mixed up, which usually makes it useless.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.vgdb" xreflabel="--vgdb">
    <term>
      <option><![CDATA[--vgdb=<no|yes|full> [default: yes] ]]></option>
    </term>
    <listitem>
      
      <para>Valgrind will provide "gdbserver" functionality when
      <option>--vgdb=yes</option> or <option>--vgdb=full</option> is
      specified.  This allows an external GNU GDB debugger to control
      and debug your program when it runs on Valgrind.
      <option>--vgdb=full</option> incurs significant performance
      overheads, but provides more precise breakpoints and
      watchpoints. See <xref linkend="manual-core-adv.gdbserver"/> for
      a detailed description.
      </para>

      <para> If the embedded gdbserver is enabled but no gdb is
      currently being used, the <xref linkend="manual-core-adv.vgdb"/>
      command line utility can send "monitor commands" to Valgrind
      from a shell.  The Valgrind core provides a set of
      <xref linkend="manual-core-adv.valgrind-monitor-commands"/>. A tool
      can optionally provide tool specific monitor commands, which are
      documented in the tool specific chapter.
      </para>

    </listitem>
  </varlistentry>

  <varlistentry id="opt.vgdb-error" xreflabel="--vgdb-error">
    <term>
      <option><![CDATA[--vgdb-error=<number> [default: 999999999] ]]></option>
    </term>
    <listitem>
      <para> Use this option when the Valgrind gdbserver is enabled with
      <option>--vgdb=yes</option> or <option>--vgdb=full</option>.
      Tools that report errors will wait
      for "<computeroutput>number</computeroutput>" errors to be
      reported before freezing the program and waiting for you to
      connect with GDB.  It follows that a value of zero will cause
      the gdbserver to be started before your program is executed.
      This is typically used to insert GDB breakpoints before
      execution, and also works with tools that do not report
      errors, such as Massif.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.track-fds" xreflabel="--track-fds">
    <term>
      <option><![CDATA[--track-fds=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will print out a list of open file
      descriptors on exit or on request, via the gdbserver monitor
      command <varname>v.info open_fds</varname>.  Along with each
      file descriptor is printed a stack backtrace of where the file
      was opened and any details relating to the file descriptor such
      as the file name or socket details.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.time-stamp" xreflabel="--time-stamp">
    <term>
      <option><![CDATA[--time-stamp=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, each message is preceded with an indication of
      the elapsed wallclock time since startup, expressed as days,
      hours, minutes, seconds and milliseconds.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.log-fd" xreflabel="--log-fd">
    <term>
      <option><![CDATA[--log-fd=<number> [default: 2, stderr] ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send all of its messages to
      the specified file descriptor.  The default, 2, is the standard
      error channel (stderr).  Note that this may interfere with the
      client's own use of stderr, as Valgrind's output will be
      interleaved with any output that the client sends to
      stderr.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.log-file" xreflabel="--log-file">
    <term>
      <option><![CDATA[--log-file=<filename> ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send all of its messages to
      the specified file.  If the file name is empty, it causes an abort.
      There are three special format specifiers that can be used in the file
      name.</para>

      <para><option>%p</option> is replaced with the current process ID.
      This is very useful for program that invoke multiple processes.
      WARNING: If you use <option>--trace-children=yes</option> and your
      program invokes multiple processes OR your program forks without
      calling exec afterwards, and you don't use this specifier
      (or the <option>%q</option> specifier below), the Valgrind output from
      all those processes will go into one file, possibly jumbled up, and
      possibly incomplete.</para>

      <para><option>%q{FOO}</option> is replaced with the contents of the
      environment variable <varname>FOO</varname>.  If the
      <option>{FOO}</option> part is malformed, it causes an abort.  This
      specifier is rarely needed, but very useful in certain circumstances
      (eg. when running MPI programs).  The idea is that you specify a
      variable which will be set differently for each process in the job,
      for example <computeroutput>BPROC_RANK</computeroutput> or whatever is
      applicable in your MPI setup.  If the named environment variable is not
      set, it causes an abort.  Note that in some shells, the
      <option>{</option> and <option>}</option> characters may need to be
      escaped with a backslash.</para>

      <para><option>%%</option> is replaced with <option>%</option>.</para>
      
      <para>If an <option>%</option> is followed by any other character, it
      causes an abort.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.log-socket" xreflabel="--log-socket">
    <term>
      <option><![CDATA[--log-socket=<ip-address:port-number> ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send all of its messages to
      the specified port at the specified IP address.  The port may be
      omitted, in which case port 1500 is used.  If a connection cannot
      be made to the specified socket, Valgrind falls back to writing
      output to the standard error (stderr).  This option is intended to
      be used in conjunction with the
      <computeroutput>valgrind-listener</computeroutput> program.  For
      further details, see 
      <link linkend="&vg-comment-id;">the commentary</link>
      in the manual.</para>
    </listitem>
  </varlistentry>

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

</sect2>


<sect2 id="manual-core.erropts" xreflabel="Error-related Options">
<title>Error-related Options</title>

<!-- start of xi:include in the manpage -->
<para id="error-related.opts.para">These options are used by all tools
that can report errors, e.g. Memcheck, but not Cachegrind.</para>

<variablelist id="error-related.opts.list">

  <varlistentry id="opt.xml" xreflabel="--xml">
    <term>
      <option><![CDATA[--xml=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, the important parts of the output (e.g. tool error
      messages) will be in XML format rather than plain text.  Furthermore,
      the XML output will be sent to a different output channel than the
      plain text output.  Therefore, you also must use one of
      <option>--xml-fd</option>, <option>--xml-file</option> or
      <option>--xml-socket</option> to specify where the XML is to be sent.
      </para>
      
      <para>Less important messages will still be printed in plain text, but
      because the XML output and plain text output are sent to different
      output channels (the destination of the plain text output is still
      controlled by <option>--log-fd</option>, <option>--log-file</option>
      and <option>--log-socket</option>) this should not cause problems.
      </para>

      <para>This option is aimed at making life easier for tools that consume
      Valgrind's output as input, such as GUI front ends.  Currently this
      option works with Memcheck, Helgrind, DRD and SGcheck.  The output
      format is specified in the file
      <computeroutput>docs/internals/xml-output-protocol4.txt</computeroutput>
      in the source tree for Valgrind 3.5.0 or later.</para>

      <para>The recommended options for a GUI to pass, when requesting
      XML output, are: <option>--xml=yes</option> to enable XML output,
      <option>--xml-file</option> to send the XML output to a (presumably
      GUI-selected) file, <option>--log-file</option> to send the plain
      text output to a second GUI-selected file,
      <option>--child-silent-after-fork=yes</option>, and
      <option>-q</option> to restrict the plain text output to critical
      error messages created by Valgrind itself.  For example, failure to
      read a specified suppressions file counts as a critical error message.
      In this way, for a successful run the text output file will be empty.
      But if it isn't empty, then it will contain important information
      which the GUI user should be made aware
      of.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.xml-fd" xreflabel="--xml-fd">
    <term>
      <option><![CDATA[--xml-fd=<number> [default: -1, disabled] ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send its XML output to the
      specified file descriptor.  It must be used in conjunction with
      <option>--xml=yes</option>.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.xml-file" xreflabel="--xml-file">
    <term>
      <option><![CDATA[--xml-file=<filename> ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send its XML output
      to the specified file.  It must be used in conjunction with
      <option>--xml=yes</option>.  Any <option>%p</option> or
      <option>%q</option> sequences appearing in the filename are expanded
      in exactly the same way as they are for <option>--log-file</option>.
      See the description of <option>--log-file</option> for details.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.xml-socket" xreflabel="--xml-socket">
    <term>
      <option><![CDATA[--xml-socket=<ip-address:port-number> ]]></option>
    </term>
    <listitem>
      <para>Specifies that Valgrind should send its XML output the
      specified port at the specified IP address.  It must be used in
      conjunction with <option>--xml=yes</option>.  The form of the argument
      is the same as that used by <option>--log-socket</option>.
      See the description of <option>--log-socket</option>
      for further details.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.xml-user-comment" xreflabel="--xml-user-comment">
    <term>
      <option><![CDATA[--xml-user-comment=<string> ]]></option>
    </term>
    <listitem>
      <para>Embeds an extra user comment string at the start of the XML
      output.  Only works when <option>--xml=yes</option> is specified;
      ignored otherwise.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.demangle" xreflabel="--demangle">
    <term>
      <option><![CDATA[--demangle=<yes|no> [default: yes] ]]></option>
    </term>
    <listitem>
      <para>Enable/disable automatic demangling (decoding) of C++ names.
      Enabled by default.  When enabled, Valgrind will attempt to
      translate encoded C++ names back to something approaching the
      original.  The demangler handles symbols mangled by g++ versions
      2.X, 3.X and 4.X.</para>

      <para>An important fact about demangling is that function names
      mentioned in suppressions files should be in their mangled form.
      Valgrind does not demangle function names when searching for
      applicable suppressions, because to do otherwise would make
      suppression file contents dependent on the state of Valgrind's
      demangling machinery, and also slow down suppression matching.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.num-callers" xreflabel="--num-callers">
    <term>
      <option><![CDATA[--num-callers=<number> [default: 12] ]]></option>
    </term>
    <listitem>
      <para>Specifies the maximum number of entries shown in stack traces
      that identify program locations.  Note that errors are commoned up
      using only the top four function locations (the place in the current
      function, and that of its three immediate callers).  So this doesn't
      affect the total number of errors reported.</para>

      <para>The maximum value for this is 500. Note that higher settings
      will make Valgrind run a bit more slowly and take a bit more
      memory, but can be useful when working with programs with
      deeply-nested call chains.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.unw-stack-scan-thresh"
                xreflabel="--unw-stack-scan-thresh">
    <term>
      <option><![CDATA[--unw-stack-scan-thresh=<number> [default: 0] ]]></option>
    </term>
    <term>
      <option><![CDATA[--unw-stack-scan-frames=<number> [default: 5] ]]></option>
    </term>
    <listitem>
      <para>Stack-scanning support is available only on ARM
      targets.</para>

      <para>These flags enable and control stack unwinding by stack
      scanning.  When the normal stack unwinding mechanisms -- usage
      of Dwarf CFI records, and frame-pointer following -- fail, stack
      scanning may be able to recover a stack trace.</para>

      <para>Note that stack scanning is an imprecise, heuristic
      mechanism that may give very misleading results, or none at all.
      It should be used only in emergencies, when normal unwinding
      fails, and it is important to nevertheless have stack
      traces.</para>

      <para>Stack scanning is a simple technique: the unwinder reads
      words from the stack, and tries to guess which of them might be
      return addresses, by checking to see if they point just after
      ARM or Thumb call instructions.  If so, the word is added to the
      backtrace.</para>

      <para>The main danger occurs when a function call returns,
      leaving its return address exposed, and a new function is
      called, but the new function does not overwrite the old address.
      The result of this is that the backtrace may contain entries for
      functions which have already returned, and so be very
      confusing.</para>

      <para>A second limitation of this implementation is that it will
      scan only the page (4KB, normally) containing the starting stack
      pointer.  If the stack frames are large, this may result in only
      a few (or not even any) being present in the trace.  Also, if
      you are unlucky and have an initial stack pointer near the end
      of its containing page, the scan may miss all interesting
      frames.</para>

      <para>By default stack scanning is disabled.  The normal use
      case is to ask for it when a stack trace would otherwise be very
      short.  So, to enable it,
      use <computeroutput>--unw-stack-scan-thresh=number</computeroutput>.
      This requests Valgrind to try using stack scanning to "extend"
      stack traces which contain fewer
      than <computeroutput>number</computeroutput> frames.</para>

      <para>If stack scanning does take place, it will only generate
      at most the number of frames specified
      by <computeroutput>--unw-stack-scan-frames</computeroutput>.
      Typically, stack scanning generates so many garbage entries that
      this value is set to a low value (5) by default.  In no case
      will a stack trace larger than the value specified
      by <computeroutput>--num-callers</computeroutput> be
      created.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.error-limit" xreflabel="--error-limit">
    <term>
      <option><![CDATA[--error-limit=<yes|no> [default: yes] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind stops reporting errors after 10,000,000
      in total, or 1,000 different ones, have been seen.  This is to
      stop the error tracking machinery from becoming a huge performance
      overhead in programs with many errors.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.error-exitcode" xreflabel="--error-exitcode">
    <term>
      <option><![CDATA[--error-exitcode=<number> [default: 0] ]]></option>
    </term>
    <listitem>
      <para>Specifies an alternative exit code to return if Valgrind
      reported any errors in the run.  When set to the default value
      (zero), the return value from Valgrind will always be the return 
      value of the process being simulated.  When set to a nonzero value,
      that value is returned instead, if Valgrind detects any errors.
      This is useful for using Valgrind as part of an automated test
      suite, since it makes it easy to detect test cases for which
      Valgrind has reported errors, just by inspecting return codes.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.sigill-diagnostics" xreflabel="--sigill-diagnostics">
    <term>
      <option><![CDATA[--sigill-diagnostics=<yes|no> [default: yes] ]]></option>
    </term>
    <listitem>
      <para>Enable/disable printing of illegal instruction diagnostics.
      Enabled by default, but defaults to disabled when
      <option>--quiet</option> is given. The default can always be explicitly
      overridden by giving this option.</para>

      <para>When enabled, a warning message will be printed, along with some
      diagnostics, whenever an instruction is encountered that Valgrind
      cannot decode or translate, before the program is given a SIGILL signal.
      Often an illegal instruction indicates a bug in the program or missing
      support for the particular instruction in Valgrind.  But some programs
      do deliberately try to execute an instruction that might be missing
      and trap the SIGILL signal to detect processor features.  Using
      this flag makes it possible to avoid the diagnostic output
      that you would otherwise get in such cases.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.show-below-main" xreflabel="--show-below-main">
    <term>
      <option><![CDATA[--show-below-main=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>By default, stack traces for errors do not show any
      functions that appear beneath <function>main</function> because
      most of the time it's uninteresting C library stuff and/or
      gobbledygook.  Alternatively, if <function>main</function> is not
      present in the stack trace, stack traces will not show any functions
      below <function>main</function>-like functions such as glibc's
      <function>__libc_start_main</function>.   Furthermore, if
      <function>main</function>-like functions are present in the trace,
      they are normalised as <function>(below main)</function>, in order to
      make the output more deterministic.</para>
      
      <para>If this option is enabled, all stack trace entries will be
      shown and <function>main</function>-like functions will not be
      normalised.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.fullpath-after" xreflabel="--fullpath-after">
    <term>
      <option><![CDATA[--fullpath-after=<string>
              [default: don't show source paths] ]]></option>
    </term>
    <listitem>
      <para>By default Valgrind only shows the filenames in stack
      traces, but not full paths to source files.  When using Valgrind
      in large projects where the sources reside in multiple different
      directories, this can be inconvenient.
      <option>--fullpath-after</option> provides a flexible solution
      to this problem.  When this option is present, the path to each
      source file is shown, with the following all-important caveat:
      if <option>string</option> is found in the path, then the path
      up to and including <option>string</option> is omitted, else the
      path is shown unmodified.  Note that <option>string</option> is
      not required to be a prefix of the path.</para>

      <para>For example, consider a file named
      <computeroutput>/home/janedoe/blah/src/foo/bar/xyzzy.c</computeroutput>.
      Specifying <option>--fullpath-after=/home/janedoe/blah/src/</option>
      will cause Valgrind to show the name
      as <computeroutput>foo/bar/xyzzy.c</computeroutput>.</para>

      <para>Because the string is not required to be a prefix,
      <option>--fullpath-after=src/</option> will produce the same
      output.  This is useful when the path contains arbitrary
      machine-generated characters.  For example, the
      path
      <computeroutput>/my/build/dir/C32A1B47/blah/src/foo/xyzzy</computeroutput>
      can be pruned to <computeroutput>foo/xyzzy</computeroutput>
      using
      <option>--fullpath-after=/blah/src/</option>.</para>

      <para>If you simply want to see the full path, just specify an
      empty string: <option>--fullpath-after=</option>.  This isn't a
      special case, merely a logical consequence of the above rules.</para>

      <para>Finally, you can use <option>--fullpath-after</option>
      multiple times.  Any appearance of it causes Valgrind to switch
      to producing full paths and applying the above filtering rule.
      Each produced path is compared against all
      the <option>--fullpath-after</option>-specified strings, in the
      order specified.  The first string to match causes the path to
      be truncated as described above.  If none match, the full path
      is shown.  This facilitates chopping off prefixes when the
      sources are drawn from a number of unrelated directories.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.extra-debuginfo-path" xreflabel="--extra-debuginfo-path">
    <term>
      <option><![CDATA[--extra-debuginfo-path=<path> [default: undefined and unused] ]]></option>
    </term>
    <listitem>
      <para>By default Valgrind searches in several well-known paths
      for debug objects, such
      as <computeroutput>/usr/lib/debug/</computeroutput>.</para>

      <para>However, there may be scenarios where you may wish to put
      debug objects at an arbitrary location, such as external storage
      when running Valgrind on a mobile device with limited local
      storage.  Another example might be a situation where you do not
      have permission to install debug object packages on the system
      where you are running Valgrind.</para>

      <para>In these scenarios, you may provide an absolute path as an extra,
      final place for Valgrind to search for debug objects by specifying
      <option>--extra-debuginfo-path=/path/to/debug/objects</option>.
      The given path will be prepended to the absolute path name of
      the searched-for object.  For example, if Valgrind is looking
      for the debuginfo
      for <computeroutput>/w/x/y/zz.so</computeroutput>
      and <option>--extra-debuginfo-path=/a/b/c</option> is specified,
      it will look for a debug object at
      <computeroutput>/a/b/c/w/x/y/zz.so</computeroutput>.</para>

      <para>This flag should only be specified once.  If it is
      specified multiple times, only the last instance is
      honoured.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.debuginfo-server" xreflabel="--debuginfo-server">
    <term>
      <option><![CDATA[--debuginfo-server=ipaddr:port [default: undefined and unused]]]></option>
    </term>
    <listitem>
      <para>This is a new, experimental, feature introduced in version
      3.9.0.</para>

      <para>In some scenarios it may be convenient to read debuginfo
      from objects stored on a different machine.  With this flag,
      Valgrind will query a debuginfo server running
      on <computeroutput>ipaddr</computeroutput> and listening on
      port <computeroutput>port</computeroutput>, if it cannot find
      the debuginfo object in the local filesystem.</para>

      <para>The debuginfo server must accept TCP connections on
      port <computeroutput>port</computeroutput>.  The debuginfo
      server is contained in the source
      file <computeroutput>auxprogs/valgrind-di-server.c</computeroutput>.
      It will only serve from the directory it is started
      in.  <computeroutput>port</computeroutput> defaults to 1500 in
      both client and server if not specified.</para>

      <para>If Valgrind looks for the debuginfo for
      <computeroutput>/w/x/y/zz.so</computeroutput> by using the
      debuginfo server, it will strip the pathname components and
      merely request <computeroutput>zz.so</computeroutput> on the
      server.  That in turn will look only in its current working
      directory for a matching debuginfo object.</para>

      <para>The debuginfo data is transmitted in small fragments (8
      KB) as requested by Valgrind.  Each block is compressed using
      LZO to reduce transmission time.  The implementation has been
      tuned for best performance over a single-stage 802.11g (WiFi)
      network link.</para>

      <para>Note that checks for matching primary vs debug objects,
      using GNU debuglink CRC scheme, are performed even when using
      the debuginfo server.  To disable such checking, you need to
      also specify
      <computeroutput>--allow-mismatched-debuginfo=yes</computeroutput>.
      </para>

      <para>By default the Valgrind build system will
      build <computeroutput>valgrind-di-server</computeroutput> for
      the target platform, which is almost certainly not what you
      want.  So far we have been unable to find out how to get
      automake/autoconf to build it for the build platform.  If
      you want to use it, you will have to recompile it by hand using
      the command shown at the top
      of <computeroutput>auxprogs/valgrind-di-server.c</computeroutput>.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.allow-mismatched-debuginfo"
                xreflabel="--allow-mismatched-debuginfo">
    <term>
      <option><![CDATA[--allow-mismatched-debuginfo=no|yes [no] ]]></option>
    </term>
    <listitem>
      <para>When reading debuginfo from separate debuginfo objects,
      Valgrind will by default check that the main and debuginfo
      objects match, using the GNU debuglink mechanism.  This
      guarantees that it does not read debuginfo from out of date
      debuginfo objects, and also ensures that Valgrind can't crash as
      a result of mismatches.</para>

      <para>This check can be overridden using 
      <computeroutput>--allow-mismatched-debuginfo=yes</computeroutput>.
      This may be useful when the debuginfo and main objects have not
      been split in the proper way.  Be careful when using this,
      though: it disables all consistency checking, and Valgrind has
      been observed to crash when the main and debuginfo objects don't
      match.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.suppressions" xreflabel="--suppressions">
    <term>
      <option><![CDATA[--suppressions=<filename> [default: $PREFIX/lib/valgrind/default.supp] ]]></option>
    </term>
    <listitem>
      <para>Specifies an extra file from which to read descriptions of
      errors to suppress.  You may use up to 100 extra suppression
      files.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.gen-suppressions" xreflabel="--gen-suppressions">
    <term>
      <option><![CDATA[--gen-suppressions=<yes|no|all> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When set to <varname>yes</varname>, Valgrind will pause
      after every error shown and print the line:
      <literallayout><computeroutput>    ---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----</computeroutput></literallayout>

      The prompt's behaviour is the same as for the
      <option>--db-attach</option> option (see below).</para>

      <para>If you choose to, Valgrind will print out a suppression for
      this error.  You can then cut and paste it into a suppression file
      if you don't want to hear about the error in the future.</para>

      <para>When set to <varname>all</varname>, Valgrind will print a
      suppression for every reported error, without querying the
      user.</para>

      <para>This option is particularly useful with C++ programs, as it
      prints out the suppressions with mangled names, as
      required.</para>

      <para>Note that the suppressions printed are as specific as
      possible.  You may want to common up similar ones, by adding
      wildcards to function names, and by using frame-level wildcards.
      The wildcarding facilities are powerful yet flexible, and with a
      bit of careful editing, you may be able to suppress a whole
      family of related errors with only a few suppressions.  
      <!-- commented out because it causes broken links in the man page
      For details on how to do this, see
      <xref linkend="manual-core.suppress"/>.
      -->
      </para>

      <para>Sometimes two different errors
      are suppressed by the same suppression, in which case Valgrind
      will output the suppression more than once, but you only need to
      have one copy in your suppression file (but having more than one
      won't cause problems).  Also, the suppression name is given as
      <computeroutput>&lt;insert a suppression name
      here&gt;</computeroutput>; the name doesn't really matter, it's
      only used with the <option>-v</option> option which prints out all
      used suppression records.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.db-attach" xreflabel="--db-attach">
    <term>
      <option><![CDATA[--db-attach=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will pause after every error shown
      and print the line:
      <literallayout><computeroutput>    ---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ----</computeroutput></literallayout>

      Pressing <varname>Ret</varname>, or <varname>N Ret</varname> or
      <varname>n Ret</varname>, causes Valgrind not to start a debugger
      for this error.</para>

      <para>Pressing <varname>Y Ret</varname> or
      <varname>y Ret</varname> causes Valgrind to start a debugger for
      the program at this point. When you have finished with the
      debugger, quit from it, and the program will continue. Trying to
      continue from inside the debugger doesn't work.</para>

      <para>
      Note: if you use GDB, more powerful debugging support is
      provided by the <option>--vgdb=</option> <varname>yes</varname>
      or <varname>full</varname> value.  This activates Valgrind's
      internal gdbserver, which provides more-or-less full GDB-style
      control of the application: insertion of breakpoints, continuing 
      from inside GDB, inferior function calls, and much more.
      </para> 

      <para><varname>C Ret</varname> or <varname>c Ret</varname> causes
      Valgrind not to start a debugger, and not to ask again.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.db-command" xreflabel="--db-command">
    <term>
      <option><![CDATA[--db-command=<command> [default: gdb -nw %f %p] ]]></option>
    </term>
    <listitem>
      <para>Specify the debugger to use with the
      <option>--db-attach</option> command. The default debugger is
      GDB. This option is a template that is expanded by Valgrind at
      runtime.  <literal>%f</literal> is replaced with the executable's
      file name and <literal>%p</literal> is replaced by the process ID
      of the executable.</para>

      <para>This specifies how Valgrind will invoke the debugger.  By
      default it will use whatever GDB is detected at build time, which
      is usually <computeroutput>/usr/bin/gdb</computeroutput>.  Using
      this command, you can specify some alternative command to invoke
      the debugger you want to use.</para>

      <para>The command string given can include one or instances of the
      <literal>%p</literal> and <literal>%f</literal> expansions. Each
      instance of <literal>%p</literal> expands to the PID of the
      process to be debugged and each instance of <literal>%f</literal>
      expands to the path to the executable for the process to be
      debugged.</para>

      <para>Since <computeroutput>&lt;command&gt;</computeroutput> is likely
      to contain spaces, you will need to put this entire option in
      quotes to ensure it is correctly handled by the shell.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.input-fd" xreflabel="--input-fd">
    <term>
      <option><![CDATA[--input-fd=<number> [default: 0, stdin] ]]></option>
    </term>
    <listitem>
      <para>When using <option>--db-attach=yes</option> or
      <option>--gen-suppressions=yes</option>, Valgrind will stop so as
      to read keyboard input from you when each error occurs.  By
      default it reads from the standard input (stdin), which is
      problematic for programs which close stdin.  This option allows
      you to specify an alternative file descriptor from which to read
      input.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.dsymutil" xreflabel="--dsymutil">
    <term>
      <option><![CDATA[--dsymutil=no|yes [no] ]]></option>
    </term>
    <listitem>
      <para>This option is only relevant when running Valgrind on
      Mac OS X.</para>

      <para>Mac OS X uses a deferred debug information (debuginfo)
      linking scheme.  When object files containing debuginfo are
      linked into a <computeroutput>.dylib</computeroutput> or an
      executable, the debuginfo is not copied into the final file.
      Instead, the debuginfo must be linked manually by
      running <computeroutput>dsymutil</computeroutput>, a
      system-provided utility, on the executable
      or <computeroutput>.dylib</computeroutput>.  The resulting
      combined debuginfo is placed in a directory alongside the
      executable or <computeroutput>.dylib</computeroutput>, but with
      the extension <computeroutput>.dSYM</computeroutput>.</para>

      <para>With <option>--dsymutil=no</option>, Valgrind
      will detect cases where the
      <computeroutput>.dSYM</computeroutput> directory is either
      missing, or is present but does not appear to match the
      associated executable or <computeroutput>.dylib</computeroutput>,
      most likely because it is out of date.  In these cases, Valgrind
      will print a warning message but take no further action.</para>

      <para>With <option>--dsymutil=yes</option>, Valgrind
      will, in such cases, automatically
      run <computeroutput>dsymutil</computeroutput> as necessary to
      bring the debuginfo up to date.  For all practical purposes, if
      you always use <option>--dsymutil=yes</option>, then
      there is never any need to
      run <computeroutput>dsymutil</computeroutput> manually or as part
      of your applications's build system, since Valgrind will run it
      as necessary.</para>

      <para>Valgrind will not attempt to
      run <computeroutput>dsymutil</computeroutput> on any 
      executable or library in
      <computeroutput>/usr/</computeroutput>,
      <computeroutput>/bin/</computeroutput>,
      <computeroutput>/sbin/</computeroutput>,
      <computeroutput>/opt/</computeroutput>,
      <computeroutput>/sw/</computeroutput>,
      <computeroutput>/System/</computeroutput>,
      <computeroutput>/Library/</computeroutput> or
      <computeroutput>/Applications/</computeroutput>
      since <computeroutput>dsymutil</computeroutput> will always fail
      in such situations.  It fails both because the debuginfo for
      such pre-installed system components is not available anywhere,
      and also because it would require write privileges in those
      directories.</para>

      <para>Be careful when
      using <option>--dsymutil=yes</option>, since it will
      cause pre-existing <computeroutput>.dSYM</computeroutput>
      directories to be silently deleted and re-created.  Also note that
      <computeroutput>dsymutil</computeroutput> is quite slow, sometimes
      excessively so.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.max-stackframe" xreflabel="--max-stackframe">
    <term>
      <option><![CDATA[--max-stackframe=<number> [default: 2000000] ]]></option>
    </term>
    <listitem>
      <para>The maximum size of a stack frame.  If the stack pointer moves by
      more than this amount then Valgrind will assume that
      the program is switching to a different stack.</para>

      <para>You may need to use this option if your program has large
      stack-allocated arrays.  Valgrind keeps track of your program's
      stack pointer.  If it changes by more than the threshold amount,
      Valgrind assumes your program is switching to a different stack,
      and Memcheck behaves differently than it would for a stack pointer
      change smaller than the threshold.  Usually this heuristic works
      well.  However, if your program allocates large structures on the
      stack, this heuristic will be fooled, and Memcheck will
      subsequently report large numbers of invalid stack accesses.  This
      option allows you to change the threshold to a different
      value.</para>

      <para>You should only consider use of this option if Valgrind's
      debug output directs you to do so.  In that case it will tell you
      the new threshold you should specify.</para>

      <para>In general, allocating large structures on the stack is a
      bad idea, because you can easily run out of stack space,
      especially on systems with limited memory or which expect to
      support large numbers of threads each with a small stack, and also
      because the error checking performed by Memcheck is more effective
      for heap-allocated data than for stack-allocated data.  If you
      have to use this option, you may wish to consider rewriting your
      code to allocate on the heap rather than on the stack.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.main-stacksize" xreflabel="--main-stacksize">
    <term>
      <option><![CDATA[--main-stacksize=<number>
               [default: use current 'ulimit' value] ]]></option>
    </term>
    <listitem>
      <para>Specifies the size of the main thread's stack.</para>

      <para>To simplify its memory management, Valgrind reserves all
      required space for the main thread's stack at startup.  That
      means it needs to know the required stack size at
      startup.</para>

      <para>By default, Valgrind uses the current "ulimit" value for
      the stack size, or 16 MB, whichever is lower.  In many cases
      this gives a stack size in the range 8 to 16 MB, which almost
      never overflows for most applications.</para>

      <para>If you need a larger total stack size,
      use <option>--main-stacksize</option> to specify it.  Only set
      it as high as you need, since reserving far more space than you
      need (that is, hundreds of megabytes more than you need)
      constrains Valgrind's memory allocators and may reduce the total
      amount of memory that Valgrind can use.  This is only really of
      significance on 32-bit machines.</para>

      <para>On Linux, you may request a stack of size up to 2GB.
      Valgrind will stop with a diagnostic message if the stack cannot
      be allocated.</para>

      <para><option>--main-stacksize</option> only affects the stack
      size for the program's initial thread.  It has no bearing on the
      size of thread stacks, as Valgrind does not allocate
      those.</para>

      <para>You may need to use both <option>--main-stacksize</option>
      and <option>--max-stackframe</option> together.  It is important
      to understand that <option>--main-stacksize</option> sets the
      maximum total stack size,
      whilst <option>--max-stackframe</option> specifies the largest
      size of any one stack frame.  You will have to work out
      the <option>--main-stacksize</option> value for yourself
      (usually, if your applications segfaults).  But Valgrind will
      tell you the needed <option>--max-stackframe</option> size, if
      necessary.</para>

      <para>As discussed further in the description
      of <option>--max-stackframe</option>, a requirement for a large
      stack is a sign of potential portability problems.  You are best
      advised to place all large data in heap-allocated memory.</para>
    </listitem>
  </varlistentry>

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

</sect2>


<sect2 id="manual-core.mallocopts" xreflabel="malloc-related Options">
<title>malloc-related Options</title>

<!-- start of xi:include in the manpage -->
<para id="malloc-related.opts.para">For tools that use their own version of
<computeroutput>malloc</computeroutput> (e.g. Memcheck,
Massif, Helgrind, DRD), the following options apply.</para>

<variablelist id="malloc-related.opts.list">

  <varlistentry id="opt.alignment" xreflabel="--alignment">
    <term>
      <option><![CDATA[--alignment=<number> [default: 8 or 16, depending on the platform] ]]></option>
    </term>
    <listitem>
      <para>By default Valgrind's <function>malloc</function>,
      <function>realloc</function>, etc, return a block whose starting
      address is 8-byte aligned or 16-byte aligned (the value depends on the
      platform and matches the platform default).  This option allows you to
      specify a different alignment.  The supplied value must be greater
      than or equal to the default, less than or equal to 4096, and must be
      a power of two.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.redzone-size" xreflabel="--redzone-size">
    <term>
      <option><![CDATA[--redzone-size=<number> [default: depends on the tool] ]]></option>
    </term>
    <listitem>
      <para> Valgrind's <function>malloc, realloc,</function> etc, add
      padding blocks before and after each heap block allocated by the
      program being run. Such padding blocks are called redzones.  The
      default value for the redzone size depends on the tool.  For
      example, Memcheck adds and protects a minimum of 16 bytes before
      and after each block allocated by the client.  This allows it to
      detect block underruns or overruns of up to 16 bytes.
      </para>
      <para>Increasing the redzone size makes it possible to detect
      overruns of larger distances, but increases the amount of memory
      used by Valgrind.  Decreasing the redzone size will reduce the
      memory needed by Valgrind but also reduces the chances of
      detecting over/underruns, so is not recommended.</para>
    </listitem>
  </varlistentry>

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

</sect2>


<sect2 id="manual-core.rareopts" xreflabel="Uncommon Options">
<title>Uncommon Options</title>

<!-- start of xi:include in the manpage -->
<para id="uncommon.opts.para">These options apply to all tools, as they
affect certain obscure workings of the Valgrind core.  Most people won't
need to use them.</para>

<variablelist id="uncommon.opts.list">

  <varlistentry id="opt.smc-check" xreflabel="--smc-check">
    <term>
      <option><![CDATA[--smc-check=<none|stack|all|all-non-file> [default: stack] ]]></option>
    </term>
    <listitem>
      <para>This option controls Valgrind's detection of self-modifying
      code.  If no checking is done, if a program executes some code, then
      overwrites it with new code, and executes the new code, Valgrind will
      continue to execute the translations it made for the old code.  This
      will likely lead to incorrect behaviour and/or crashes.</para>
      
      <para>Valgrind has four levels of self-modifying code detection:
      no detection, detect self-modifying code on the stack (which is used by
      GCC to implement nested functions), detect self-modifying code
      everywhere, and detect self-modifying code everywhere except in
      file-backed mappings.

      Note that the default option will catch the vast majority
      of cases.  The main case it will not catch is programs such as JIT
      compilers that dynamically generate code <emphasis>and</emphasis>
      subsequently overwrite part or all of it.  Running with
      <varname>all</varname> will slow Valgrind down noticeably.
      Running with
      <varname>none</varname> will rarely speed things up, since very little
      code gets put on the stack for most programs.  The
      <function>VALGRIND_DISCARD_TRANSLATIONS</function> client
      request is an alternative to <option>--smc-check=all</option>
      that requires more programmer effort but allows Valgrind to run
      your program faster, by telling it precisely when translations
      need to be re-made.
      <!-- commented out because it causes broken links in the man page
      ;  see <xref
      linkend="manual-core-adv.clientreq"/> for more details.
      -->
      </para>

      <para><option>--smc-check=all-non-file</option> provides a
      cheaper but more limited version
      of <option>--smc-check=all</option>.  It adds checks to any
      translations that do not originate from file-backed memory
      mappings.  Typical applications that generate code, for example
      JITs in web browsers, generate code into anonymous mmaped areas,
      whereas the "fixed" code of the browser always lives in
      file-backed mappings.  <option>--smc-check=all-non-file</option>
      takes advantage of this observation, limiting the overhead of
      checking to code which is likely to be JIT generated.</para>

      <para>Some architectures (including ppc32, ppc64, ARM and MIPS)
      require programs which create code at runtime to flush the
      instruction cache in between code generation and first use.
      Valgrind observes and honours such instructions.  Hence, on
      ppc32/Linux, ppc64/Linux and ARM/Linux, Valgrind always provides
      complete, transparent support for self-modifying code.  It is
      only on platforms such as x86/Linux, AMD64/Linux, x86/Darwin and
      AMD64/Darwin that you need to use this option.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.read-var-info" xreflabel="--read-var-info">
    <term>
      <option><![CDATA[--read-var-info=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will read information about
      variable types and locations from DWARF3 debug info.
      This slows Valgrind down and makes it use more memory, but for
      the tools that can take advantage of it (Memcheck, Helgrind,
      DRD) it can result in more precise error messages.  For example,
      here are some standard errors issued by Memcheck:</para>
<programlisting><![CDATA[
==15516== Uninitialised byte(s) found during client check request
==15516==    at 0x400633: croak (varinfo1.c:28)
==15516==    by 0x4006B2: main (varinfo1.c:55)
==15516==  Address 0x60103b is 7 bytes inside data symbol "global_i2"
==15516== 
==15516== Uninitialised byte(s) found during client check request
==15516==    at 0x400633: croak (varinfo1.c:28)
==15516==    by 0x4006BC: main (varinfo1.c:56)
==15516==  Address 0x7fefffefc is on thread 1's stack]]></programlisting>

      <para>And here are the same errors with
      <option>--read-var-info=yes</option>:</para>

<programlisting><![CDATA[
==15522== Uninitialised byte(s) found during client check request
==15522==    at 0x400633: croak (varinfo1.c:28)
==15522==    by 0x4006B2: main (varinfo1.c:55)
==15522==  Location 0x60103b is 0 bytes inside global_i2[7],
==15522==  a global variable declared at varinfo1.c:41
==15522== 
==15522== Uninitialised byte(s) found during client check request
==15522==    at 0x400633: croak (varinfo1.c:28)
==15522==    by 0x4006BC: main (varinfo1.c:56)
==15522==  Location 0x7fefffefc is 0 bytes inside local var "local"
==15522==  declared at varinfo1.c:46, in frame #1 of thread 1]]></programlisting>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.vgdb-poll" xreflabel="--vgdb-poll">
    <term>
      <option><![CDATA[--vgdb-poll=<number> [default: 5000] ]]></option>
    </term>
    <listitem>
      <para> As part of its main loop, the Valgrind scheduler will
      poll to check if some activity (such as an external command or
      some input from a gdb) has to be handled by gdbserver.  This
      activity poll will be done after having run the given number of
      basic blocks (or slightly more than the given number of basic
      blocks). This poll is quite cheap so the default value is set
      relatively low. You might further decrease this value if vgdb
      cannot use ptrace system call to interrupt Valgrind if all
      threads are (most of the time) blocked in a system call.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.vgdb-shadow-registers" xreflabel="--vgdb-shadow-registers">
    <term>
      <option><![CDATA[--vgdb-shadow-registers=no|yes [default: no] ]]></option>
    </term>
    <listitem>
      <para> When activated, gdbserver will expose the Valgrind shadow registers
      to GDB. With this, the value of the Valgrind shadow registers can be examined
      or changed using GDB. Exposing shadow registers only works with GDB version
      7.1 or later.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.vgdb-prefix" xreflabel="--vgdb-prefix">
    <term>
      <option><![CDATA[--vgdb-prefix=<prefix> [default: /tmp/vgdb-pipe] ]]></option>
    </term>
    <listitem>
      <para> To communicate with gdb/vgdb, the Valgrind gdbserver
      creates 3 files (2 named FIFOs and a mmap shared memory
      file). The prefix option controls the directory and prefix for
      the creation of these files.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.run-libc-freeres" xreflabel="--run-libc-freeres">
    <term>
      <option><![CDATA[--run-libc-freeres=<yes|no> [default: yes] ]]></option>
    </term>
    <listitem>
      <para>This option is only relevant when running Valgrind on Linux.</para>

      <para>The GNU C library (<function>libc.so</function>), which is
      used by all programs, may allocate memory for its own uses.
      Usually it doesn't bother to free that memory when the program
      ends&mdash;there would be no point, since the Linux kernel reclaims
      all process resources when a process exits anyway, so it would
      just slow things down.</para>

      <para>The glibc authors realised that this behaviour causes leak
      checkers, such as Valgrind, to falsely report leaks in glibc, when
      a leak check is done at exit.  In order to avoid this, they
      provided a routine called <function>__libc_freeres</function>
      specifically to make glibc release all memory it has allocated.
      Memcheck therefore tries to run
      <function>__libc_freeres</function> at exit.</para>

      <para>Unfortunately, in some very old versions of glibc,
      <function>__libc_freeres</function> is sufficiently buggy to cause
      segmentation faults.  This was particularly noticeable on Red Hat
      7.1.  So this option is provided in order to inhibit the run of
      <function>__libc_freeres</function>.  If your program seems to run
      fine on Valgrind, but segfaults at exit, you may find that
      <option>--run-libc-freeres=no</option> fixes that, although at the
      cost of possibly falsely reporting space leaks in
      <filename>libc.so</filename>.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.sim-hints" xreflabel="--sim-hints">
    <term>
      <option><![CDATA[--sim-hints=hint1,hint2,... ]]></option>
    </term>
    <listitem>
      <para>Pass miscellaneous hints to Valgrind which slightly modify
      the simulated behaviour in nonstandard or dangerous ways, possibly
      to help the simulation of strange features.  By default no hints
      are enabled.  Use with caution!  Currently known hints are:</para>
      <itemizedlist>
        <listitem>
          <para><option>lax-ioctls: </option> Be very lax about ioctl
          handling; the only assumption is that the size is
          correct. Doesn't require the full buffer to be initialized
          when writing.  Without this, using some device drivers with a
          large number of strange ioctl commands becomes very
          tiresome.</para>
        </listitem>
        <listitem>
          <para><option>enable-outer: </option> Enable some special
          magic needed when the program being run is itself
          Valgrind.</para>
        </listitem>
        <listitem>
          <para><option>no-inner-prefix: </option> Disable printing
          a prefix <option>&gt;</option> in front of each stdout or
          stderr output line in an inner Valgrind being run by an
          outer Valgrind. This is useful when running Valgrind
          regression tests in an outer/inner setup. Note that the
          prefix <option>&gt;</option> will always be printed in
          front of the inner debug logging lines.</para>
        </listitem>
        <listitem>
          <para><option>fuse-compatible: </option> Enable special
            handling for certain system calls that may block in a FUSE
            file-system.  This may be necessary when running Valgrind
            on a multi-threaded program that uses one thread to manage
            a FUSE file-system and another thread to access that
            file-system.
          </para>
        </listitem>
      </itemizedlist>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.fair-sched" xreflabel="--fair-sched">
    <term>
      <option><![CDATA[--fair-sched=<no|yes|try>    [default: no] ]]></option>
    </term>

    <listitem> <para>The <option>--fair-sched</option> option controls
      the locking mechanism used by Valgrind to serialise thread
      execution.  The locking mechanism controls the way the threads
      are scheduled, and different settings give different trade-offs
      between fairness and performance. For more details about the
      Valgrind thread serialisation scheme and its impact on
      performance and thread scheduling, see
      <xref linkend="&vg-pthreads-perf-sched-id;"/>.</para>

      <itemizedlist>
        <listitem> <para>The value <option>--fair-sched=yes</option>
          activates a fair scheduler.  In short, if multiple threads are
          ready to run, the threads will be scheduled in a round robin
          fashion.  This mechanism is not available on all platforms or
          Linux versions.  If not available,
          using <option>--fair-sched=yes</option> will cause Valgrind to
          terminate with an error.</para>
        <para>You may find this setting improves overall
          responsiveness if you are running an interactive
          multithreaded program, for example a web browser, on
          Valgrind.</para>
        </listitem>
        
        <listitem> <para>The value <option>--fair-sched=try</option>
          activates fair scheduling if available on the
          platform.  Otherwise, it will automatically fall back
          to <option>--fair-sched=no</option>.</para>
        </listitem>
        
        <listitem> <para>The value <option>--fair-sched=no</option> activates
          a scheduler which does not guarantee fairness
          between threads ready to run, but which in general gives the
         highest performance.</para>
        </listitem>
      </itemizedlist>
    </listitem>

  </varlistentry>

  <varlistentry id="opt.kernel-variant" xreflabel="--kernel-variant">
    <term>
      <option>--kernel-variant=variant1,variant2,...</option>
    </term>
    <listitem>
      <para>Handle system calls and ioctls arising from minor variants
      of the default kernel for this platform.  This is useful for
      running on hacked kernels or with kernel modules which support
      nonstandard ioctls, for example.  Use with caution.  If you don't
      understand what this option does then you almost certainly don't
      need it.  Currently known variants are:</para>
      <itemizedlist>
        <listitem>
          <para><option>bproc: </option> Support the
          <function>sys_broc</function> system call on x86.  This is for
          running on BProc, which is a minor variant of standard Linux which
          is sometimes used for building clusters.</para>
        </listitem>
      </itemizedlist>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.merge-recursive-frames" xreflabel="--merge-recursive-frames">
    <term>
      <option><![CDATA[--merge-recursive-frames=<number> [default: 0] ]]></option>
    </term>
    <listitem>
      <para>Some recursive algorithms, for example balanced binary
      tree implementations, create many different stack traces, each
      containing cycles of calls.  A cycle is defined as two identical
      program counter values separated by zero or more other program
      counter values.  Valgrind may then use a lot of memory to store
      all these stack traces.  This is a poor use of memory
      considering that such stack traces contain repeated
      uninteresting recursive calls instead of more interesting
      information such as the function that has initiated the
      recursive call.
      </para>
      <para>The option <option>--merge-recursive-frames=&lt;number&gt;</option>
      instructs Valgrind to detect and merge recursive call cycles
      having a size of up to <option>&lt;number&gt;</option>
      frames. When such a cycle is detected, Valgrind records the
      cycle in the stack trace as a unique program counter.
      </para>
      <para>
      The value 0 (the default) causes no recursive call merging.
      A value of 1 will cause stack traces of simple recursive algorithms
      (for example, a factorial implementation) to be collapsed.
      A value of 2 will usually be needed to collapse stack traces produced
      by recursive algorithms such as binary trees, quick sort, etc.
      Higher values might be needed for more complex recursive algorithms.
      </para>
      <para>Note: recursive calls are detected by analysis of program
      counter values.  They are not detected by looking at function
      names.</para>
   </listitem>
  </varlistentry>

  <varlistentry id="opt.num-transtab-sectors" xreflabel="--num-transtab-sectors">
    <term>
      <option><![CDATA[--num-transtab-sectors=<number> [default: 6
      for Android platforms, 16 for all others] ]]></option>
    </term>
    <listitem>
      <para>Valgrind translates and instruments your program's machine
      code in small fragments. The translations are stored in a
      translation cache that is divided into a number of sections
      (sectors). If the cache is full, the sector containing the
      oldest translations is emptied and reused. If these old
      translations are needed again, Valgrind must re-translate and
      re-instrument the corresponding machine code, which is
      expensive.  If the "executed instructions" working set of a
      program is big, increasing the number of sectors may improve
      performance by reducing the number of re-translations needed.
      Sectors are allocated on demand.  Once allocated, a sector can
      never be freed, and occupies considerable space, depending on the tool
      (about 40 MB per sector for Memcheck).  Use the
      option <option>--stats=yes</option> to obtain precise
      information about the memory used by a sector and the allocation
      and recycling of sectors.</para>
   </listitem>
  </varlistentry>

  <varlistentry id="opt.show-emwarns" xreflabel="--show-emwarns">
    <term>
      <option><![CDATA[--show-emwarns=<yes|no> [default: no] ]]></option>
    </term>
    <listitem>
      <para>When enabled, Valgrind will emit warnings about its CPU
      emulation in certain cases.  These are usually not
      interesting.</para>
   </listitem>
  </varlistentry>

  <varlistentry id="opt.require-text-symbol"
        xreflabel="--require-text-symbol">
    <term>
      <option><![CDATA[--require-text-symbol=:sonamepatt:fnnamepatt]]></option>
    </term>
    <listitem>
      <para>When a shared object whose soname
      matches <varname>sonamepatt</varname> is loaded into the
      process, examine all the text symbols it exports.  If none of
      those match <varname>fnnamepatt</varname>, print an error
      message and abandon the run.  This makes it possible to ensure
      that the run does not continue unless a given shared object
      contains a particular function name.
      </para>
      <para>
      Both <varname>sonamepatt</varname> and
      <varname>fnnamepatt</varname> can be written using the usual
      <varname>?</varname> and <varname>*</varname> wildcards.  For
      example: <varname>":*libc.so*:foo?bar"</varname>.  You may use
      characters other than a colon to separate the two patterns.  It
      is only important that the first character and the separator
      character are the same.  For example, the above example could
      also be written <varname>"Q*libc.so*Qfoo?bar"</varname>.
      Multiple <varname> --require-text-symbol</varname> flags are
      allowed, in which case shared objects that are loaded into
      the process will be checked against all of them.
      </para>
      <para>
      The purpose of this is to support reliable usage of marked-up
      libraries.  For example, suppose we have a version of GCC's
      <varname>libgomp.so</varname> which has been marked up with
      annotations to support Helgrind.  It is only too easy and
      confusing to load the wrong, un-annotated
      <varname>libgomp.so</varname> into the application.  So the idea
      is: add a text symbol in the marked-up library, for
      example <varname>annotated_for_helgrind_3_6</varname>, and then
      give the flag
      <varname>--require-text-symbol=:*libgomp*so*:annotated_for_helgrind_3_6</varname>
      so that when <varname>libgomp.so</varname> is loaded, Valgrind
      scans its symbol table, and if the symbol isn't present the run
      is aborted, rather than continuing silently with the
      un-marked-up library.  Note that you should put the entire flag
      in quotes to stop shells expanding up the <varname>*</varname>
      and <varname>?</varname> wildcards.
      </para>
   </listitem>
  </varlistentry>

  <varlistentry id="opt.soname-synonyms"
        xreflabel="--soname-synonyms">
    <term>
      <option><![CDATA[--soname-synonyms=syn1=pattern1,syn2=pattern2,...]]></option>
    </term>
    <listitem>
      <para>When a shared library is loaded, Valgrind checks for 
      functions in the library that must be replaced or wrapped.
      For example, Memcheck replaces all malloc related
      functions (malloc, free, calloc, ...) with its own versions.
      Such replacements are done by default only in shared libraries whose
      soname matches a predefined soname pattern (e.g.
      <varname>libc.so*</varname> on linux).
      By default, no replacement is done for a statically linked
      library or for alternative libraries such as tcmalloc.
      In some cases, the replacements allow
      <option>--soname-synonyms</option> to specify one additional
      synonym pattern, giving flexibility in the replacement. </para>

      <para>Currently, this flexibility is only allowed for the
      malloc related functions, using the
      synonym <varname>somalloc</varname>.  This synonym is usable for
      all tools doing standard replacement of malloc related functions
      (e.g. memcheck, massif, drd, helgrind, exp-dhat, exp-sgcheck).
      </para>

      <itemizedlist>
        <listitem>

          <para>Alternate malloc library: to replace the malloc
          related functions in an alternate library with
          soname <varname>mymalloclib.so</varname>, give the
          option <option>--soname-synonyms=somalloc=mymalloclib.so</option>.
          A pattern can be used to match multiple libraries sonames.
          For
          example, <option>--soname-synonyms=somalloc=*tcmalloc*</option>
          will match the soname of all variants of the tcmalloc library
          (native, debug, profiled, ... tcmalloc variants). </para>
          <para>Note: the soname of a elf shared library can be
          retrieved using the readelf utility. </para>

        </listitem>

        <listitem>
          <para>Replacements in a statically linked library are done by
          using the <varname>NONE</varname> pattern. For example, if
          you link with <varname>libtcmalloc.a</varname>, memcheck 
          will properly work when you give the
          option <option>--soname-synonyms=somalloc=NONE</option>.  Note
          that a NONE pattern will match the main executable and any
          shared library having no soname. </para>
        </listitem>

        <listitem>
          <para>To run a "default" Firefox build for Linux, in which
          JEMalloc is linked in to the main executable,
          use <option>--soname-synonyms=somalloc=NONE</option>.
          </para>
        </listitem>

      </itemizedlist>
   </listitem>
  </varlistentry>


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

</sect2>


<sect2 id="manual-core.debugopts" xreflabel="Debugging Options">
<title>Debugging Options</title>

<!-- start of xi:include in the manpage -->
<para id="debug.opts.para">There are also some options for debugging
Valgrind itself.  You shouldn't need to use them in the normal run of
things.  If you wish to see the list, use the
<option>--help-debug</option> option.</para>

<para>If you wish to debug your program rather than debugging
Valgrind itself, then you should use the options
<option>--vgdb=yes</option> or <option>--vgdb=full</option>
or <option>--db-attach=yes</option>.
</para>

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

</sect2>


<sect2 id="manual-core.defopts" xreflabel="Setting Default Options">
<title>Setting Default Options</title>

<para>Note that Valgrind also reads options from three places:</para>

  <orderedlist>
   <listitem>
    <para>The file <computeroutput>~/.valgrindrc</computeroutput></para>
   </listitem>

   <listitem>
    <para>The environment variable
    <computeroutput>$VALGRIND_OPTS</computeroutput></para>
   </listitem>

   <listitem>
    <para>The file <computeroutput>./.valgrindrc</computeroutput></para>
   </listitem>
  </orderedlist>

<para>These are processed in the given order, before the
command-line options.  Options processed later override those
processed earlier; for example, options in
<computeroutput>./.valgrindrc</computeroutput> will take
precedence over those in
<computeroutput>~/.valgrindrc</computeroutput>.
</para>

<para>Please note that the <computeroutput>./.valgrindrc</computeroutput>
file is ignored if it is marked as world writeable or not owned 
by the current user. This is because the
<computeroutput>./.valgrindrc</computeroutput> can contain options that are
potentially harmful or can be used by a local attacker to execute code under
your user account.
</para>

<para>Any tool-specific options put in
<computeroutput>$VALGRIND_OPTS</computeroutput> or the
<computeroutput>.valgrindrc</computeroutput> files should be
prefixed with the tool name and a colon.  For example, if you
want Memcheck to always do leak checking, you can put the
following entry in <literal>~/.valgrindrc</literal>:</para>

<programlisting><![CDATA[
--memcheck:leak-check=yes]]></programlisting>

<para>This will be ignored if any tool other than Memcheck is
run.  Without the <computeroutput>memcheck:</computeroutput>
part, this will cause problems if you select other tools that
don't understand
<option>--leak-check=yes</option>.</para>

</sect2>

</sect1>



<sect1 id="manual-core.pthreads" xreflabel="Support for Threads">
<title>Support for Threads</title>

<para>Threaded programs are fully supported.</para>

<para>The main thing to point out with respect to threaded programs is
that your program will use the native threading library, but Valgrind
serialises execution so that only one (kernel) thread is running at a
time.  This approach avoids the horrible implementation problems of
implementing a truly multithreaded version of Valgrind, but it does
mean that threaded apps never use more than one CPU simultaneously,
even if you have a multiprocessor or multicore machine.</para>

<para>Valgrind doesn't schedule the threads itself.  It merely ensures
that only one thread runs at once, using a simple locking scheme.  The
actual thread scheduling remains under control of the OS kernel.  What
this does mean, though, is that your program will see very different
scheduling when run on Valgrind than it does when running normally.
This is both because Valgrind is serialising the threads, and because
the code runs so much slower than normal.</para>

<para>This difference in scheduling may cause your program to behave
differently, if you have some kind of concurrency, critical race,
locking, or similar, bugs.  In that case you might consider using the
tools Helgrind and/or DRD to track them down.</para>

<para>On Linux, Valgrind also supports direct use of the
<computeroutput>clone</computeroutput> system call,
<computeroutput>futex</computeroutput> and so on.
<computeroutput>clone</computeroutput> is supported where either
everything is shared (a thread) or nothing is shared (fork-like); partial
sharing will fail.
</para>

<!-- Referenced from both the manual and manpage -->
<sect2 id="&vg-pthreads-perf-sched-id;" xreflabel="&vg-pthreads-perf-sched-label;">
<title>Scheduling and Multi-Thread Performance</title>

<para>A thread executes code only when it holds the abovementioned
lock.  After executing some number of instructions, the running thread
will release the lock.  All threads ready to run will then compete to
acquire the lock.</para>

<para>The <option>--fair-sched</option> option controls the locking mechanism
used to serialise thread execution.</para>

<para>The default pipe based locking mechanism
(<option>--fair-sched=no</option>) is available on all
platforms.  Pipe based locking does not guarantee fairness between
threads: it is quite likely that a thread that has just released the
lock reacquires it immediately, even though other threads are ready to
run.  When using pipe based locking, different runs of the same
multithreaded application might give very different thread
scheduling.</para>

<para>An alternative locking mechanism, based on futexes, is available
on some platforms.  If available, it is activated
by <option>--fair-sched=yes</option> or
<option>--fair-sched=try</option>.  Futex based locking ensures
fairness (round-robin scheduling) between threads: if multiple threads
are ready to run, the lock will be given to the thread which first
requested the lock.  Note that a thread which is blocked in a system
call (e.g. in a blocking read system call) has not (yet) requested the
lock: such a thread requests the lock only after the system call is
finished.</para>

<para> The fairness of the futex based locking produces better
reproducibility of thread scheduling for different executions of a
multithreaded application. This better reproducibility is particularly
helpful when using Helgrind or DRD.</para>

<para>Valgrind's use of thread serialisation implies that only one
thread at a time may run.  On a multiprocessor/multicore system, the
running thread is assigned to one of the CPUs by the OS kernel
scheduler.  When a thread acquires the lock, sometimes the thread will
be assigned to the same CPU as the thread that just released the
lock.  Sometimes, the thread will be assigned to another CPU.  When
using pipe based locking, the thread that just acquired the lock
will usually be scheduled on the same CPU as the thread that just
released the lock.  With the futex based mechanism, the thread that
just acquired the lock will more often be scheduled on another
CPU.</para>

<para>Valgrind's thread serialisation and CPU assignment by the OS
kernel scheduler can interact badly with the CPU frequency scaling
available on many modern CPUs.  To decrease power consumption, the
frequency of a CPU or core is automatically decreased if the CPU/core
has not been used recently.  If the OS kernel often assigns the thread
which just acquired the lock to another CPU/core, it is quite likely
that this CPU/core is currently at a low frequency.  The frequency of
this CPU will be increased after some time.  However, during this
time, the (only) running thread will have run at the low frequency.
Once this thread has run for some time, it will release the lock.
Another thread will acquire this lock, and might be scheduled again on
another CPU whose clock frequency was decreased in the
meantime.</para>

<para>The futex based locking causes threads to change CPUs/cores more
often.  So, if CPU frequency scaling is activated, the futex based
locking might decrease significantly the performance of a
multithreaded app running under Valgrind.  Performance losses of up to
50% degradation have been observed, as compared to running on a
machine for which CPU frequency scaling has been disabled.  The pipe
based locking locking scheme also interacts badly with CPU frequency
scaling, with performance losses in the range 10..20% having been
observed.</para>

<para>To avoid such performance degradation, you should indicate to
the kernel that all CPUs/cores should always run at maximum clock
speed.  Depending on your Linux distribution, CPU frequency scaling
may be controlled using a graphical interface or using command line
such as
<computeroutput>cpufreq-selector</computeroutput> or
<computeroutput>cpufreq-set</computeroutput>.
</para>

<para>An alternative way to avoid these problems is to tell the
OS scheduler to tie a Valgrind process to a specific (fixed) CPU using the
<computeroutput>taskset</computeroutput> command.  This should ensure
that the selected CPU does not fall below its maximum frequency
setting so long as any thread of the program has work to do.
</para>

</sect2>


</sect1>

<sect1 id="manual-core.signals" xreflabel="Handling of Signals">
<title>Handling of Signals</title>

<para>Valgrind has a fairly complete signal implementation.  It should be
able to cope with any POSIX-compliant use of signals.</para>
 
<para>If you're using signals in clever ways (for example, catching
SIGSEGV, modifying page state and restarting the instruction), you're
probably relying on precise exceptions.  In this case, you will need
to use <option>--vex-iropt-register-updates=allregs-at-mem-access</option>
or <option>--vex-iropt-register-updates=allregs-at-each-insn</option>.
</para>

<para>If your program dies as a result of a fatal core-dumping signal,
Valgrind will generate its own core file
(<computeroutput>vgcore.NNNNN</computeroutput>) containing your program's
state.  You may use this core file for post-mortem debugging with GDB or
similar.  (Note: it will not generate a core if your core dump size limit is
0.)  At the time of writing the core dumps do not include all the floating
point register information.</para>

<para>In the unlikely event that Valgrind itself crashes, the operating system
will create a core dump in the usual way.</para>

</sect1>








<sect1 id="manual-core.install" xreflabel="Building and Installing">
<title>Building and Installing Valgrind</title>

<para>We use the standard Unix
<computeroutput>./configure</computeroutput>,
<computeroutput>make</computeroutput>, <computeroutput>make
install</computeroutput> mechanism.  Once you have completed 
<computeroutput>make install</computeroutput> you may then want 
to run the regression tests
with <computeroutput>make regtest</computeroutput>.
</para>

<para>In addition to the usual
<option>--prefix=/path/to/install/tree</option>, there are three
 options which affect how Valgrind is built:
<itemizedlist>

  <listitem>
    <para><option>--enable-inner</option></para>
    <para>This builds Valgrind with some special magic hacks which make
     it possible to run it on a standard build of Valgrind (what the
     developers call "self-hosting").  Ordinarily you should not use
     this option as various kinds of safety checks are disabled.
   </para>
  </listitem>

  <listitem>
    <para><option>--enable-only64bit</option></para>
    <para><option>--enable-only32bit</option></para>
    <para>On 64-bit platforms (amd64-linux, ppc64-linux,
     amd64-darwin), Valgrind is by default built in such a way that
     both 32-bit and 64-bit executables can be run.  Sometimes this
     cleverness is a problem for a variety of reasons.  These two
     options allow for single-target builds in this situation.  If you
     issue both, the configure script will complain.  Note they are
     ignored on 32-bit-only platforms (x86-linux, ppc32-linux,
     arm-linux, x86-darwin).
   </para>
  </listitem>

</itemizedlist>
</para>

<para>The <computeroutput>configure</computeroutput> script tests
the version of the X server currently indicated by the current
<computeroutput>$DISPLAY</computeroutput>.  This is a known bug.
The intention was to detect the version of the current X
client libraries, so that correct suppressions could be selected
for them, but instead the test checks the server version.  This
is just plain wrong.</para>

<para>If you are building a binary package of Valgrind for
distribution, please read <literal>README_PACKAGERS</literal>
<xref linkend="dist.readme-packagers"/>.  It contains some
important information.</para>

<para>Apart from that, there's not much excitement here.  Let us
know if you have build problems.</para>

</sect1>



<sect1 id="manual-core.problems" xreflabel="If You Have Problems">
<title>If You Have Problems</title>

<para>Contact us at <ulink url="&vg-url;">&vg-url;</ulink>.</para>

<para>See <xref linkend="manual-core.limits"/> for the known
limitations of Valgrind, and for a list of programs which are
known not to work on it.</para>

<para>All parts of the system make heavy use of assertions and 
internal self-checks.  They are permanently enabled, and we have no 
plans to disable them.  If one of them breaks, please mail us!</para>

<para>If you get an assertion failure
in <filename>m_mallocfree.c</filename>, this may have happened because
your program wrote off the end of a heap block, or before its
beginning, thus corrupting heap metadata.  Valgrind hopefully will have
emitted a message to that effect before dying in this way.</para>

<para>Read the <xref linkend="FAQ"/> for more advice about common problems, 
crashes, etc.</para>

</sect1>



<sect1 id="manual-core.limits" xreflabel="Limitations">
<title>Limitations</title>

<para>The following list of limitations seems long.  However, most
programs actually work fine.</para>

<para>Valgrind will run programs on the supported platforms
subject to the following constraints:</para>

 <itemizedlist>
  <listitem>
   <para>On x86 and amd64, there is no support for 3DNow!
   instructions.  If the translator encounters these, Valgrind will
   generate a SIGILL when the instruction is executed.  Apart from
   that, on x86 and amd64, essentially all instructions are supported,
   up to and including AVX and AES in 64-bit mode and SSSE3 in 32-bit
   mode.  32-bit mode does in fact support the bare minimum SSE4
   instructions to needed to run programs on MacOSX 10.6 on 32-bit
   targets.
   </para>
  </listitem>

  <listitem>
   <para>On ppc32 and ppc64, almost all integer, floating point and
   Altivec instructions are supported.  Specifically: integer and FP
   insns that are mandatory for PowerPC, the "General-purpose
   optional" group (fsqrt, fsqrts, stfiwx), the "Graphics optional"
   group (fre, fres, frsqrte, frsqrtes), and the Altivec (also known
   as VMX) SIMD instruction set, are supported.  Also, instructions
   from the Power ISA 2.05 specification, as present in POWER6 CPUs,
   are supported.</para>
  </listitem>

  <listitem>
   <para>On ARM, essentially the entire ARMv7-A instruction set
    is supported, in both ARM and Thumb mode.  ThumbEE and Jazelle are
    not supported.  NEON, VFPv3 and ARMv6 media support is fairly
    complete.
   </para>
  </listitem>

  <listitem>
   <para>If your program does its own memory management, rather than
   using malloc/new/free/delete, it should still work, but Memcheck's
   error checking won't be so effective.  If you describe your
   program's memory management scheme using "client requests" (see
   <xref linkend="manual-core-adv.clientreq"/>), Memcheck can do
   better.  Nevertheless, using malloc/new and free/delete is still
   the best approach.</para>
  </listitem>

  <listitem>
   <para>Valgrind's signal simulation is not as robust as it could be.
   Basic POSIX-compliant sigaction and sigprocmask functionality is
   supplied, but it's conceivable that things could go badly awry if you
   do weird things with signals.  Workaround: don't.  Programs that do
   non-POSIX signal tricks are in any case inherently unportable, so
   should be avoided if possible.</para>
  </listitem>

  <listitem>
   <para>Machine instructions, and system calls, have been implemented
   on demand.  So it's possible, although unlikely, that a program will
   fall over with a message to that effect.  If this happens, please
   report all the details printed out, so we can try and implement the
   missing feature.</para>
  </listitem>

  <listitem>
   <para>Memory consumption of your program is majorly increased
   whilst running under Valgrind's Memcheck tool.  This is due to the
   large amount of administrative information maintained behind the
   scenes.  Another cause is that Valgrind dynamically translates the
   original executable.  Translated, instrumented code is 12-18 times
   larger than the original so you can easily end up with 150+ MB of
   translations when running (eg) a web browser.</para>
  </listitem>

  <listitem>
   <para>Valgrind can handle dynamically-generated code just fine.  If
   you regenerate code over the top of old code (ie. at the same
   memory addresses), if the code is on the stack Valgrind will
   realise the code has changed, and work correctly.  This is
   necessary to handle the trampolines GCC uses to implemented nested
   functions.  If you regenerate code somewhere other than the stack,
   and you are running on an 32- or 64-bit x86 CPU, you will need to
   use the <option>--smc-check=all</option> option, and Valgrind will
   run more slowly than normal.  Or you can add client requests that
   tell Valgrind when your program has overwritten code.
   </para>
   <para> On other platforms (ARM, PowerPC) Valgrind observes and
   honours the cache invalidation hints that programs are obliged to
   emit to notify new code, and so self-modifying-code support should
   work automatically, without the need
   for <option>--smc-check=all</option>.</para>
  </listitem>

  <listitem>
   <para>Valgrind has the following limitations
   in its implementation of x86/AMD64 floating point relative to 
   IEEE754.</para>

   <para>Precision: There is no support for 80 bit arithmetic.
   Internally, Valgrind represents all such "long double" numbers in 64
   bits, and so there may be some differences in results.  Whether or
   not this is critical remains to be seen.  Note, the x86/amd64
   fldt/fstpt instructions (read/write 80-bit numbers) are correctly
   simulated, using conversions to/from 64 bits, so that in-memory
   images of 80-bit numbers look correct if anyone wants to see.</para>

   <para>The impression observed from many FP regression tests is that
   the accuracy differences aren't significant.  Generally speaking, if
   a program relies on 80-bit precision, there may be difficulties
   porting it to non x86/amd64 platforms which only support 64-bit FP
   precision.  Even on x86/amd64, the program may get different results
   depending on whether it is compiled to use SSE2 instructions (64-bits
   only), or x87 instructions (80-bit).  The net effect is to make FP
   programs behave as if they had been run on a machine with 64-bit IEEE
   floats, for example PowerPC.  On amd64 FP arithmetic is done by
   default on SSE2, so amd64 looks more like PowerPC than x86 from an FP
   perspective, and there are far fewer noticeable accuracy differences
   than with x86.</para>

   <para>Rounding: Valgrind does observe the 4 IEEE-mandated rounding
   modes (to nearest, to +infinity, to -infinity, to zero) for the
   following conversions: float to integer, integer to float where
   there is a possibility of loss of precision, and float-to-float
   rounding.  For all other FP operations, only the IEEE default mode
   (round to nearest) is supported.</para>

   <para>Numeric exceptions in FP code: IEEE754 defines five types of
   numeric exception that can happen: invalid operation (sqrt of
   negative number, etc), division by zero, overflow, underflow,
   inexact (loss of precision).</para>

   <para>For each exception, two courses of action are defined by IEEE754:
   either (1) a user-defined exception handler may be called, or (2) a
   default action is defined, which "fixes things up" and allows the
   computation to proceed without throwing an exception.</para>

   <para>Currently Valgrind only supports the default fixup actions.
   Again, feedback on the importance of exception support would be
   appreciated.</para>

   <para>When Valgrind detects that the program is trying to exceed any
   of these limitations (setting exception handlers, rounding mode, or
   precision control), it can print a message giving a traceback of
   where this has happened, and continue execution.  This behaviour used
   to be the default, but the messages are annoying and so showing them
   is now disabled by default.  Use <option>--show-emwarns=yes</option> to see
   them.</para>

   <para>The above limitations define precisely the IEEE754 'default'
   behaviour: default fixup on all exceptions, round-to-nearest
   operations, and 64-bit precision.</para>
  </listitem>
   
  <listitem>
   <para>Valgrind has the following limitations in
   its implementation of x86/AMD64 SSE2 FP arithmetic, relative to 
   IEEE754.</para>

   <para>Essentially the same: no exceptions, and limited observance of
   rounding mode.  Also, SSE2 has control bits which make it treat
   denormalised numbers as zero (DAZ) and a related action, flush
   denormals to zero (FTZ).  Both of these cause SSE2 arithmetic to be
   less accurate than IEEE requires.  Valgrind detects, ignores, and can
   warn about, attempts to enable either mode.</para>
  </listitem>

  <listitem>
   <para>Valgrind has the following limitations in
   its implementation of ARM VFPv3 arithmetic, relative to 
   IEEE754.</para>

   <para>Essentially the same: no exceptions, and limited observance
   of rounding mode.  Also, switching the VFP unit into vector mode
   will cause Valgrind to abort the program -- it has no way to
   emulate vector uses of VFP at a reasonable performance level.  This
   is no big deal given that non-scalar uses of VFP instructions are
   in any case deprecated.</para>
  </listitem>

  <listitem>
   <para>Valgrind has the following limitations
   in its implementation of PPC32 and PPC64 floating point 
   arithmetic, relative to IEEE754.</para>

   <para>Scalar (non-Altivec): Valgrind provides a bit-exact emulation of
   all floating point instructions, except for "fre" and "fres", which are
   done more precisely than required by the PowerPC architecture specification.
   All floating point operations observe the current rounding mode.
   </para>

   <para>However, fpscr[FPRF] is not set after each operation.  That could
   be done but would give measurable performance overheads, and so far
   no need for it has been found.</para>

   <para>As on x86/AMD64, IEEE754 exceptions are not supported: all floating
   point exceptions are handled using the default IEEE fixup actions.
   Valgrind detects, ignores, and can warn about, attempts to unmask 
   the 5 IEEE FP exception kinds by writing to the floating-point status 
   and control register (fpscr).
   </para>

   <para>Vector (Altivec, VMX): essentially as with x86/AMD64 SSE/SSE2: 
   no exceptions, and limited observance of rounding mode.  
   For Altivec, FP arithmetic
   is done in IEEE/Java mode, which is more accurate than the Linux default
   setting.  "More accurate" means that denormals are handled properly, 
   rather than simply being flushed to zero.</para>
  </listitem>
 </itemizedlist>

 <para>Programs which are known not to work are:</para>
 <itemizedlist>
  <listitem>
   <para>emacs starts up but immediately concludes it is out of
   memory and aborts.  It may be that Memcheck does not provide
   a good enough emulation of the 
   <computeroutput>mallinfo</computeroutput> function.
   Emacs works fine if you build it to use
   the standard malloc/free routines.</para>
  </listitem>
 </itemizedlist>

</sect1>


<sect1 id="manual-core.example" xreflabel="An Example Run">
<title>An Example Run</title>

<para>This is the log for a run of a small program using Memcheck.
The program is in fact correct, and the reported error is as the
result of a potentially serious code generation bug in GNU g++
(snapshot 20010527).</para>

<programlisting><![CDATA[
sewardj@phoenix:~/newmat10$ ~/Valgrind-6/valgrind -v ./bogon 
==25832== Valgrind 0.10, a memory error detector for x86 RedHat 7.1.
==25832== Copyright (C) 2000-2001, and GNU GPL'd, by Julian Seward.
==25832== Startup, with flags:
==25832== --suppressions=/home/sewardj/Valgrind/redhat71.supp
==25832== reading syms from /lib/ld-linux.so.2
==25832== reading syms from /lib/libc.so.6
==25832== reading syms from /mnt/pima/jrs/Inst/lib/libgcc_s.so.0
==25832== reading syms from /lib/libm.so.6
==25832== reading syms from /mnt/pima/jrs/Inst/lib/libstdc++.so.3
==25832== reading syms from /home/sewardj/Valgrind/valgrind.so
==25832== reading syms from /proc/self/exe
==25832== 
==25832== Invalid read of size 4
==25832==    at 0x8048724: BandMatrix::ReSize(int,int,int) (bogon.cpp:45)
==25832==    by 0x80487AF: main (bogon.cpp:66)
==25832==  Address 0xBFFFF74C is not stack'd, malloc'd or free'd
==25832==
==25832== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==25832== malloc/free: in use at exit: 0 bytes in 0 blocks.
==25832== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==25832== For a detailed leak analysis, rerun with: --leak-check=yes
]]></programlisting>

<para>The GCC folks fixed this about a week before GCC 3.0
shipped.</para>

</sect1>


<sect1 id="manual-core.warnings" xreflabel="Warning Messages">
<title>Warning Messages You Might See</title>

<para>Some of these only appear if you run in verbose mode
(enabled by <option>-v</option>):</para>

 <itemizedlist>

  <listitem>
    <para><computeroutput>More than 100 errors detected.  Subsequent
    errors will still be recorded, but in less detail than
    before.</computeroutput></para>

    <para>After 100 different errors have been shown, Valgrind becomes
    more conservative about collecting them.  It then requires only the
    program counters in the top two stack frames to match when deciding
    whether or not two errors are really the same one.  Prior to this
    point, the PCs in the top four frames are required to match.  This
    hack has the effect of slowing down the appearance of new errors
    after the first 100.  The 100 constant can be changed by recompiling
    Valgrind.</para>
  </listitem>

  <listitem>
    <para><computeroutput>More than 1000 errors detected.  I'm not
    reporting any more.  Final error counts may be inaccurate.  Go fix
    your program!</computeroutput></para>

    <para>After 1000 different errors have been detected, Valgrind
    ignores any more.  It seems unlikely that collecting even more
    different ones would be of practical help to anybody, and it avoids
    the danger that Valgrind spends more and more of its time comparing
    new errors against an ever-growing collection.  As above, the 1000
    number is a compile-time constant.</para>
  </listitem>

  <listitem>
    <para><computeroutput>Warning: client switching stacks?</computeroutput></para>

    <para>Valgrind spotted such a large change in the stack pointer
    that it guesses the client is switching to a different stack.  At
    this point it makes a kludgey guess where the base of the new
    stack is, and sets memory permissions accordingly.  At the moment
    "large change" is defined as a change of more that 2000000 in the
    value of the stack pointer register.  If Valgrind guesses wrong,
    you may get many bogus error messages following this and/or have
    crashes in the stack trace recording code.  You might avoid these
    problems by informing Valgrind about the stack bounds using
    VALGRIND_STACK_REGISTER client request. </para>

  </listitem>

  <listitem>
    <para><computeroutput>Warning: client attempted to close Valgrind's
    logfile fd &lt;number&gt;</computeroutput></para>

    <para>Valgrind doesn't allow the client to close the logfile,
    because you'd never see any diagnostic information after that point.
    If you see this message, you may want to use the
    <option>--log-fd=&lt;number&gt;</option> option to specify a
    different logfile file-descriptor number.</para>
  </listitem>

  <listitem>
    <para><computeroutput>Warning: noted but unhandled ioctl
    &lt;number&gt;</computeroutput></para>

    <para>Valgrind observed a call to one of the vast family of
    <computeroutput>ioctl</computeroutput> system calls, but did not
    modify its memory status info (because nobody has yet written a 
    suitable wrapper).  The call will still have gone through, but you may get
    spurious errors after this as a result of the non-update of the
    memory info.</para>
  </listitem>

  <listitem>
    <para><computeroutput>Warning: set address range perms: large range
    &lt;number></computeroutput></para>

    <para>Diagnostic message, mostly for benefit of the Valgrind
    developers, to do with memory permissions.</para>
  </listitem>

 </itemizedlist>

</sect1>






</chapter>