docs/xml/quick-start-guide.xml

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<book id="QuickStart" xreflabel="Valgrind Quick Start Guide">

<bookinfo>
  <title>The Valgrind Quick Start Guide</title>
  <releaseinfo>&rel-type; &rel-version; &rel-date;</releaseinfo>
  <copyright>
    <year>&vg-lifespan;</year>
    <holder><ulink url="&vg-developers;">Valgrind Developers</ulink></holder>
  </copyright>
  <legalnotice>
    <para>Email: <ulink url="mailto:&vg-vemail;">&vg-vemail;</ulink></para>
  </legalnotice>
</bookinfo>


<article id="quick-start">
<title>The Valgrind Quick Start Guide</title>


<sect1 id="quick-start.intro" xreflabel="Introduction">
<title>Introduction</title>

<para>The Valgrind distribution has multiple tools.  The most popular is
the memory checking tool (called Memcheck) which can detect many common
memory errors such as:</para>

<itemizedlist>
  <listitem>
    <para>touching memory you shouldn't (eg. overrunning heap block
    boundaries);</para>
  </listitem>
  <listitem>
    <para>using values before they have been initialized;</para>
  </listitem>
  <listitem>
    <para>incorrect freeing of memory, such as double-freeing heap
    blocks;</para>
  </listitem>
  <listitem>
    <para>memory leaks.</para>
  </listitem>
</itemizedlist>

<para>What follows is the minimum information you need to start
detecting memory errors in your program with Memcheck.  Note that this
guide applies to Valgrind version 2.4.0 and later; some of the
information is not quite right for earlier versions.</para>

</sect1>


<sect1 id="quick-start.prepare" xreflabel="Preparing your program">
<title>Preparing your program</title>

<para>Compile your program with <option>-g</option> to include debugging
information so that Memcheck's error messages include exact line
numbers.  Using <computeroutput>-O0</computeroutput> is also a good
idea, if you can tolerate the slowdown.  With
<computeroutput>-O1</computeroutput> line numbers in error messages can
be inaccurate, although generally speaking Memchecking code compiled at
<computeroutput>-O1</computeroutput> works fairly well.  Use of
<computeroutput>-O2</computeroutput> and above is not recommended as
Memcheck occasionally reports uninitialised-value errors which don't
really exist.</para>

</sect1>


<sect1 id="quick-start.mcrun" xreflabel="Running your program under Memcheck">
<title>Running your program under Memcheck</title>

<para>If you normally run your program like this:
<programlisting>    myprog arg1 arg2
</programlisting>

Use this command line:
<programlisting>    valgrind --leak-check=yes myprog arg1 arg2
</programlisting>

Memcheck is the default tool.  The <option>--leak-check</option> option
turns on the detailed memory leak detector.</para>

<para>Your program will run much slower (eg. 20 to 30 times) than
normal, and use a lot more memory.  Memcheck will issue messages about
memory errors and leaks that it detects.</para>

</sect1>


<sect1 id="quick-start.interpret" 
       xreflabel="Interpreting Memcheck's output">
<title>Interpreting Memcheck's output</title>
<para>Here's an example C program with a memory error and a memory leak.

<programlisting>
  #include &lt;stdlib.h&gt;

  void f(void)
  {
     int* x = malloc(10 * sizeof(int));
     x[10] = 0;        // problem 1: heap block overrun
  }                    // problem 2: memory leak -- x not freed

  int main(void)
  {
     f();
     return 0;
  }
</programlisting>

Most error messages look like the following, which describes problem 1,
the heap block overrun:

<programlisting>
  ==19182== Invalid write of size 4
  ==19182==    at 0x804838F: f (example.c:6)
  ==19182==    by 0x80483AB: main (example.c:11)
  ==19182==  Address 0x1BA45050 is 0 bytes after a block of size 40 alloc'd
  ==19182==    at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
  ==19182==    by 0x8048385: f (example.c:5)
  ==19182==    by 0x80483AB: main (example.c:11)
</programlisting>

Things to notice:

<itemizedlist>
  <listitem>
   <para>There is a lot of information in each error message; read it
   carefully.</para>
  </listitem>
  <listitem>
    <para>The 19182 is the process ID;  it's usually unimportant.</para>
  </listitem>
  <listitem>
   <para>The first line ("Invalid write...") tells you what kind of
   error it is.  Here, the program wrote to some memory it should not
   have due to a heap block overrun.</para>
  </listitem>
  <listitem>
    <para>Below the first line is a stack trace telling you where the
    problem occurred.  Stack traces can get quite large, and be
    confusing, especially if you are using the C++ STL.  Reading them
    from the bottom up can help.  If the stack trace is not big enough,
    use the <option>--num-callers</option> option to make it
    bigger.</para>
  </listitem>
  <listitem>
   <para>The code addresses (eg. 0x804838F) are usually unimportant, but
   occasionally crucial for tracking down weirder bugs.</para>
  </listitem>
  <listitem>
    <para>Some error messages have a second component which describes
    the memory address involved.  This one shows that the written memory
    is just past the end of a block allocated with malloc() on line 5 of
    example.c.</para>
  </listitem>
</itemizedlist>

It's worth fixing errors in the order they are reported, as later errors
can be caused by earlier errors.</para>

<para>Memory leak messages look like this:

<programlisting>
  ==19182== 40 bytes in 1 blocks are definitely lost in loss record 1 of 1
  ==19182==    at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
  ==19182==    by 0x8048385: f (a.c:5)
  ==19182==    by 0x80483AB: main (a.c:11)
</programlisting>

The stack trace tells you where the leaked memory was allocated.
Memcheck cannot tell you why the memory leaked, unfortunately.  (Ignore
the "vg_replace_malloc.c", that's an implementation detail.)</para>

<para>There are several kinds of leaks; the two most important
categories are:

<itemizedlist>
  <listitem>
    <para>"definitely lost": your program is leaking memory -- fix
    it!</para>
  </listitem>
  <listitem>
    <para>"probably lost": your program is leaking memory, unless you're
    doing funny things with pointers (such as moving them to point to
    the middle of a heap block).</para>
  </listitem>
</itemizedlist>

If you don't understand an error message, please consult 
<xref linkend="mc-manual.errormsgs"/> in the <xref linkend="manual"/>
which has examples of all the error messages Memcheck produces.</para>

</sect1>


<sect1 id="quick-start.caveats" xreflabel="Caveats">
<title>Caveats</title>

<para>Memcheck is not perfect;  it occasionally produces false positives,
and there are mechanisms for suppressing these (see 
<xref linkend="manual-core.suppress"/> in the <xref linkend="manual"/>).
However, it is typically right 99% of the time, so you should be wary of
ignoring its error messages.  After all, you wouldn't ignore warning
messages produced by a compiler, right?  The suppression mechanism is
also useful if Memcheck is reporting errors in library code that you
cannot change; the default suppression set hides a lot of these, but you
may come across more.</para>

<para>Memcheck also cannot detect every memory error your program has.
For example, it can't detect if you overrun the bounds of an array that
is allocated statically or on the stack.  But it should detect every
error that could crash your program (eg. cause a segmentation
fault).</para>

</sect1>


<sect1 id="quick-start.info" xreflabel="More Information">
<title>More information</title>

<para>Please consult the <xref linkend="FAQ"/> and the 
<xref linkend="manual"/>, which have much more information.  Note that
the other tools in the Valgrind distribution can be invoked with the
<option>--tool</option> option.</para>

</sect1>


</article>
</book>