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<a name="Introduction-to-Profiling"></a>
<h2 class="chapter">1 Introduction to Profiling</h2>


<p>Profiling allows you to learn where your program spent its time and which
functions called which other functions while it was executing.  This
information can show you which pieces of your program are slower than you
expected, and might be candidates for rewriting to make your program
execute faster.  It can also tell you which functions are being called more
or less often than you expected.  This may help you spot bugs that had
otherwise been unnoticed.
</p>
<p>Since the profiler uses information collected during the actual execution
of your program, it can be used on programs that are too large or too
complex to analyze by reading the source.  However, how your program is run
will affect the information that shows up in the profile data.  If you
don&rsquo;t use some feature of your program while it is being profiled, no
profile information will be generated for that feature.
</p>
<p>Profiling has several steps:
</p>
<ul>
<li> You must compile and link your program with profiling enabled.
See <a href="Compiling.html#Compiling">Compiling a Program for Profiling</a>.

</li><li> You must execute your program to generate a profile data file.
See <a href="Executing.html#Executing">Executing the Program</a>.

</li><li> You must run <code>gprof</code> to analyze the profile data.
See <a href="Invoking.html#Invoking"><code>gprof</code> Command Summary</a>.
</li></ul>

<p>The next three chapters explain these steps in greater detail.
</p>

<p>Several forms of output are available from the analysis.
</p>
<p>The <em>flat profile</em> shows how much time your program spent in each function,
and how many times that function was called.  If you simply want to know
which functions burn most of the cycles, it is stated concisely here.
See <a href="Flat-Profile.html#Flat-Profile">The Flat Profile</a>.
</p>
<p>The <em>call graph</em> shows, for each function, which functions called it, which
other functions it called, and how many times.  There is also an estimate
of how much time was spent in the subroutines of each function.  This can
suggest places where you might try to eliminate function calls that use a
lot of time.  See <a href="Call-Graph.html#Call-Graph">The Call Graph</a>.
</p>
<p>The <em>annotated source</em> listing is a copy of the program&rsquo;s
source code, labeled with the number of times each line of the
program was executed.  See <a href="Annotated-Source.html#Annotated-Source">The Annotated Source
Listing</a>.
</p>
<p>To better understand how profiling works, you may wish to read
a description of its implementation.
See <a href="Implementation.html#Implementation">Implementation of Profiling</a>.
</p>
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