<?xml version="1.0"?> <!-- -*- sgml -*- -->
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<chapter id="hg-manual" xreflabel="Helgrind: a data-race detector">
  <title>Helgrind: a data-race detector</title>

<para>Helgrind is a Valgrind tool for detecting data races in C
and C++ programs that use the Pthreads library.</para>

<para>Note: Helgrind does not work in Valgrind 3.1.0.  We hope
to reinstate in version 3.2.0.</para>

<para>To use this tool, you must specify
<computeroutput>--tool=helgrind</computeroutput> on the Valgrind
command line.</para>

<para>It uses the Eraser algorithm described in:

 <address>Eraser: A Dynamic Data Race Detector for Multithreaded Programs
  Stefan Savage, Michael Burrows, Greg Nelson, Patrick Sobalvarro and Thomas Anderson
  ACM Transactions on Computer Systems, 15(4):391-411
  November 1997.
 </address>
</para>

<para>We also incorporate significant improvements from this paper:

 <address>Runtime Checking of Multithreaded Applications with Visual Threads
  Jerry J. Harrow, Jr.
  Proceedings of the 7th International SPIN Workshop on Model Checking of Software
  Stanford, California, USA
  August 2000
  LNCS 1885, pp331--342
  K. Havelund, J. Penix, and W. Visser, editors.
 </address>
</para>

<para>Basically what Helgrind does is to look for memory
locations which are accessed by more than one thread.  For each
such location, Helgrind records which of the program's
(pthread_mutex_)locks were held by the accessing thread at the
time of the access.  The hope is to discover that there is indeed
at least one lock which is used by all threads to protect that
location.  If no such lock can be found, then there is
(apparently) no consistent locking strategy being applied for
that location, and so a possible data race might result.</para>

<para>Helgrind also allows for "thread segment lifetimes".  If
the execution of two threads cannot overlap -- for example, if
your main thread waits on another thread with a
<computeroutput>pthread_join()</computeroutput> operation -- they
can both access the same variable without holding a lock.</para>

<para>There's a lot of other sophistication in Helgrind, aimed at
reducing the number of false reports, and at producing useful
error reports.  We hope to have more documentation one
day...</para>

</chapter>