为什么dlmalloc分配的块标头包含先前分配的块的4个字节

Yes块确实重叠。从前，记忆是非常昂贵的。这是dlmalloc、ptmalloc和glibcmalloc中的一个特性。

代码中有一个相当不错的解释：

This struct declaration is misleading (but accurate and necessary).
It declares a "view" into memory allowing access to necessary
fields at known offsets from a given base. See explanation below.
struct malloc_chunk {
 INTERNAL_SIZE_T      prev_size;  /* Size of previous chunk (if free).  */
 INTERNAL_SIZE_T      size;       /* Size in bytes, including overhead. */
 struct malloc_chunk* fd;         /* double links -- used only if free. */
 struct malloc_chunk* bk;
};

malloc_chunk详细信息：

(The following includes lightly edited explanations by Colin Plumb.)
Chunks of memory are maintained using a `boundary tag' method as
described in e.g., Knuth or Standish.  (See the paper by Paul
Wilson ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps for a
survey of such techniques.)  Sizes of free chunks are stored both
in the front of each chunk and at the end.  This makes
consolidating fragmented chunks into bigger chunks very fast.  The
size fields also hold bits representing whether chunks are free or
in use.
An allocated chunk looks like this:

 chunk->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Size of previous chunk, if allocated            | |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Size of chunk, in bytes                         |P|
  mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             User data starts here...                          .
        .                                                               .
        .             (malloc_usable_space() bytes)                     .
        .                                                               |
next  ->+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Size of chunk                                     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Where "chunk" is the front of the chunk for the purpose of most of
the malloc code, but "mem" is the pointer that is returned to the
user.  "Nextchunk" is the beginning of the next contiguous chunk.
Chunks always begin on even word boundries, so the mem portion
(which is returned to the user) is also on an even word boundary, and
thus at least double-word aligned.
Free chunks are stored in circular doubly-linked lists, and look like this:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Size of previous chunk                            |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`head:' |             Size of chunk, in bytes                         |P|
  mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Forward pointer to next chunk in list             |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Back pointer to previous chunk in list            |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |             Unused space (may be 0 bytes long)                .
        .                                                               .
        .                                                               |
 next-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`foot:' |             Size of chunk, in bytes                           |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The P (PREV_INUSE) bit, stored in the unused low-order bit of the
chunk size (which is always a multiple of two words), is an in-use
bit for the *previous* chunk.  If that bit is *clear*, then the
word before the current chunk size contains the previous chunk
size, and can be used to find the front of the previous chunk.
The very first chunk allocated always has this bit set,
preventing access to non-existent (or non-owned) memory. If
prev_inuse is set for any given chunk, then you CANNOT determine
the size of the previous chunk, and might even get a memory
addressing fault when trying to do so.
Note that the `foot' of the current chunk is actually represented
as the prev_size of the NEXT chunk. This makes it easier to
deal with alignments etc but can be very confusing when trying
to extend or adapt this code.
The two exceptions to all this are
 1. The special chunk `top' doesn't bother using the
    trailing size field since there is no next contiguous chunk
    that would have to index off it. After initialization, `top'
    is forced to always exist.  If it would become less than
    MINSIZE bytes long, it is replenished.
 2. Chunks allocated via mmap, which have the second-lowest-order
    bit (IS_MMAPPED) set in their size fields.  Because they are
    allocated one-by-one, each must contain its own trailing size field.

我没有专门研究dlmalloc，但这里有一个可能的解释：

在具有需要16字节对齐的对象的体系结构上（与Intel SSE一样），返回的地址必须是16的倍数。如果报头具有12个字节的信息（包含块的大小）和一些链接信息以将块与前一个块合并，则报头可以被定义为具有16的长度，前四个字节用于前一个分配的块的末尾。如果前一个区块是空闲的，那么分配器可以使用这个空间进行优化。