Unit testing involving C free()

Question

I'm working in Unix/Linux with C. I have a basic understanding of how memory allocation works, enough to know that if I malloc() then free(), I'm not likely going to actually free an entire page; thus if I use getrusage() before and after a free() I'm not likely going to see any difference.

I'd like to write a unit test for a function which destroys a data structure to see that the memory regions involved has actually been freed. I'm open to an OS-dependent solution, in which case my primary platform is

Linux beast 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

with OS X and FreeBSD as secondaries. I'm also open to a drop in replacement malloc() if there's a solution that makes checking free() relatively easy.

To be clear, I'm testing a routine that is to delete a large data structure, and I want to make sure that all the allocated regions are actually freed, in essence a unit test that the particular unit does not have a basic memory leak. I'm going to assume that free() does its job, I'm just making sure that my code actually calls free on all allocated regions it's responsible for.

In this particular case it's a tree structure and for each piece of data in the tree the structure is responsible for calling the routine that deletes the data stored in the tree as well, which might be some other arbitrary thing...

Answer 1

I hate to do this again, but after a night of sleep I found an explicit answer, it comes from SVID (does anyone even remember System V?), but is incorporated in glibc on Linux, likely through it's use in dlmalloc; hence it can be used on other system by using dlmalloc() as a drop in replacement malloc.

use the routine

struct mallinfo mallinfo(void);

and struct mallinfo is

struct mallinfo
{
  int arena;    /* non-mmapped space allocated from system */
  int ordblks;  /* number of free chunks */
  int smblks;   /* number of fastbin blocks */
  int hblks;    /* number of mmapped regions */
  int hblkhd;   /* space in mmapped regions */
  int usmblks;  /* maximum total allocated space */
  int fsmblks;  /* space available in freed fastbin blocks */
  int uordblks; /* total allocated space */
  int fordblks; /* total free space */
  int keepcost; /* top-most, releasable (via malloc_trim) space */
};

In particular arena and uordblks will give you the number of bytes allocated by malloc() independent of the size of the pages requested from the OS using sbrk() or mmap(), which are given by arena and hblkhd.