ftmemsim-valgrind/coregrind/docs/coregrind_core.html

<a name="core"></a>
<h2>2&nbsp; Using and understanding the Valgrind core</h2>

This section describes the Valgrind core services, flags and behaviours.  That
means it is relevant regardless of what particular tool you are using.
A point of terminology: most references to "valgrind" in the rest of
this section (Section 2) refer to the valgrind core services.


<a name="core-whatdoes"></a>
<h3>2.1&nbsp; What it does with your program</h3>

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.

Simply put <code>valgrind --tool=<i>tool_name</i></code> at the start of
the command line normally used to run the program.  For example,
if want to run the command <code>ls -l</code>
using the heavyweight memory-checking tool Memcheck, issue the command:

  <blockquote>
  <code>valgrind --tool=memcheck ls -l</code>.  
  </blockquote>

<p>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 (if that is
appropriate)

<p>
Your program is then run on a synthetic x86 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.

<p>
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, increasing the size of the
code at least 12 times, and making it run 25-50 times slower than
natively.  At the other end of the spectrum, the ultra-trivial "none"
tool (a.k.a. Nulgrind) adds no instrumentation at all and causes in total
"only" about a 4 times slowdown.  

<p>
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
(<code>.so</code>-format) libraries, including the GNU C library, the
X client libraries, Qt, if you work with KDE, and so on.  

<p>
If you're using one of the error-detection tools, Valgrind will often
detect errors in 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 noo 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 attempts to select suppressions which give reasonable
behaviour for the libc and XFree86 versions detected on your machine.
To make it easier to write suppressions, you can use the
<code>--gen-suppressions=yes</code> option which tells Valgrind to print
out a suppression for each error that appears, which you can then copy
into a suppressions file.

<p>
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.


<a name="started"></a>
<h3>2.2&nbsp; Getting started</h3>

First off, consider whether it might be beneficial to recompile your
application and supporting libraries with debugging info enabled (the
<code>-g</code> flag).  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 <code>-g</code>, you'll hopefully get messages
which point directly to the relevant source code lines.

<p>
Another flag you might like to consider, if you are working with 
C++, is <code>-fno-inline</code>.  That makes it easier to see the
function-call chain, which can help reduce confusion when navigating
around large C++ apps.  For whatever it's worth, debugging
OpenOffice.org with Memcheck is a bit easier when using this flag.

<p>
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.

<p>
This paragraph applies only if you plan to use Memcheck:
On rare occasions, optimisation levels
at <code>-O2</code> and above have been observed to generate code which
fools Memcheck into wrongly reporting 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 unmanagably
slow, a plausible compromise is to use <code>-O</code>.  This gets 
you the majority of the benefits of higher optimisation levels whilst
keeping relatively small the chances of false complaints from Memcheck.
All other tools (as far as we know) are unaffected by optimisation
level.

<p>
Valgrind understands both the older "stabs" debugging format, used by
gcc versions prior to 3.1, and the newer DWARF2 format used by gcc 3.1
and later.  We continue to refine and debug our debug-info readers,
although the majority of effort will naturally enough go into the 
newer DWARF2 reader.

<p>
When you're ready to roll, just run your application as you would
normally, but place <code>valgrind --tool=<i>tool_name</i></code> in
front of your usual command-line invocation.  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 <code>/bin/sh</code>, <code>/usr/bin/perl</code>, 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 flag
<code>--trace-children=yes</code>, but confusion is still likely.


<a name="comment"></a>
<h3>2.3&nbsp; The commentary</h3>

Valgrind tools write a commentary, a stream of text, detailing error
reports and other significant events.  All lines in the commentary
have following form:<br>
<pre>
  ==12345== some-message-from-Valgrind
</pre>

<p>The <code>12345</code>  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.

<p>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 <code>-v</code> flag to Valgrind.

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

<ul>
<li>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 <code>--log-fd=9</code>.
<p>
<li>A less intrusive option is to write the commentary to a file, 
    which you specify by <code>--log-file=filename</code>.  Note 
    carefully that the commentary is <b>not</b> written to the file
    you specify, but instead to one called
    <code>filename.pid12345</code>, if for example the pid of the
    traced process is 12345.  This is helpful when valgrinding a whole
    tree of processes at once, since it means that each process writes
    to its own logfile, rather than the result being jumbled up in one
    big logfile.
<p>
<li>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: <code>--log-socket=192.168.0.1:12345</code> if you
    want to send the output to host IP 192.168.0.1 port 12345 (I have
    no idea if 12345 is a port of pre-existing significance).  You can
    also omit the port number: <code>--log-socket=192.168.0.1</code>, 
    in which case a default port of 1500 is used.  This default is
    defined by the constant <code>VG_CLO_DEFAULT_LOGPORT</code>
    in the sources.
    <p>
    Note, unfortunately, that you have to use an IP address here, rather
    than a hostname.
    <p>
    Writing to a network socket is pretty useless if you don't have
    something listening at the other end.  We provide a simple
    listener program, <code>valgrind-listener</code>, 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.
    <p>
    valgrind-listener 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.  
    <p>
    valgrind-listener accepts two command-line flags:
    <ul>
    <li><code>-e</code> or <code>--exit-at-zero</code>: 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.
    <p>
    <li><code>portnumber</code>: 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 <code>--log-socket=</code> to Valgrind itself.
    </ul>
    <p>
    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.
</ul>
<p>
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
<code>--log-*</code> 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.


<a name="report"></a>
<h3>2.4&nbsp; Reporting of errors</h3>

When one of the error-checking tools (Memcheck, Addrcheck, Helgrind)
detects something bad happening in the program, an error message is
written to the commentary.  For example:<br>
<pre>
  ==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==    by 0x40371E5E: __libc_start_main (libc-start.c:129)
  ==25832==    by 0x80485D1: (within /home/sewardj/newmat10/bogon)
  ==25832==    Address 0xBFFFF74C is not stack'd, malloc'd or free'd
</pre>

<p>
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 currently malloc'd or free'd
blocks.  The read is happening at line 45 of <code>bogon.cpp</code>,
called from line 66 of the same file, etc.  For errors associated with
an identified malloc'd/free'd block, for example reading free'd
memory, Valgrind reports not only the location where the error
happened, but also where the associated block was malloc'd/free'd.

<p>
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.

<p>
If you want to know how many times each error occurred, run with the
<code>-v</code> 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.

<p>
Errors are reported before the associated operation actually happens.
If you're using a tool (Memcheck, Addrcheck) which does address
checking, and your program attempts to read from address zero, the
tool will emit a message to this effect, and the program will then
duly die with a segmentation fault.

<p>
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.

<p>
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 here,
Valgrind will simply stop collecting errors after 300 different errors
have been seen, or 30000 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 300/30000 limits apply after suppressed errors are removed.  These
limits are defined in <code>vg_include.h</code> and can be increased
if necessary.

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


<a name="suppress"></a>
<h3>2.5&nbsp; Suppressing errors</h3>

The error-checking tools detect numerous problems in the base
libraries, such as the GNU C library, and the XFree86 client
libraries, which come pre-installed on your GNU/Linux system.  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 cooked up by the
<code>./configure</code> script when the system is built.

<p>
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.

<p>
<b>Note:</b> By far the easiest way to add suppressions is to use the
<code>--gen-suppressions=yes</code> flag described in <a href="#flags">this
section</a>.

<p>
Each error to be suppressed is described very specifically, to
minimise the possibility that a suppression-directive inadvertantly
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.

<p>
If you use the <code>-v</code> flag, at the end of execution, Valgrind
prints out one line for each used suppression, giving its name and the
number of times it got used.  Here's the suppressions used by a run of
<code>valgrind --tool=memcheck ls -l</code>:
<pre>
  --27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getgrgid_r
  --27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getpwuid_r
  --27579-- supp: 6 strrchr/_dl_map_object_from_fd/_dl_map_object
</pre>

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

<p>
If you want to understand more about suppressions, look at an existing
suppressions file whilst reading the following documentation.  The file
<code>glibc-2.2.supp</code>, in the source distribution, provides some good
examples.

<p>Each suppression has the following components:<br>
<ul>
  <li>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.
      </li>
      <p>

  <li>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, eg:
      <pre>
      tool_name1,tool_name2:suppression_name
      </pre>
      (Nb: no spaces are allowed).
      <p>      
      Recall that Valgrind-2.0.X 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.
      <p>
      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.
      <p>
      Valgrind's core can detect certain PThreads API errors, for which this
      line reads:
      <pre>
      core:PThread
      </pre>

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

  <li>Remaining lines: This is the calling context for the error -- the chain
      of function calls that led to it.  There can be up to four of these lines.
      <p>
      Locations may be either names of shared objects/executables or wildcards
      matching function names.  They begin <code>obj:</code> and
      <code>fun:</code> respectively.  Function and object names to match
      against may use the wildcard characters <code>*</code> and
      <code>?</code>.
      <p>
      <b>Important note:</b> C++ function names must be <b>mangled</b>.  If
      you are writing suppressions by hand, use the <code>--demangle=no</code>
      option to get the mangled names in your error messages.
      <p>
    
  <li>Finally, the entire suppression must be between curly braces. Each
      brace must be the first character on its own line.
</ul>

<p>

A suppression only suppresses an error when the error matches all the
details in the suppression.  Here's an example:
<pre>
  {
    __gconv_transform_ascii_internal/__mbrtowc/mbtowc
    Memcheck:Value4
    fun:__gconv_transform_ascii_internal
    fun:__mbr*toc
    fun:mbtowc
  }
</pre>

<p>What is means is: for Memcheck only, suppress a
use-of-uninitialised-value error, when the data size is 4, when it
occurs in the function <code>__gconv_transform_ascii_internal</code>,
when that is called from any function of name matching
<code>__mbr*toc</code>, when that is called from <code>mbtowc</code>.
It doesn't apply under any other circumstances.  The string by which
this suppression is identified to the user is
__gconv_transform_ascii_internal/__mbrtowc/mbtowc.
<p>
(See <a href="mc_main.html#suppfiles">this section</a> for more details on
the specifics of Memcheck's suppression kinds.)

<p>Another example, again for the Memcheck tool:
<pre>
  {
    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
  }
</pre>

<p>Suppress any size 4 uninitialised-value error which occurs anywhere
in <code>libX11.so.6.2</code>, when called from anywhere in the same
library, when called from anywhere in <code>libXaw.so.7.0</code>.  The
inexact specification of locations is regrettable, but is about all
you can hope for, given that the X11 libraries shipped with Red Hat
7.2 have had their symbol tables removed.

<p>Note -- since the above two examples did not make it clear -- that
you can freely mix the <code>obj:</code> and <code>fun:</code>
styles of description within a single suppression record.
<p>

<a name="flags"></a>
<h3>2.6&nbsp; Command-line flags for the Valgrind core</h3>


As mentioned above, Valgrind's core accepts a common set of flags.
The tools also accept tool-specific flags, which are documented
seperately for each tool.  

You invoke Valgrind like this:
<pre>
  valgrind --tool=<i>tool_name</i> [options-for-Valgrind] your-prog [options for your-prog]
</pre>

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

<h4>Tool-selection option</h4>
The single most important option.
<ul>
  <li><code>--tool=<i>name</i></code><br>
      <p>Run the Valgrind tool called <i>name</i>, e.g. Memcheck, Addrcheck,
      Cachegrind, etc.
      </li><br><p>
</ul>

<h4>Basic Options</h4>
These options work with all tools.

<ul>
  <li><code>--help</code><br>
      <p>Show help for all options, both for the core and for the
      selected tool. </li><br><p>

  <li><code>--help-debug</code><br>
      <p>Same as <code>--help</code>, but also lists debugging options which
      usually are only of use to developers.</li><br><p>

  <li><code>--version</code><br> <p>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.  </li><br><p>

  <li><code>-v --verbose</code><br> <p>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 flag increases the verbosity
      level.  </li><br><p>

  <li><code>-q --quiet</code><br>
      <p>Run silently, and only print error messages.  Useful if you
      are running regression tests or have some other automated test
      machinery.
      </li><br><p>

  <li><code>--trace-children=no</code> [default]<br>
      <code>--trace-children=yes</code>
      <p>When enabled, Valgrind will trace into child processes.  This
      is confusing and often not what you want, so is disabled by
      default.

      <p>Note that the name of this option is slightly misleading.
      It actually controls whether programs started with
      <code>exec()</code> are run under Valgrind's control.  If your
      program calls <code>fork()</code>, both the parent and the child
      will run under Valgrind's control.
      </li><br><p>

  <li><code>--log-fd=&lt;number&gt;</code> [default: 2, stderr]
      <p>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.  
      </li><br><p>

  <li><code>--log-file=&lt;filename&gt;</code>
      <p>Specifies that Valgrind should send all of its
      messages to the specified file.  In fact, the file name used
      is created by concatenating the text <code>filename</code>,
      ".pid" and the process ID, so as to create a file per process.
      The specified file name may not be the empty string.
      </li><br><p>

  <li><code>--log-socket=&lt;ip-address:port-number&gt;</code>
      <p>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
      <code>valgrind-listener</code> program.  For further details,
      see section <a href="#core-comment">2.3</a>.
      </li><br><p>
</ul>

<h4>Error-related options</h4>
These options are used by all tools that can report errors, e.g. Memcheck, but
not Cachegrind.
<ul>
  <li><code>--demangle=no</code><br>
      <code>--demangle=yes</code> [default]
      <p>Disable/enable automatic demangling (decoding) of C++ names.
      Enabled by default.  When enabled, Valgrind will attempt to
      translate encoded C++ procedure names back to something
      approaching the original.  The demangler handles symbols mangled
      by g++ versions 2.X and 3.X.

      <p>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
      suppressions file contents dependent on the state of Valgrind's
      demangling machinery, and would also be slow and pointless.
      </li><br><p>

  <li><code>--num-callers=&lt;number&gt;</code> [default=4]<br>
      <p>By default, Valgrind shows four levels of function call names
      to help you identify program locations.  You can change that
      number with this option.  This can help in determining the
      program's location in deeply-nested call chains.  Note that errors
      are commoned up using only the top three function locations (the
      place in the current function, and that of its two immediate
      callers).  So this doesn't affect the total number of errors
      reported.  
      <p>
      The maximum value for this is 50.  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.  
      </li><br><p>

  <li><code>--error-limit=yes</code> [default]<br>
      <code>--error-limit=no</code> <p>When enabled, Valgrind stops
      reporting errors after 30000 in total, or 300 different ones,
      have been seen.  This is to stop the error tracking machinery
      from becoming a huge performance overhead in programs with many
      errors.  
      </li><br><p>

  <li><code>--show-below-main=yes</code><br>
      <code>--show-below-main=no</code>  [default]
      <p>By default, stack traces for errors do not show any functions that
      appear beneath <code>main()</code>;  most of the time it's uninteresting
      C library stuff.  If this option is enabled, these entries below
      <code>main()</code> will be shown.
      </li><br><p>

  <li><code>--suppressions=&lt;filename&gt;</code> 
      [default: $PREFIX/lib/valgrind/default.supp]
      <p>Specifies an extra
      file from which to read descriptions of errors to suppress.  You
      may use as many extra suppressions files as you
      like.
      </li><br><p>

  <li><code>--gen-suppressions=no</code> [default]<br>
      <code>--gen-suppressions=yes</code>
      <p>When enabled, Valgrind will pause after every error shown,
      and print the line
      <br>
      <code>---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----</code>
      <p>
      The prompt's behaviour is the same as for the <code>--db-attach</code>
      option.
      <p>
      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.
      <p>
      This option is particularly useful with C++ programs, as it prints out
      the suppressions with mangled names, as required.
      <p>
      Note that the suppressions printed are as specific as possible.  You
      may want to common up similar ones, eg. by adding wildcards to function
      names.  Also, 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 <code>&lt;insert a suppression name here&gt;</code>;
      the name doesn't really matter, it's only used with the
      <code>-v</code> option which prints out all used suppression records.
      </li><br><p>

  <li><code>--track-fds=no</code> [default]<br>
      <code>--track-fds=yes</code>
      <p>When enabled, Valgrind will print out a list of open file
      descriptors on exit.  Along with each file descriptor, Valgrind
      prints out 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.
      <br><p>

  <li><code>--db-attach=no</code> [default]<br>
      <code>--db-attach=yes</code>
      <p>When enabled, Valgrind will pause after every error shown,
      and print the line
      <br>
      <code>---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ----</code>
      <p>
      Pressing <code>Ret</code>, or <code>N</code> <code>Ret</code>
      or <code>n</code> <code>Ret</code>, causes Valgrind not to
      start a debugger for this error.
      <p>
      <code>Y</code> <code>Ret</code>
      or <code>y</code> <code>Ret</code> 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.
      <p>
      <code>C</code> <code>Ret</code>
      or <code>c</code> <code>Ret</code> causes Valgrind not to
      start a debugger, and not to ask again.
      <p>
      <code>--db-attach=yes</code> conflicts with
      <code>--trace-children=yes</code>.  You can't use them together.
      Valgrind refuses to start up in this situation.  1 May 2002:
      this is a historical relic which could be easily fixed if it
      gets in your way.  Mail me and complain if this is a problem for
      you.
      <p>
      Nov 2002: if you're sending output to a logfile or to a network
      socket, I guess this option doesn't make any sense.  Caveat emptor.
      </li><br><p>

  <li><code>--db-command=&lt;command&gt;</code> [default: gdb -nw %f %p]<br>
      <p>This specifies how Valgrind will invoke the debugger.  By
      default it will use whatever GDB is detected at build time, 
      which is usually <code>/usr/bin/gdb</code>.  Using this command,
      you can specify some alternative command to invoke the debugger
      you want to use.
      <p>
      The command string given can include one or instances of the
      %p and %f expansions. Each instance of %p expands to the PID of
      the process to be debugged and each instance of %f expands to
      the path to the executable for the process to be debugged.
      </li><br><p>

  <li><code>--input-fd=&lt;number&gt;</code> [default=0, stdin]<br>
      <p>When using <code>--db-attach=yes</code> and 
         <code>--gen-suppressions=yes</code>, 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.  
      </li><br><p>
</ul>

<h4><code>malloc()</code>-related options</h4>
For tools that use their own version of <code>malloc()</code> (e.g. Memcheck
and Addrcheck), the following options apply.
<ul>
  <li><code>--alignment=&lt;number&gt;</code> [default: 8]<br> <p>By
      default Valgrind's <code>malloc</code>, <code>realloc</code>,
      etc, return 4-byte aligned addresses.  These are suitable for
      any accesses on x86 processors. 
      Some programs might however assume that <code>malloc</code> et
      al return 8- or more aligned memory.  The supplied value must be
      between 4 and 4096 inclusive, and must be a power of two.</li><br><p>

  <li><code>--sloppy-malloc=no</code> [default]<br>
      <code>--sloppy-malloc=yes</code>
      <p>When enabled, all requests for malloc/calloc are rounded up
      to a whole number of machine words -- in other words, made
      divisible by 4.  For example, a request for 17 bytes of space
      would result in a 20-byte area being made available.  This works
      around bugs in sloppy libraries which assume that they can
      safely rely on malloc/calloc requests being rounded up in this
      fashion.  Without the workaround, these libraries tend to
      generate large numbers of errors when they access the ends of
      these areas.  
      <p>
      Valgrind snapshots dated 17 Feb 2002 and later are
      cleverer about this problem, and you should no longer need to 
      use this flag.  To put it bluntly, if you do need to use this
      flag, your program violates the ANSI C semantics defined for
      <code>malloc</code> and <code>free</code>, even if it appears to
      work correctly, and you should fix it, at least if you hope for
      maximum portability.
      </li><br><p>
</ul>

<h4>Rare options</h4>
These options apply to all tools, as they affect certain obscure workings of
the Valgrind core.  Most people won't need to use these.
<ul>
  <li><code>--run-libc-freeres=yes</code> [default]<br>
      <code>--run-libc-freeres=no</code>
      <p>The GNU C library (<code>libc.so</code>), 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 - 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.
      <p>
      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 <code>__libc_freeres</code>
      specifically to make glibc release all memory it has allocated.
      Memcheck and Addrcheck therefore try and run
      <code>__libc_freeres</code> at exit.
      <p>
      Unfortunately, in some versions of glibc,
      <code>__libc_freeres</code> is sufficiently buggy to cause
      segmentation faults.  This is particularly noticeable on Red Hat
      7.1.  So this flag is provided in order to inhibit the run of
      <code>__libc_freeres</code>.  If your program seems to run fine
      on Valgrind, but segfaults at exit, you may find that
      <code>--run-libc-freeres=no</code> fixes that, although at the
      cost of possibly falsely reporting space leaks in
      <code>libc.so</code>.
      </li><br><p>

  <li><code>--weird-hacks=hack1,hack2,...</code>
      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 hacks
      are enabled.  Use with caution!  Currently known hacks are:
      <p>
      <ul>
      <li><code>lax-ioctls</code> 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.
      </ul>
      </li><br><p>

  <li><code>--signal-polltime=&lt;time&gt;</code> [default=50]<br>
      <p>How often to poll for signals (in milliseconds).  Only applies for
      older kernels that need signal routing.
      </li><br><p>

  <li><code>--lowlat-signals=no</code> [default]<br>
      <code>--lowlat-signals=yes</code><br>
      <p>Improve wake-up latency when a thread receives a signal.
      </li><br><p>

  <li><code>--lowlat-syscalls=no</code> [default]<br>
      <code>--lowlat-syscalls=yes</code><br>
      <p>Improve wake-up latency when a thread's syscall completes. 
      </li><br><p>

</ul>

There are also some options for debugging Valgrind itself.  You
shouldn't need to use them in the normal run of things.  Nevertheless:

<ul>

  <li><code>--single-step=no</code> [default]<br>
      <code>--single-step=yes</code>
      <p>When enabled, each x86 insn is translated separately into
      instrumented code.  When disabled, translation is done on a
      per-basic-block basis, giving much better translations.</li><br>
      <p>

  <li><code>--optimise=no</code><br>
      <code>--optimise=yes</code> [default]
      <p>When enabled, various improvements are applied to the
      intermediate code, mainly aimed at allowing the simulated CPU's
      registers to be cached in the real CPU's registers over several
      simulated instructions.</li><br>
      <p>

  <li><code>--profile=no</code><br>
      <code>--profile=yes</code> [default]
      <p>When enabled, does crude internal profiling of Valgrind 
      itself.  This is not for profiling your programs.  Rather it is
      to allow the developers to assess where Valgrind is spending
      its time.  The tools must be built for profiling for this to
      work.
      </li><br><p>

  <li><code>--trace-syscalls=no</code> [default]<br>
      <code>--trace-syscalls=yes</code>
      <p>Enable/disable tracing of system call intercepts.</li><br>
      <p>

  <li><code>--trace-signals=no</code> [default]<br>
      <code>--trace-signals=yes</code>
      <p>Enable/disable tracing of signal handling.</li><br>
      <p>

  <li><code>--trace-sched=no</code> [default]<br>
      <code>--trace-sched=yes</code>
      <p>Enable/disable tracing of thread scheduling events.</li><br>
      <p>

  <li><code>--trace-pthread=none</code> [default]<br>
      <code>--trace-pthread=some</code> <br>
      <code>--trace-pthread=all</code>
      <p>Specifies amount of trace detail for pthread-related events.</li><br>
      <p>

  <li><code>--trace-symtab=no</code> [default]<br>
      <code>--trace-symtab=yes</code>
      <p>Enable/disable tracing of symbol table reading.</li><br>
      <p>

  <li><code>--trace-malloc=no</code> [default]<br>
      <code>--trace-malloc=yes</code>
      <p>Enable/disable tracing of malloc/free (et al) intercepts.
      </li><br>
      <p>

  <li><code>--trace-codegen=XXXXX</code> [default: 00000]
      <p>Enable/disable tracing of code generation.  Code can be printed
      at five different stages of translation;  each <code>X</code> element
      must be 0 or 1.
      </li><br>
      <p>

  <li><code>--dump-error=&lt;number></code> [default: inactive]
      <p>After the program has exited, show gory details of the
      translation of the basic block containing the &lt;number>'th
      error context.  When used with <code>--single-step=yes</code>,
      can show the exact x86 instruction causing an error.  This is
      all fairly dodgy and doesn't work at all if threads are
      involved.</li><br>
      <p>
</ul>

<h4>Setting default options</h4>

<p>Note that Valgrind also reads options from three places:
<ul>
<li>The file <code>~/.valgrindrc</code>
<li>The environment variable <code>$VALGRIND_OPTS</code>
<li>The file <code>./.valgrindrc</code>
</ul>
These are processed in the given order, before the command-line options.
Options processed later override those processed earlier;  for example,
options in <code>./.valgrindrc</code> will take precedence over those in
<code>~/.valgrindrc</code>.  The first two are particularly useful for
setting the default tool to use.
<p>
Any tool-specific options put in <code>$VALGRIND_OPTS</code> or the
<code>.valgrindrc</code> 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 <code>~/.valgrindrc</code>:

<pre>
    --memcheck:leak-check=yes
</pre>

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


<a name="clientreq"></a>
<h3>2.7&nbsp; The Client Request mechanism</h3>

Valgrind has a trapdoor mechanism via which the client program can
pass all manner of requests and queries to Valgrind and the current tool.
Internally, this is used extensively to make malloc, free, signals, threads,
etc, work, although you don't see that.
<p>
For your convenience, a subset of these so-called client requests is
provided to allow you to tell Valgrind facts about the behaviour of
your program, and conversely to make queries.  In particular, your
program can tell Valgrind about changes in memory range permissions
that Valgrind would not otherwise know about, and so allows clients to
get Valgrind to do arbitrary custom checks.
<p>
Clients need to include a header file to make this work.  Which header file
depends on which client requests you use.  Some client requests are handled by
the core, and are defined in the header file <code>valgrind.h</code>.
Tool-specific header files are named after the tool, e.g.
<code>memcheck.h</code>.  All header files can be found in the
<code>include</code> directory of wherever Valgrind was installed.
<p>
The macros in these header files have the magical property that
they generate code in-line which Valgrind can spot.  However, the code
does nothing when not run on Valgrind, so you are not forced to run
your program on Valgrind just because you use the macros in this file.
Also, you are not required to link your program with any extra
supporting libraries.
<p>
Here is a brief description of the macros available in
<code>valgrind.h</code>, which work with more than one tool (see the
tool-specific documentation for explanations of the tool-specific macros).
<ul>
<li><code>RUNNING_ON_VALGRIND</code>: returns 1 if running on
    Valgrind, 0 if running on the real CPU.
<p>
<li><code>VALGRIND_DISCARD_TRANSLATIONS</code>: discard translations
    of code in the specified address range.  Useful if you are
    debugging a JITter or some other dynamic code generation system.
    After this call, attempts to execute code in the invalidated
    address range will cause Valgrind to make new translations of that
    code, which is probably the semantics you want.  Note that this is
    implemented naively, and involves checking all 200191 entries in
    the translation table to see if any of them overlap the specified
    address range.  So try not to call it often, or performance will
    nosedive.  Note that you can be clever about this: you only need
    to call it when an area which previously contained code is
    overwritten with new code.  You can choose to write code into
    fresh memory, and just call this occasionally to discard large
    chunks of old code all at once.
    <p>
    Warning: minimally tested, especially for tools other than Memcheck.
<p>
<li><code>VALGRIND_COUNT_ERRORS</code>: returns the number of errors
    found so far by Valgrind.  Can be useful in test harness code when
    combined with the <code>--log-fd=-1</code> option;  this runs
    Valgrind silently, but the client program can detect when errors
    occur.  Only useful for tools that report errors, e.g. it's useful for
    Memcheck, but for Cachegrind it will always return zero because 
    Cachegrind doesn't report errors.
<p>
<li><code>VALGRIND_MALLOCLIKE_BLOCK</code>: If your program manages its own
    memory instead of using the standard
    <code>malloc()</code>/<code>new</code>/<code>new[]</code>, tools that track
    information about heap blocks will not do nearly as good a
    job.  For example, Memcheck won't detect nearly as many errors, and the
    error messages won't be as informative.  To improve this situation, use
    this macro just after your custom allocator allocates some new memory.  See
    the comments in <code>valgrind.h</code> for information on how to use it.
<p>
<li><code>VALGRIND_FREELIKE_BLOCK</code>: This should be used in conjunction 
    with <code>VALGRIND_MALLOCLIKE_BLOCK</code>.  Again, see
    <code>memcheck/memcheck.h</code> for information on how to use it.  
<p>
<li><code>VALGRIND_CREATE_MEMPOOL</code>: This is similar to
    <code>VALGRIND_MALLOCLIKE_BLOCK</code>, but is tailored towards code
    that uses memory pools.  See the comments in <code>valgrind.h</code>
    for information on how to use it.
<p>
<li><code>VALGRIND_DESTROY_MEMPOOL</code>: This should be used in
    conjunction with <code>VALGRIND_CREATE_MEMPOOL</code> Again, see the
    comments in <code>valgrind.h</code> for information on how to use it.
<p>
<li><code>VALGRIND_MEMPOOL_ALLOC</code>: This should be used in
    conjunction with <code>VALGRIND_CREATE_MEMPOOL</code> Again, see the
    comments in <code>valgrind.h</code> for information on how to use it.
<p>
<li><code>VALGRIND_MEMPOOL_FREE</code>: This should be used in
    conjunction with <code>VALGRIND_CREATE_MEMPOOL</code> Again, see the
    comments in <code>valgrind.h</code> for information on how to use it.
<p>
<li><code>VALGRIND_NON_SIMD_CALL[0123]</code>: executes a function of 0, 1, 2
     or 3 args in the client program on the <i>real</i> CPU, not the virtual
     CPU that Valgrind normally runs code on.  These are used in various ways
     internally to Valgrind.  They might be useful to client programs.
     <b>Warning:</b> Only use these if you <i>really</i> know what you are
     doing.
<p>
<li><code>VALGRIND_PRINTF(format, ...)</code>: printf a message to the
    log file when running under Valgrind.  Nothing is output if not
    running under Valgrind.  Returns the number of characters output.
<p>
<li><code>VALGRIND_PRINTF_BACKTRACE(format, ...)</code>: printf a message
    to the log file along with a stack backtrace when running under
    Valgrind.  Nothing is output if not running under Valgrind.
    Returns the number of characters output.
<p>
</ul>
Note that <code>valgrind.h</code> is included by all the tool-specific header
files (such as <code>memcheck.h</code>), so you don't need to include it in
your client if you include a tool-specific header.
<p>


<a name="pthreads"></a>
<h3>2.8&nbsp; Support for POSIX Pthreads</h3>

Valgrind supports programs which use POSIX pthreads.  Getting this to work was
technically challenging but it all works well enough for significant threaded
applications to work.
<p>
It works as follows: threaded apps are (dynamically) linked against
<code>libpthread.so</code>.  Usually this is the one installed with
your Linux distribution.  Valgrind, however, supplies its own
<code>libpthread.so</code> and automatically connects your program to
it instead.
<p>
The fake <code>libpthread.so</code> and Valgrind cooperate to
implement a user-space pthreads package.  This approach avoids the 
horrible implementation problems of implementing a truly
multiprocessor version of Valgrind, but it does mean that threaded
apps run only on one CPU, even if you have a multiprocessor machine.
<p>
Valgrind schedules your threads in a round-robin fashion, with all
threads having equal priority.  It switches threads every 50000 basic
blocks (typically around 300000 x86 instructions), which means you'll
get a much finer interleaving of thread executions than when run
natively.  This in itself may cause your program to behave differently
if you have some kind of concurrency, critical race, locking, or
similar, bugs.
<p>
As of the Valgrind-1.0 release, the state of pthread support was as follows:
<ul>
<li>Mutexes, condition variables, thread-specific data,
    <code>pthread_once</code>, reader-writer locks, semaphores,
    cleanup stacks, cancellation and thread detaching currently work.
    Various attribute-like calls are handled but ignored; you get a
    warning message.
<p>
<li>Currently the following syscalls are thread-safe (nonblocking):
    <code>write</code> <code>read</code> <code>nanosleep</code>
    <code>sleep</code> <code>select</code> <code>poll</code> 
    <code>recvmsg</code> and
    <code>accept</code>.
<p>
<li>Signals in pthreads are now handled properly(ish): 
    <code>pthread_sigmask</code>, <code>pthread_kill</code>,
    <code>sigwait</code> and <code>raise</code> are now implemented.
    Each thread has its own signal mask, as POSIX requires.
    It's a bit kludgey -- there's a system-wide pending signal set,
    rather than one for each thread.  But hey.
</ul>

As of 18 May 02, the following threaded programs now work fine on my
RedHat 7.2 box: Opera 6.0Beta2, KNode in KDE 3.0, Mozilla-0.9.2.1 and
Galeon-0.11.3, both as supplied with RedHat 7.2.  Also Mozilla 1.0RC2.
OpenOffice 1.0.  MySQL 3.something (the current stable release).



<a name="signals"></a>
<h3>2.9&nbsp; Handling of signals</h3>

Valgrind provides suitable handling of signals, so, provided you stick
to POSIX stuff, you should be ok.  Basic sigaction() and sigprocmask()
are handled.  Signal handlers may return in the normal way or do
longjmp(); both should work ok.  As specified by POSIX, a signal is
blocked in its own handler.  Default actions for signals should work
as before.  Etc, etc.

<p>Under the hood, dealing with signals is a real pain, and Valgrind's
simulation leaves much to be desired.  If your program does
way-strange stuff with signals, bad things may happen.  If so, let me
know.  I don't promise to fix it, but I'd at least like to be aware of
it.



<a name="install"></a>
<h3>2.10&nbsp; Building and installing</h3>

We now use the standard Unix <code>./configure</code>,
<code>make</code>, <code>make install</code> mechanism, and I have
attempted to ensure that it works on machines with kernel 2.2 or 2.4
and glibc 2.1.X or 2.2.X.  I don't think there is much else to say.
There are no options apart from the usual <code>--prefix</code> that
you should give to <code>./configure</code>.

<p>
The <code>configure</code> script tests the version of the X server
currently indicated by the current <code>$DISPLAY</code>.  This is a
known bug.  The intention was to detect the version of the current
XFree86 client libraries, so that correct suppressions could be
selected for them, but instead the test checks the server version.
This is just plain wrong.

<p>
If you are building a binary package of Valgrind for distribution,
please read <code>README_PACKAGERS</code>.  It contains some important
information.

<p>
Apart from that there is no excitement here.  Let me know if you have
build problems.



<a name="problems"></a>
<h3>2.11&nbsp; If you have problems</h3>
Contact us at <a href="http://valgrind.kde.org">valgrind.kde.org</a>.

<p>See <a href="#limits">this section</a> for the known limitations of
Valgrind, and for a list of programs which are known not to work on
it.

<p>The translator/instrumentor has a lot of assertions in it.  They
are permanently enabled, and I have no plans to disable them.  If one
of these breaks, please mail us!

<p>If you get an assertion failure on the expression
<code>chunkSane(ch)</code> in <code>vg_free()</code> in
<code>vg_malloc.c</code>, this may have happened because your program
wrote off the end of a malloc'd block, or before its beginning.
Valgrind should have emitted a proper message to that effect before
dying in this way.  This is a known problem which I should fix.

<p>
Read the file <code>FAQ.txt</code> in the source distribution, for
more advice about common problems, crashes, etc.

<a name="limits"></a>
<h3>2.12&nbsp; Limitations</h3>

The following list of limitations seems depressingly long.  However,
most programs actually work fine.

<p>Valgrind will run x86-GNU/Linux ELF dynamically linked binaries, on
a kernel 2.2.X or 2.4.X system, subject to the following constraints:

<ul>
  <li>No support for 3DNow instructions.  If the translator encounters
      these, Valgrind will generate a SIGILL when the instruction is
      executed.</li>
      <p>

  <li>Pthreads support is improving, but there are still significant
      limitations in that department.  See the section above on
      Pthreads.  Note that your program must be dynamically linked
      against <code>libpthread.so</code>, so that Valgrind can
      substitute its own implementation at program startup time.  If
      you're statically linked against it, things will fail
      badly.</li>
      <p>

  <li>Memcheck assumes that the floating point registers are
      not used as intermediaries in memory-to-memory copies, so it
      immediately checks definedness of values loaded from memory by
      floating-point loads.  If you want to write code which copies
      around possibly-uninitialised values, you must ensure these
      travel through the integer registers, not the FPU.</li>
      <p>

  <li>If your program does its own memory management, rather than
      using malloc/new/free/delete, it should still work, but
      Valgrind's error checking won't be so effective.
      If you describe your program's memory management scheme
      using "client requests" (Section 3.7 of this manual),
      Memcheck can do better.  Nevertheless, using malloc/new
      and free/delete is still the best approach.
      </li>
      <p>

  <li>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.</li>
      <p>

  <li>Programs which switch stacks are not well handled.  Valgrind
      does have support for this, but I don't have great faith in it.
      It's difficult -- there's no cast-iron way to decide whether a
      large change in %esp is as a result of the program switching
      stacks, or merely allocating a large object temporarily on the
      current stack -- yet Valgrind needs to handle the two situations
      differently.</li>
      <p>

  <li>x86 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.</li>
      <p>

  <li>x86 floating point works correctly, but floating-point code may
      run even more slowly than integer code, due to my simplistic
      approach to FPU emulation.</li>
      <p>

  <li>Memory consumption of your program is majorly increased whilst
      running under Valgrind.  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 14-16 times larger
      than the original (!) so you can easily end up with 30+ MB of
      translations when running (eg) a web browser.
      </li>
</ul>

Programs which are known not to work are:

<ul>
  <li>emacs starts up but immediately concludes it is out of memory
      and aborts.  Emacs has it's own memory-management scheme, but I
      don't understand why this should interact so badly with
      Valgrind.  Emacs works fine if you build it to use the standard
      malloc/free routines.</li><br>
      <p>
</ul>

Known platform-specific limitations, as of release 1.0.0:

<ul>
  <li>On Red Hat 7.3, there have been reports of link errors (at
      program start time) for threaded programs using
      <code>__pthread_clock_gettime</code> and
      <code>__pthread_clock_settime</code>.  This appears to be due to
      <code>/lib/librt-2.2.5.so</code> needing them.  Unfortunately I
      do not understand enough about this problem to fix it properly,
      and I can't reproduce it on my test RedHat 7.3 system.  Please
      mail me if you have more information / understanding.  </li><br>
      <p>
</ul>



<a name="howworks"></a>
<h3>2.13&nbsp; How it works -- a rough overview</h3>
Some gory details, for those with a passion for gory details.  You
don't need to read this section if all you want to do is use Valgrind.
What follows is an outline of the machinery.  A more detailed
(and somewhat out of date) description is to be found
<A HREF="mc_techdocs.html">here</A>.

<a name="startb"></a>
<h4>2.13.1&nbsp; Getting started</h4>

Valgrind is compiled into a shared object, valgrind.so.  The shell
script valgrind sets the LD_PRELOAD environment variable to point to
valgrind.so.  This causes the .so to be loaded as an extra library to
any subsequently executed dynamically-linked ELF binary, viz, the
program you want to debug.

<p>The dynamic linker allows each .so in the process image to have an
initialisation function which is run before main().  It also allows
each .so to have a finalisation function run after main() exits.

<p>When valgrind.so's initialisation function is called by the dynamic
linker, the synthetic CPU to starts up.  The real CPU remains locked
in valgrind.so for the entire rest of the program, but the synthetic
CPU returns from the initialisation function.  Startup of the program
now continues as usual -- the dynamic linker calls all the other .so's
initialisation routines, and eventually runs main().  This all runs on
the synthetic CPU, not the real one, but the client program cannot
tell the difference.

<p>Eventually main() exits, so the synthetic CPU calls valgrind.so's
finalisation function.  Valgrind detects this, and uses it as its cue
to exit.  It prints summaries of all errors detected, possibly checks
for memory leaks, and then exits the finalisation routine, but now on
the real CPU.  The synthetic CPU has now lost control -- permanently
-- so the program exits back to the OS on the real CPU, just as it
would have done anyway.

<p>On entry, Valgrind switches stacks, so it runs on its own stack.
On exit, it switches back.  This means that the client program
continues to run on its own stack, so we can switch back and forth
between running it on the simulated and real CPUs without difficulty.
This was an important design decision, because it makes it easy (well,
significantly less difficult) to debug the synthetic CPU.


<a name="engine"></a>
<h4>2.13.2&nbsp; The translation/instrumentation engine</h4>

Valgrind does not directly run any of the original program's code.  Only
instrumented translations are run.  Valgrind maintains a translation
table, which allows it to find the translation quickly for any branch
target (code address).  If no translation has yet been made, the
translator - a just-in-time translator - is summoned.  This makes an
instrumented translation, which is added to the collection of
translations.  Subsequent jumps to that address will use this
translation.

<p>Valgrind no longer directly supports detection of self-modifying
code.  Such checking is expensive, and in practice (fortunately)
almost no applications need it.  However, to help people who are
debugging dynamic code generation systems, there is a Client Request 
(basically a macro you can put in your program) which directs Valgrind
to discard translations in a given address range.  So Valgrind can
still work in this situation provided the client tells it when
code has become out-of-date and needs to be retranslated.

<p>The JITter translates basic blocks -- blocks of straight-line-code
-- as single entities.  To minimise the considerable difficulties of
dealing with the x86 instruction set, x86 instructions are first
translated to a RISC-like intermediate code, similar to sparc code,
but with an infinite number of virtual integer registers.  Initially
each insn is translated seperately, and there is no attempt at
instrumentation.

<p>The intermediate code is improved, mostly so as to try and cache
the simulated machine's registers in the real machine's registers over
several simulated instructions.  This is often very effective.  Also,
we try to remove redundant updates of the simulated machines's
condition-code register.

<p>The intermediate code is then instrumented, giving more
intermediate code.  There are a few extra intermediate-code operations
to support instrumentation; it is all refreshingly simple.  After
instrumentation there is a cleanup pass to remove redundant value
checks.

<p>This gives instrumented intermediate code which mentions arbitrary
numbers of virtual registers.  A linear-scan register allocator is
used to assign real registers and possibly generate spill code.  All
of this is still phrased in terms of the intermediate code.  This
machinery is inspired by the work of Reuben Thomas (Mite).

<p>Then, and only then, is the final x86 code emitted.  The
intermediate code is carefully designed so that x86 code can be
generated from it without need for spare registers or other
inconveniences.

<p>The translations are managed using a traditional LRU-based caching
scheme.  The translation cache has a default size of about 14MB.

<a name="track"></a>

<h4>2.13.3&nbsp; Tracking the status of memory</h4> Each byte in the
process' address space has nine bits associated with it: one A bit and
eight V bits.  The A and V bits for each byte are stored using a
sparse array, which flexibly and efficiently covers arbitrary parts of
the 32-bit address space without imposing significant space or
performance overheads for the parts of the address space never
visited.  The scheme used, and speedup hacks, are described in detail
at the top of the source file vg_memory.c, so you should read that for
the gory details.

<a name="sys_calls"></a>

<h4>2.13.4 System calls</h4>
All system calls are intercepted.  The memory status map is consulted
before and updated after each call.  It's all rather tiresome.  See
coregrind/vg_syscalls.c for details.

<a name="sys_signals"></a>

<h4>2.13.5&nbsp; Signals</h4>
All system calls to sigaction() and sigprocmask() are intercepted.  If
the client program is trying to set a signal handler, Valgrind makes a
note of the handler address and which signal it is for.  Valgrind then
arranges for the same signal to be delivered to its own handler.

<p>When such a signal arrives, Valgrind's own handler catches it, and
notes the fact.  At a convenient safe point in execution, Valgrind
builds a signal delivery frame on the client's stack and runs its
handler.  If the handler longjmp()s, there is nothing more to be said.
If the handler returns, Valgrind notices this, zaps the delivery
frame, and carries on where it left off before delivering the signal.

<p>The purpose of this nonsense is that setting signal handlers
essentially amounts to giving callback addresses to the Linux kernel.
We can't allow this to happen, because if it did, signal handlers
would run on the real CPU, not the simulated one.  This means the
checking machinery would not operate during the handler run, and,
worse, memory permissions maps would not be updated, which could cause
spurious error reports once the handler had returned.

<p>An even worse thing would happen if the signal handler longjmp'd
rather than returned: Valgrind would completely lose control of the
client program.

<p>Upshot: we can't allow the client to install signal handlers
directly.  Instead, Valgrind must catch, on behalf of the client, any
signal the client asks to catch, and must delivery it to the client on
the simulated CPU, not the real one.  This involves considerable
gruesome fakery; see vg_signals.c for details.
<p>



<a name="example"></a>
<h3>2.14&nbsp; An example run</h3>
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).
<pre>
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== loaded 5950 symbols, 142333 line number locations
==25832== 
==25832== Invalid read of size 4
==25832==    at 0x8048724: _ZN10BandMatrix6ReSizeEiii (bogon.cpp:45)
==25832==    by 0x80487AF: main (bogon.cpp:66)
==25832==    by 0x40371E5E: __libc_start_main (libc-start.c:129)
==25832==    by 0x80485D1: (within /home/sewardj/newmat10/bogon)
==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
==25832==
==25832== exiting, did 1881 basic blocks, 0 misses.
==25832== 223 translations, 3626 bytes in, 56801 bytes out.
</pre>
<p>The GCC folks fixed this about a week before gcc-3.0 shipped.
<p>

<a name="warnings"></a>
<h3>2.15&nbsp; Warning messages you might see</h3>

Most of these only appear if you run in verbose mode (enabled by
<code>-v</code>):
<ul>
<li> <code>More than 50 errors detected.  Subsequent errors
     will still be recorded, but in less detail than before.</code>
     <br>
     After 50 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 50.  The 50 constant can
     be changed by recompiling Valgrind.
<p>
<li> <code>More than 300 errors detected.  I'm not reporting any more.
     Final error counts may be inaccurate.  Go fix your
     program!</code>
     <br>
     After 300 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 300
     number is a compile-time constant.
<p>
<li> <code>Warning: client switching stacks?</code>
     <br>
     Valgrind spotted such a large change in the stack pointer, %esp,
     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.  You may get
     many bogus error messages following this, if Valgrind guesses
     wrong.  At the moment "large change" is defined as a change of
     more that 2000000 in the value of the %esp (stack pointer)
     register.
<p>
<li> <code>Warning: client attempted to close Valgrind's logfile fd &lt;number>
     </code>
     <br>
     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 <code>--log-fd=&lt;number></code> 
     option to specify a different logfile file-descriptor number.
     Or 
<p>
<li> <code>Warning: noted but unhandled ioctl &lt;number></code>
     <br>
     Valgrind observed a call to one of the vast family of
     <code>ioctl</code> system calls, but did not modify its
     memory status info (because I have not yet got round to it).
     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.
<p>
<li> <code>Warning: set address range perms: large range &lt;number></code>
     <br> 
     Diagnostic message, mostly for benefit of the valgrind
     developers, to do with memory permissions.
</ul>

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