How to get memory block length after malloc?

Question

I thought that I couldn't retrieve the length of an allocated memory block like the simple .length function in Java. However, I now know that when malloc() allocates the block, it allocates extra bytes to hold an integer containing the size of the block. This integer is located at the beginning of the block; the address actually returned to the caller points to the location just past this length value. The problem is, I can't access that address to retrieve the block length.

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
        char *str;
        str = (char*) malloc(sizeof(char)*1000);
        int *length;
        length = str-4; /*because on 32 bit system, an int is 4 bytes long*/
        printf("Length of str:%d\n", *length);
        free(str);
}

**Edit: I finally did it. The problem is, it keeps giving 0 as the length instead of the size on my system is because my Ubuntu is 64 bit. I changed str-4 to str-8, and it works now.

If I change the size to 2000, it produces 2017 as the length. However, when I change to 3000, it gives 3009. I am using GCC.

Answer 1

You don't have to track it by your self!

size_t malloc_usable_size (void *ptr);

But it returns the real size of the allocated memory block! Not the size you passed to malloc!

Answer 2

What you're doing is definitely wrong. While it's almost certain that the word just before the allocated block is related to the size, even so it probably contains some additional flags or information in the unused bits. Depending on the implementation, this data might even be in the high bits, which would cause you to read the entirely wrong length. Also it's possible that small allocations (e.g. 1 to 32 bytes) are packed into special small-block pages with no headers, in which case the word before the allocated block is just part of another block and has no meaning whatsoever in relation to the size of the block you're examining.

Just stop this misguided and dangerous pursuit. If you need to know the size of a block obtained by malloc, you're doing something wrong.