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<title>GNU gprof: Assumptions</title>

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<a name="Assumptions"></a>
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Previous: <a href="Sampling-Error.html#Sampling-Error" accesskey="p" rel="prev">Sampling Error</a>, Up: <a href="Inaccuracy.html#Inaccuracy" accesskey="u" rel="up">Inaccuracy</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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<a name="Estimating-children-Times"></a>
<h3 class="section">6.2 Estimating <code>children</code> Times</h3>

<p>Some of the figures in the call graph are estimates&mdash;for example, the
<code>children</code> time values and all the time figures in caller and
subroutine lines.
</p>
<p>There is no direct information about these measurements in the profile
data itself.  Instead, <code>gprof</code> estimates them by making an assumption
about your program that might or might not be true.
</p>
<p>The assumption made is that the average time spent in each call to any
function <code>foo</code> is not correlated with who called <code>foo</code>.  If
<code>foo</code> used 5 seconds in all, and 2/5 of the calls to <code>foo</code> came
from <code>a</code>, then <code>foo</code> contributes 2 seconds to <code>a</code>&rsquo;s
<code>children</code> time, by assumption.
</p>
<p>This assumption is usually true enough, but for some programs it is far
from true.  Suppose that <code>foo</code> returns very quickly when its argument
is zero; suppose that <code>a</code> always passes zero as an argument, while
other callers of <code>foo</code> pass other arguments.  In this program, all the
time spent in <code>foo</code> is in the calls from callers other than <code>a</code>.
But <code>gprof</code> has no way of knowing this; it will blindly and
incorrectly charge 2 seconds of time in <code>foo</code> to the children of
<code>a</code>.
</p>
<p>We hope some day to put more complete data into <samp>gmon.out</samp>, so that
this assumption is no longer needed, if we can figure out how.  For the
novice, the estimated figures are usually more useful than misleading.
</p>


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