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


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.

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


Bugs Found

Checked Projects
336
Collected Errors
12 743