Writing Defensive Code to Automatically Find Memory Leaks

In an earlier post, I suggested making all your memory allocations go through a single routine, and deletions through another. When you centralize allocation and deallocation like this, you gain a couple of benefits.

  • First of all, you make the memory allocation more explicit, which will tend to make programmers more careful.
  • Second, you can write “defensive code” to automatically find bugs during the development process.

I first thought hard about the concept of “defensive code” when I read the book Writing Solid Code by Steve Maguire. This book is now slightly dated, but the general concepts in it are excellent. Defensive code refers to the concept that, especially in debug code used during development, you should have validation routines that are called automatically that will call your attention to latent bugs such as memory leaks.

Once you’ve got your memory allocation and deallocations routines centralized, there are various ways you can instrument these routines during development so your code can tell you about memory allocation problems as you’re developing it.

The easiest thing to do is to keep counts of how many chunks of memory have been allocated and deleted and monitor it over time to see if the number of currently allocated chunks matches your expectation. This is lightweight enough to implement even in the most constrained environments.

A variation is to keep a copy of the pointer in the allocation routine in some kind of registry data structure. Then you can write a routine to print out the currently allocated items. This is especially helpful if a type of item is leaked only occasionally, as it lets you see exactly what items have leaked. (I have also implemented this in conjunction with Boost smart pointers, but be aware this keeps the reference count at 1, so you have to also write code to release this pointer when the reference is 1 in your printout routine or the items are never deallocated.)

In many embedded environments, you may have the source code to the C library, or even the entire operating system available. If something like valgrind is too resource-intensive or unavailable for your platform, you can look at some of the malloc debugging packages available on the net. If none meets your needs or is unavailable on your platform, you can simply modify malloc() and free() to record operations. You will want to log the address and size of each allocation and deallocation to a file or trace facility. You will also find it useful to record the addresses on the call stack, such as with GCC’s __builtin_return_address() facility. You can write a program to examine the saved allocation data and delete all the records that have matching allocation/deallocations. The results left over are malloc() calls with no matching free() calls and double calls to free(). You can then match up the calling addresses with a symbol table, the output of nm or the gdb list *address command to find out where these calls were made in your program.

Ben Mesander has more than 18 years of experience leading software development teams and implementing software. His strengths include Linux, C, C++, numerical methods, control systems and digital signal processing. His experience includes embedded software, scientific software and enterprise software development environments.

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