How can I get the size of an array from a pointer

Question

I've allocated an "array" of mystruct of size n like this:

if (NULL == (p = calloc(sizeof(struct mystruct) * n,1))) {
 /* handle error */
}

Later on, I only have access to p, and no longer have n. Is there a way to determine the length of the array given just the pointer p?

I figure it must be possible, since free(p) does just that. I know malloc() keeps track of how much memory it has allocated, and that's why it knows the length; perhaps there is a way to query for this information? Something like...

int length = askMallocLibraryHowMuchMemoryWasAlloced(p) / sizeof(mystruct)

I know I should just rework the code so that I know n, but I'd rather not if possible. Any ideas?

Answer 1

keep track of the array size yourself; free uses the malloc chain to free the block that was allocated, which does not necessarily have the same size as the array you requested

Answer 2

This is a test of my sort routine. It sets up 7 variables to hold float values, then assigns them to an array, which is used to find the max value.

The magic is in the call to myMax:

float mmax = myMax((float *)&arr,(int) sizeof(arr)/sizeof(arr[0]));

And that was magical, wasn't it?

myMax expects a float array pointer (float *) so I use &arr to get the address of the array, and cast it as a float pointer.

myMax also expects the number of elements in the array as an int. I get that value by using sizeof() to give me byte sizes of the array and the first element of the array, then divide the total bytes by the number of bytes in each element. (we should not guess or hard code the size of an int because it's 2 bytes on some system and 4 on some like my OS X Mac, and could be something else on others).

NOTE:All this is important when your data may have a varying number of samples.

Here's the test code:

#include <stdio.h>

float a, b, c, d, e, f, g;

float myMax(float *apa,int soa){
 int i;
 float max = apa[0];
 for(i=0; i< soa; i++){
  if (apa[i]>max){max=apa[i];}
  printf("on i=%d val is %0.2f max is %0.2f, soa=%d\n",i,apa[i],max,soa);
 }
 return max;
}

int main(void)
{
 a = 2.0;
 b = 1.0;
 c = 4.0;
 d = 3.0;
 e = 7.0;
 f = 9.0;
 g = 5.0;
 float arr[] = {a,b,c,d,e,f,g};

 float mmax = myMax((float *)&arr,(int) sizeof(arr)/sizeof(arr[0]));
 printf("mmax = %0.2f\n",mmax);

 return 0;
}

Answer 3

For an array of pointers you can use a NULL-terminated array. The length can then determinate like it is done with strings. In your example you can maybe use an structure attribute to mark then end. Of course that depends if there is a member that cannot be NULL. So lets say you have an attribute name, that needs to be set for every struct in your array you can then query the size by:

int size;
struct mystruct *cur;

for (cur = myarray; cur->name != NULL; cur++)
    ;

size = cur - myarray;

Btw it should be calloc(n, sizeof(struct mystruct)) in your example.

Answer 4

One of the reasons that you can't ask the malloc library how big a block is, is that the allocator will usually round up the size of your request to meet some minimum granularity requirement (for example, 16 bytes). So if you ask for 5 bytes, you'll get a block of size 16 back. If you were to take 16 and divide by 5, you would get three elements when you really only allocated one. It would take extra space for the malloc library to keep track of how many bytes you asked for in the first place, so it's best for you to keep track of that yourself.

Answer 5

Some compilers provide msize() or similar functions (_msize() etc), that let you do exactly that

Answer 6

In uClibc, there is a MALLOC_SIZE macro in malloc.h:

/* The size of a malloc allocation is stored in a size_t word
   MALLOC_HEADER_SIZE bytes prior to the start address of the allocation:

     +--------+---------+-------------------+
     | SIZE   |(unused) | allocation  ...   |
     +--------+---------+-------------------+
     ^ BASE             ^ ADDR
     ^ ADDR - MALLOC_HEADER_SIZE
*/

/* The amount of extra space used by the malloc header.  */
#define MALLOC_HEADER_SIZE          \
  (MALLOC_ALIGNMENT < sizeof (size_t)       \
   ? sizeof (size_t)                \
   : MALLOC_ALIGNMENT)

/* Set up the malloc header, and return the user address of a malloc block. */
#define MALLOC_SETUP(base, size)  \
  (MALLOC_SET_SIZE (base, size), (void *)((char *)base + MALLOC_HEADER_SIZE))
/* Set the size of a malloc allocation, given the base address.  */
#define MALLOC_SET_SIZE(base, size) (*(size_t *)(base) = (size))

/* Return base-address of a malloc allocation, given the user address.  */
#define MALLOC_BASE(addr)   ((void *)((char *)addr - MALLOC_HEADER_SIZE))
/* Return the size of a malloc allocation, given the user address. */
#define MALLOC_SIZE(addr)   (*(size_t *)MALLOC_BASE(addr))