main/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-devs-url;">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 tool suite provides a number of debugging and
profiling tools that help you make your programs faster and more correct.
The most popular of these tools is called Memcheck.  It can detect many
memory-related errors that are common in C and C++ programs and that can
lead to crashes and unpredictable behaviour.</para>

<para>The rest of this guide gives the minimum information you need to start
detecting memory errors in your program with Memcheck.  For full
documentation of Memcheck and the other tools, please read the User Manual.
</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 <option>-O0</option> is also a good
idea, if you can tolerate the slowdown.  With
<option>-O1</option> line numbers in error messages can
be inaccurate, although generally speaking running Memcheck on code compiled
at <option>-O1</option> works fairly well, and the speed improvement
compared to running <option>-O0</option> is quite significant.
Use of
<option>-O2</option> 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:</para>
<programlisting>  myprog arg1 arg2
</programlisting>

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

<para>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, in a file called a.c, with a memory error
and a memory leak.</para>

<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>

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

<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>

<para>Things to notice:</para>

<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>

<para>It's worth fixing errors in the order they are reported, as
later errors can be caused by earlier errors.  Failing to do this is a
common cause of difficulty with Memcheck.</para>

<para>Memory leak messages look like this:</para>

<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>

<para>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:</para>

<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>

<para>Memcheck also reports uses of uninitialised values, most commonly with
the message "Conditional jump or move depends on uninitialised
value(s)".  It can be difficult to determine the root cause of these errors.
Try using the <option>--track-origins=yes</option> to get extra information.
This makes Memcheck run slower, but the extra information you get often
saves a lot of time figuring out where the uninitialised values are coming
from.</para>

<para>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 cannot detect every memory error your program has.
For example, it can't detect out-of-range reads or writes to arrays
that are allocated statically or on the stack.  But it should detect many
errors that could crash your program (eg. cause a segmentation
fault).</para>

<para>Try to make your program so clean that Memcheck reports no
errors.  Once you achieve this state, it is much easier to see when
changes to the program cause Memcheck to report new errors.
Experience from several years of Memcheck use shows that it is
possible to make even huge programs run Memcheck-clean.  For example,
large parts of KDE, OpenOffice.org and Firefox are Memcheck-clean, or very
close to it.</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>