[C] get allocated size

Ephracis · 12-28-2006, 03:51 AM

Hi,

How can I get how much I have allocated with malloc? I have a function that allocates memory using malloc but how can I later on check how much that was allocated? Is there some sort of function or will I have to keep track of that myself?

tuxdev · 12-28-2006, 09:08 AM

You can, but that requires architecture-dependent black magic. You'll have to keep track of it yourself somehow, either through a separate variable or by null termination (if it is an array of pointers).

demon_vox · 12-28-2006, 09:38 AM

Hi,
tuxdev is right. The language itself doesnt provide you a standard way to see how much memory you have allocated (or where, or blah). So you should write a myCountingMalloc which calls malloc and keep track of the statistics you want. Of course, you have to write since it does not exist

Cheers!

jim mcnamara · 12-28-2006, 10:30 AM

Other languages on non-unix platforms resort to what are called descriptors, usually kept in a global array. Old VMS did something like that in the kernel, for example. User processes had string descriptors in user space as well. The BSTR in Windows is a string descriptor and is derived from the VMS model -

Code:

/* this is the general idea */
typedef struct
{
  void *ptr;  /* copy of the base address */
  size_t len; /* bytes allocated */
} desc_t;
desc_t malloc_arr[100]={NULL,0};

The descriptor table(s) require maintenance for every single malloc, free, and realloc call you make. So it's best to write a wrapper for each call that does the maintenance. It's pretty good for preventing overruns.

Ephracis · 12-28-2006, 12:55 PM

Thanks for your very fast and interesting replies. Tuxdev, where can I read about this black magic? I do not intend to use it in my code but it would be interesting just to learn about it.

tuxdev · 12-28-2006, 01:20 PM

I'm not sure about where to find such information for glibc, but I would think it is documented somewhere. For a simple implementation of malloc I did once, I stored the size of the chunk in the previous word. So, if you got "addr" from malloc, "*(addr-1)" got you the size.

Dan04 · 12-29-2006, 09:55 PM

Code:

void* MyMalloc(size_t bytes) {
   void* mem = malloc(sizeof(size_t) + bytes);

   if (mem == NULL) {
      return NULL;
   } else {
      *((size_t*) mem) = bytes;
      return mem + sizeof(size_t);
   }
}

size_t BytesAllocated(void* mem) {
   return ((size_t*) mem)[-1];
}

void MyFree(void* mem) {
   free(mem - sizeof(size_t));
}