How does realloc know how much to copy?

Question

how does realloc know the size of original data?

 void *realloc(void *ptr, size_t size);

So, if the implementation is like this:

Answer 1

But how can I then implement realloc in my code with malloc/free/..?

If you're already using malloc & free, why not just use realloc? else you can just have a look at the CRT source that ships with MSVC/gcc etc. (or just download it, in the case of GCC), and see how they implement it. If your running a custom allocator, then its a little more situational, eg: I use a binary bin with a slab type system, in which case realloc is simple:

void* Reallocate(Manager* pManager, void* pBlock, size_t nSize, const char* szFile, const DWORD dwLine)
{
    #if ( MMANAGER_NULL_TO_DEFAULT )
        if(pManager == NULL)
            pManager = MMANAGER_DEFUALT_MANAGER;
    #endif

    if(pBlock == NULL)
        return Allocate(pManager,nSize,szFile,dwLine);
    else if(nSize == 0)
    {
        Free(pManager,pBlock,szFile,dwLine);
        return NULL;
    }

    BlockHeader* pHeader = GetHeader(pBlock);
    size_t nPrevSize = pHeader->pPoolBlock->nSize;
    if(nPrevSize < nSize)
    {
        void* pNewBlock = Allocate(pManager,nSize,szFile,dwLine);
        memcpy(pNewBlock,pBlock,nPrevSize);
        PoolBlock* pPoolBlock = pHeader->pPoolBlock;
        if(pPoolBlock == NULL)
            free(pHeader);
        else
            FreeBlock(pPoolBlock,pHeader);

        return pNewBlock;
    }

    return pBlock;
}