<!DOCTYPE qhelp PUBLIC

"-//Semmle//qhelp//EN"

"qhelp.dtd">

<qhelp>

<overview>

<p>The code compares the unsigned difference with zero.

It is highly probable that the condition is wrong if the difference expression has the unsigned type.

The condition holds in all the cases when difference is not equal to zero.

It means that we may use condition not equal. But the programmer probably wanted to compare the difference of elements.</p>

<p>False positives include code in which the first difference element is always greater than or equal to the second.

For comparison, ">" such conditions are equivalent to "! =", And are recommended for replacement.

For comparison "> =", the conditions are always true and are recommended to be excluded.</p>

</overview>

<recommendation>

<p>Use a simple comparison of two elements, instead of comparing their difference to zero.</p>

</recommendation>

<example>

<p>The following example demonstrates an erroneous and corrected use of comparison.</p>

<sample src="UnsignedDifferenceExpressionComparedZero.c" />

</example>

<references>

<li>CERT C Coding Standard:

<a href="https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules">INT02-C. Understand integer conversion rules</a>.

</li>

</references>

</qhelp>

import cpp

import semmle.code.cpp.commons.Exclusions

from RelationalOperation ro, SubExpr sub

not isFromMacroDefinition(ro) and

ro.getLesserOperand().getValue().toInt() = 0 and

ro.getGreaterOperand() = sub and

sub.getFullyConverted().getUnspecifiedType().(IntegralType).isUnsigned()

select ro , " difference in condition is always greater than or equal to zero "