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V555. Expression of the 'A - B > 0' …
Analyzer diagnostics
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Additional information

V555. Expression of the 'A - B > 0' kind will work as 'A != B'.

Dec 20 2010

The analyzer detected a potential error in an expression of "A - B > 0" type. It is highly probable that the condition is wrong if the "A - B" subexpression has the unsigned type.

The "A - B > 0" condition holds in all the cases when 'A' is not equal to 'B'. It means that we may write the "A != B" expression instead of "A - B > 0". However, the programmer must have intended to implement quite a different thing.

Consider this sample:

unsigned int *B;
if (B[i]-70 > 0)

The programmer wanted to check whether the i-item of the B array is above 70. He could write it this way: "B[i] > 70". But he, proceeding from some reasons, wrote it this way: "B[i]-70 > 0" and made a mistake. He forgot that items of the 'B' array have the 'unsigned' type. It means that the "B[i]-70" expression has the 'unsigned' type too. So it turns out that the condition is always true except for the case when the 'B[i]' item equals to 70.

Let's clarify this case.

If 'B[i]' is above 70, then "B[i]-70" is above 0.

If 'B[i]' is below 70, then we will get an overflow of the unsigned type and a very large value as a result. Let B[i] == 50. Then "B[i]-70" = 50u - 70u = 0xFFFFFFECu = 4294967276. Surely, 4294967276 > 0.

A demonstration sample:

unsigned A;
A = 10; cout << "A=10 " << (A-70 > 0) << endl;
A = 70; cout << "A=70 " << (A-70 > 0) << endl;
A = 90; cout << "A=90 " << (A-70 > 0) << endl;
// Will be printed
A=10 1
A=70 0
A=90 1

The first way to correct the code:

unsigned int *B;
if (B[i] > 70)

The second way to correct the code:

int *B;
if (B[i]-70 > 0)

Note that an expression of the "A - B > 0" type far not always signals an error. Consider a sample where the analyzer generates a false alarm:

// Functions GetLength() and GetPosition() return
// value of size_t type.
while ((inStream.GetLength() - inStream.GetPosition()) > 0)
{ ... }

GetLength() is always above or equal to GetPosition() here, so the code is correct. To suppress the false alarm, we may add the comment //-V555 or rewrite the code in the following way:

while (inStream.GetLength() != inStream.GetPosition())
{ ... }

Here is another case when no error occurs.

__int64 A;
__uint32 B;
if (A - B > 0)

The "A - B" subexpression here has the signed type __int64 and no error occurs. The analyzer does not generate warnings in such cases.

This diagnostic is classified as:

You can look at examples of errors detected by the V555 diagnostic.

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