CodeSport (March 2011)

Dealing with coding errors

Welcome to CodeSport!  In this month’s column, we turn our attention to software coding errors, which lead to buggy code. We will discuss different types of software bugs caused by software coding errors, and the various static and runtime bug-detection tools used to catch these software errors.

Programming/coding errors are a major cause of the software defects encountered after the software has been commercially released and is in use. Late detection of such coding errors after they have manifested themselves to end-users impacts software maintenance costs. Detecting a programmer’s error during development itself leads to lowered software maintenance costs, and helps improve the customer experience of the released software. Therefore, detecting bugs early in the development cycle is important.

There are certain kinds of common coding errors, which occur often, and a developer needs to keep a vigilant eye to avoid these. Let’s look at some of the more common variety.

Common coding errors

Null pointers

Typically, programmers forget to check whether a pointer can be NULL before dereferencing it. This is known as a null pointer dereferencing error. Consider the following code snippet:

int* bar(int cond)
 {
     int* alloc = 0;
      if (cond == 0)
         return 0;
 else
 {
    alloc = (int*) malloc(sizeof(int) * cond);
     if (alloc == 0)
         return 0;
    return alloc;
 }
}
       
int foo(int a)
{
      int *p = bar(a);

      int result = *p;
      return result;
}

In the function foo, the variable result is set to the value obtained by dereferencing the pointer p. However, there is no check to see if p is non-null before dereferencing it. This is a bad coding error, since p is obtained as the return value from the function bar, and can be zero in certain cases.

This error, wherein the programmer does not check for a pointer being non-null before dereferencing it, is known as a “potential null pointer dereference error”. This is a difficult bug to catch at run-time, since depending on the inputs, p can be null or non-null. If the set of inputs we are using to test the code do not result in p being null, then this issue cannot be caught by testing. However, most static source-code-analysis tools can detect this issue.

Memory access/usage errors

Another area where programmers frequently make coding errors is in dealing with dynamically allocated memory. There are two classes of errors that frequently occur. One is the case of memory leaks, wherein the programmer fails to release the dynamically allocated memory. The other is when the programmer uses the memory after having released it, known as a “Use after free” error.

In managed environments like Java, which have automatic garbage collection support, the programmer does not face the burden of explicitly having to free dynamically allocated memory after its use is over. However, statically compiled languages like C/C++, which do not have automatic garbage collection support, require the programmer to explicitly free the dynamically allocated memory.

It is important to correctly remember to free the memory after its use, while at the same time, you should not end up freeing it too early, lest you run into a “Use after free” error. Consider the code snippet given below:

void foo(int a);
{
     int* p = malloc(sizeof(int) * a);
      assert (p != null);
     *p = a;
     int result =  bar();
     if (result == 0)
          printf("result was zero\n");
     else
     {
        free(p);
      }
      printf("the value stored in 'p' is %d\n", *p);
      return;
}

In this snippet, the code allocates memory dynamically, and the address is stored in the variable p. Note that there is one path in function foo in which the allocated memory is freed, whereas there is another path in which the programmer has omitted to free the allocated memory. This constitutes a memory leak.

On the other hand, consider the printf statement just before the return statement in function foo. This is a possible dereference of the pointer p after it has been freed, since if we reach here after passing through the else clause, p  would already have been freed — an example of a “Use after free” error.

Leaks are not limited to dynamically allocated memory, but can occur with any system resource such as file handles, file descriptors, locks, etc. For instance, the programmer may open a file using a file descriptor, but forget to close the file using the file descriptor. This would result in a file descriptor leak. Similarly, it is possible for the programmer to acquire a lock, but forget to release it once the critical section is done.

Failure to release a lock can even lead to a deadlock. Therefore, programmers need to ensure that the resources they acquire are released as soon as their use is over, and no sooner than that. A release done too early, or too late (or perhaps never) can lead to either correctness issues, and/or performance problems.

There are two other important classes of memory access errors — namely, “Uninitialised memory reads”, and out-of-bounds access. Consider the following code snippet:

initialise(int* p, int size, int init_val)
{
      for (int i = 0; i <= size; i++)
           *(p+i) = init_val;
}

Can you figure out the coding error in the above code snippet? Well, the programmer has made a mistake in specifying the terminating condition for the for loop. Assume that the variable p points to an array of size 10. The calling code passed the argument p and the size as 10. However, the for loop in the function initialise actually tries to initialise 11 elements instead of 10, and performs an out-of-bounds access when it tries to initialise *(p+10).

While the above code snippet is trivial enough for the error to be detected by visual inspection, such out-of-bounds accesses in real-life applications are extremely difficult to find. Tools that can detect such errors need to perform sophisticated inter-procedural analysis to detect these types of errors.

While out-of-bounds errors are typically caused by an off-by-one error in loop bound calculation, another major class of memory access error is “Uninitialised memory read”. Consider the code snippet below:

int foo(int size)
{
     int* p = (int*) malloc(sizeof(int) * size);
     assert(p != 0);
     int result = bar(p);
     return result;
}
int bar(int* p)
{
     int my_val = *p;
     //do something with my_val
}

Note that while the memory pointed to by the variable p is allocated in the function foo, it was not initialised in foo. However, the code in the function bar tries to read from the memory pointed to by p. This is an “Uninitialised  memory read” (UMR).

Detecting UMR errors requires tracking when a memory location is initialised. Detecting UMR errors through static analysis is difficult, and hence there are runtime detection tools to spot memory access errors like UMR. In next month’s column, we will discuss the various static and runtime analysis tools to detect software bugs.

This month’s takeaway question

The question comes from one of our readers, Sindoo. Here is a small code snippet, where getObject() is a method that returns an object of some class created with the “new” operator:

void* temp = getObject();
if(temp)
{
   delete temp;
   temp = NULL;
}

This code shows that the delete called on a void* does not know the type of the object. What would happen in this case?

My ‘must-read book’ for this month

This month’s “must-read book” is actually not a single book, but a series of columns from the C++ guru, Herb Sutter, titled “Effective Concurrency”, published in Dr Dobbs programming journal. These articles are available from Sutter’s Web page; look for articles titled “Effective Concurrency”. In fact, I would recommend all of Herb Sutter’s articles, if you want to be an effective programmer.

If you have a favourite programming book that you think is a must-read for every programmer, please do send me a note with the book’s name, and a short write-up on why you think it is useful, so I can mention it in this column. This would help many readers who want to improve their coding skills.

If you have any favourite programming puzzles that you would like to discuss on this forum, please send them to me, along with your solutions and feedback, at sandyasm_AT_yahoo_DOT_com. Till we meet again next month, happy programming and here’s wishing you the very best!

  • Abhijit

    The behavior of the code mentioned in “months takeaway question” is undefined. It is undefined as C++ essentially requires type of the object to free it and there is no object of type void.

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