为什么dlmalloc分配的块头包含4个字节的先前分配的块

I was going through dynamic memory allocator called doug lea memory allocator which uses best fit method to allocate memory on heap. The algorithm is a basis for more other algorithms but I found that in case of an allocated chunk the header of that chunk contains last 4 bytes of data of the previous chunk. I checked the algorithm explanation but couldn't find the reason. I want to know what is the purpose of this allocation of 4 bytes of previous chunk? I also came up with an explanation of this as the allocation of .dtors section in other chunk for synchronisation and proper use of space but want to know the details.I was going through dynamic memory allocator ca