exp-omega/docs/o-manual.xml

<?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="o-manual" xreflabel="Omega">

<title>Omega: an instant memory leak detector</title>

<para>Omega is a Valgrind tool that traces pointers to memory
      allocations and produces debug output as the last reference
      is removed to a block that has not been de-allocated.</para>

<para>Omega works by tracking pointers with the use of p-bits.
      In similar fashion to the methods used by memcheck and
      addrcheck, each memory location is flagged by a p-bit which
      indicates if the location is holding a pointer to an allocated
      block. Since pointers are multibyte, we gain by only requiring
      a single p-bit per pointer thus needing one p-bit per 4 bytes
      on 32bit machines and one p-bit per 8 bytes on 64bit
      architectures.</para>

<para>Whilst this simplifies and reduces the memory footprint of
      one aspect of the tracking methodology, the other aspect can be
      a bit of a memory hog. As each block is allocated, we need to
      maintain data about the block and hold back-references to any
      live pointers. There are various ways of doing this and as the
      tool evolves further upon its release, doubtless you will all
      improve upon it (and that's why I love open source).</para>

<para>This tool has been checked against the simple test cases in
      the tests directory. Again, you will all have many more that
      can be added.</para>

<para>Note that in order to reduce the overheads that this tool
      incurs, it is assumed that the only problems left are memory
      leaks. This assumption allows many corner cases to be left
      un-coded. As we stumble upon those that cannot be avoided,
      they can be added. A major assumption in the tracking is
      that all pointers are aligned in memory. If this one fails,
      the tracking code will have to be re-jigged.</para>

<para>Bryan "Brain Murders" Meredith, 2006</para>

</chapter>