Question

How free will calculate no of bytes it need to free ??

Answer 1

Cant stop myself to answer this question -

Say you are allocating 10 Bytes, system allocates 4+10 bytes and store the value 14 in first 4 bytes. Then return the pointer after moving 4 bytes forward. So the user get only 10 bytes.

In free you provide the pointer, control goes back by 4 bytes and read the first 4 bytes to get the length and free the 14 bytes.

Comments

Can we corrupt this 4 extra bytes that has size information ?

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Its in the process space so yes...

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Value of K depende on OS kit.

Actually malloc allocates memory multiple of  "K".
Means
if k=4 and you want to allocate 1 byte malloc will allocate 4 byte + no of bytes to store allocated bytes
if k=4 and you want to allocate 3 byte malloc will allocate 4 byte + no of bytes to store allocated bytes.
if k=4 and you want to allocate 5 Or 7 byte malloc will allocate 8 byte + no of bytes to store allocated bytes.

This is the reason some time segmentation fault does not comes when we access bye more than the number given in malloc.

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hi salil thanks, and can you point me the book that related to these stuffs...

Answer 2

See the code below .
First 4 byes contains the no of bytes need to free.

void *malloc_flags(size_t size, lmm_flags_t flags)
{
      oskit_size_t *chunk;
      chunk = in_alloc(size + sizeof(oskit_size_t), flags);
      if (chunk)
                *chunk++ = size;
        return chunk;
}

----------------------------------------------------------------------

void free_flags(void *chunk)
{
     oskit_size_t size;
     /* Posix says free of NULL does nothing */
    if (! chunk)
          return;
     (oskit_size_t*)chunk -= 1;
     size = *((oskit_size_t*)chunk);
     in_free(chunk, size + sizeof(oskit_size_t));
  }

=====================================================================

void *malloc(size_t size)
{
       return malloc_flags(size,0);
}
---------------------------------------------------------------------

void free(void *chunk)
{
        return free_flags(chunk);
}

Answer 3

Actually while allocating memory through mallac function. system allocates memory a bit more what you aksed for ? This extra memory is being used to store information like size of allocated memory and etc.

When free is called for a vaild address, it search through the list of allocated block. if It finds then deallocate the memory with the stored size. For a invalid address, it might get crash, I am not sure for crash thing.