Examples of software bugs

Question

Possible Duplicate:
Software bugs examples

I am looking for examples of bugs one must check in his code...

But I do not mean logical bugs (such as Adding instead of Subtracting), I am talking about bugs that can be discovered (may be guessed) just by looking into code without knowing its purpose....

Examples:

• infinite loops
• opening connections without closing it (files, database)
• write to shared variable in multi threaded code without locking..

Please help me in finding more bugs like these...

Thanks

Answer 1

This one is quite famous, the ol' assignment instead of the intended conditional:

if(a = b) {
  ...
}

Answer 2

if ( obj == null && ! obj.IsValid() ) // bad short circuit behaviour

Answer 3

A classic c and c++ one: Dereferencing NULL pointers.

Similarly: Trying to access variables that have gone out of scope.

Answer 4

Improper loops:

for (int i = 0; i < 10; i++);
{
   //some code
}

Answer 5

Look at the laundry list for any static analysis tool. That's all they do; look for known bugs in code they don't understand. In non-GC environments, memory management problems (double free, no free, use uninitialized memory, don't check for NULL from malloc() etc.) are common.

Answer 6

A most egregious issue in c++:

if (1 < x < 10)

This was peppered throughout some code I just inherited.

To clarify why this is an issue, first 1 < x is evaluated giving true for x is 2. Then true < 10 will be evaluated, becoming 1 < 10, which isn't comparing x to 10.

Suppose x were 0. The first comparison is false. So it becomes 0 < 10. True. So no matter what the value of x, it is always true.

Answer 7

In Java :

if (someString == otherString) { /*...*/ }

Answer 8

if ( 1 )
   if ( 0 )
       printf("foo");
else
   printf("bar");

Programmer expected no output, but got "bar".

Answer 9

Switch statement without breaks.

Example:

switch (/*condition*/)
{
case 1: retCode = SUCCESS;
//...
default: retCode = UNKNOWN_ERROR;
}
return retCode; // Always UNKNOWN_ERROR

Answer 10

Well, there are many, many things you have to check. Here are a few ones, based on your examples:

• Loops

Check that at least one of the variables used in the loop condition is modified by the loop body. If not, you're looking at an infinite loop.

Also look out for stray semicolons, as in:

for (int i = 0; i < length; ++i);
{
    // This code will only run once.
}

• Leaked connections

Don't introduce these bugs in the first place by using RAII throughout your project. Most, if not all, languages have some kind of construct that guarantees resources are freed when leaving the scope (e.g. C++'s destructors, C#'s using, Python's with, Common Lisp's (unwind-protect), etc.).

• Missing locks in concurrent variable access

Hopefully the shared variables are clearly visible (e.g. they share some naming convention, or there's an up-to-date list somewhere, or they're all defined in the same module). Then you can grep the source for those variables and verify locks are put in place around all stores to them.

Of course, it's always better to encapsulate write access to shared variables inside functions or methods or whole classes that perform the necessary locking themselves. See Herb Sutter's Active Objects for an very good abstraction of this concept.

Answer 11

And also some nice ones can be found here.

Answer 12

All the answers are great, but as you intend to develop a "bug detector", I strongly recommend you to go through this FindBugs bug description list on SourceForge.

This resource and other resources in the website will be an awesome starting point for your project.

Answer 13

improper use of properties:

class Foo
{
    private Int32 bar;

    public Int32 Bar
    {
        get { return Bar; }
    }            
}

Answer 14

The software bug.

Answer 15

Not handling a divide by zero condition that may occur ..

Overflow and underflow error