# V666. Value may not correspond with the length of a string passed with YY argument. Consider inspecting the NNth argument of the 'Foo' function.

The analyzer suspects that an incorrect argument has been passed into a function. An argument whose numerical value doesn't coincide with the string length found in the previous argument is considered incorrect. The analyzer draws this conclusion examining pairs of arguments consisting of a string literal and an integer constant. Analysis is performed over all the function calls of the same name.

Here's an example of incorrect code:

    if (!_strnicmp(szDir, "My Documents", 11)) // <<== Error!
      nFolder = 1;
    if (!_strnicmp(szDir, "Desktop", 7))
      nFolder = 2;
    if (!_strnicmp(szDir, "Network Favorites", 17))
      nFolder = 3;

In this case, the value 11 in the first function call is incorrect. Because of that, comparison will be successful if the 'szDir' variable points to the string literal "My Document". To fix the code you should just change the string length to a correct value, i.e. 12.

This is the fixed code:

    if (!_strnicmp(szDir, "My Documents", 12))
      nFolder = 1;

The V666 diagnostic is of empirical character. If you want to understand the point of it, you will have to read a complicated explanation. It's not obligatory, but if you choose not to read, then please check the function arguments very attentively. If you are sure that the code is absolutely correct, you may disable the diagnostic message output by adding the comment "//-V666".

Let's try to figure out how this diagnostic rule works. Look at the following code:

    foo("1234", 1, 4);
    foo("123", 2, 3);
    foo("321", 2, 2);

The analyzer will choose pairs of arguments: a string literal and a numerical value. For these, the analyzer will examine all the calls of this function and build a table of coincidence between the string length and numerical argument.

{ { "1234", 1 }, { "1234", 4 } } -> { false, true }

{ { "123", 2 }, { "123", 3 } } -> { false, true }

{ { "321", 2 }, { "321", 2 } } -> { false, false }

The first column is of no interest to us. It doesn't seem to be the string length. But the second column seems to represent the string length, and one of the calls contains an error.

This description is pretty sketchy, of course, but it allows you to grasp the general principle behind the diagnostic. Such an analysis is certainly not ideal, and false positives are inevitable. But it also lets you find interesting bugs sometimes.

This diagnostic is classified as:

CWE-805 CERT-EXP37-C

You can look at examples of errors detected by the V666 diagnostic.