* Quickstart

  This library is all in one file to simplify the most common usage:
  ftp it, compile it (-O3), and link it into another program. All of
  the compile-time options default to reasonable values for use on
  most platforms.  You might later want to step through various
  compile-time and dynamic tuning options.

  For convenience, an include file for code using this malloc is at:
     ftp://gee.cs.oswego.edu/pub/misc/malloc-2.8.6.h
  You don't really need this .h file unless you call functions not
  defined in your system include files.  The .h file contains only the
  excerpts from this file needed for using this malloc on ANSI C/C++
  systems, so long as you haven't changed compile-time options about
  naming and tuning parameters.  If you do, then you can create your
  own malloc.h that does include all settings by cutting at the point
  indicated below. Note that you may already by default be using a C
  library containing a malloc that is based on some version of this
  malloc (for example in linux). You might still want to use the one
  in this file to customize settings or to avoid overheads associated
  with library versions.
 

這個庫由1個單檔案構成，編譯時使用-O3優化等級。使用時需要與使用者自己的程式碼進行連結。

一般情況下，不需要直接包含此標頭檔案。系統中可能已經有地方包含了此標頭檔案。

Supported pointer/size_t representation:       4 or 8 bytes
       size_t MUST be an unsigned type of the same width as
       pointers. (If you are using an ancient system that declares
       size_t as a signed type, or need it to be a different width
       than pointers, you can use a previous release of this malloc
       (e.g. 2.7.2) supporting these.)
 

size_t和和指標變數的長度必須保持一致。都是4位元組或者8位元組。如果你的系統不滿足這個約束，那麼本庫需要回退到2.7.2以前的版本才支援。

Alignment:                                     8 bytes (minimum)
       This suffices for nearly all current machines and C compilers.
       However, you can define MALLOC_ALIGNMENT to be wider than this
       if necessary (up to 128bytes), at the expense of using more space.
 

可以定義64位元組或者128位元組對齊。128位元組對齊會稍微浪費更多的空間

Minimum overhead per allocated chunk:   4 or  8 bytes (if 4byte sizes)
                                          8 or 16 bytes (if 8byte sizes)
       Each malloced chunk has a hidden word of overhead holding size
       and status information, and additional cross-check word
       if FOOTERS is defined.

每一次申請記憶體至少要比使用者需求多消耗4位元組~到16位元組記憶體。用於維護此記憶體塊的資訊。

Minimum allocated size: 4-byte ptrs:  16 bytes    (including overhead)
                          8-byte ptrs:  32 bytes    (including overhead)

       Even a request for zero bytes (i.e., malloc(0)) returns a
       pointer to something of the minimum allocatable size.
       The maximum overhead wastage (i.e., number of extra bytes
       allocated than were requested in malloc) is less than or equal
       to the minimum size, except for requests >= mmap_threshold that
       are serviced via mmap(), where the worst case wastage is about
       32 bytes plus the remainder from a system page (the minimal
       mmap unit); typically 4096 or 8192 bytes.

最下的申請記憶體長度為16位元組或者32位元組（已包含記憶體塊自身維護資料在內）

Security: static-safe; optionally more or less
       The "security" of malloc refers to the ability of malicious
       code to accentuate the effects of errors (for example, freeing
       space that is not currently malloc'ed or overwriting past the
       ends of chunks) in code that calls malloc.  This malloc
       guarantees not to modify any memory locations below the base of
       heap, i.e., static variables, even in the presence of usage
       errors.  The routines additionally detect most improper frees
       and reallocs.  All this holds as long as the static bookkeeping
       for malloc itself is not corrupted by some other means.  This
       is only one aspect of security -- these checks do not, and
       cannot, detect all possible programming errors.

       If FOOTERS is defined nonzero, then each allocated chunk
       carries an additional check word to verify that it was malloced
       from its space.  These check words are the same within each
       execution of a program using malloc, but differ across
       executions, so externally crafted fake chunks cannot be
       freed. This improves security by rejecting frees/reallocs that
       could corrupt heap memory, in addition to the checks preventing
       writes to statics that are always on.  This may further improve
       security at the expense of time and space overhead.  (Note that
       FOOTERS may also be worth using with MSPACES.)

       By default detected errors cause the program to abort (calling
       "abort()"). You can override this to instead proceed past
       errors by defining PROCEED_ON_ERROR.  In this case, a bad free
       has no effect, and a malloc that encounters a bad address
       caused by user overwrites will ignore the bad address by
       dropping pointers and indices to all known memory. This may
       be appropriate for programs that should continue if at all
       possible in the face of programming errors, although they may
       run out of memory because dropped memory is never reclaimed.

       If you don't like either of these options, you can define
       CORRUPTION_ERROR_ACTION and USAGE_ERROR_ACTION to do anything
       else. And if if you are sure that your program using malloc has
       no errors or vulnerabilities, you can define INSECURE to 1,
       which might (or might not) provide a small performance improvement.

       It is also possible to limit the maximum total allocatable
       space, using malloc_set_footprint_limit. This is not
       designed as a security feature in itself (calls to set limits
       are not screened or privileged), but may be useful as one
       aspect of a secure implementation.

安全性考慮。

靜態安全性，或多或少有一些。

如應對釋放沒有申請的記憶體，或者記憶體寫越界這樣的問題，稱為安全性問題。

在本演算法中，靜態變數是不會被亂寫的。內部會檢測釋放和重分配是否合法。這些檢查無法避免所有的錯誤。

如果FOOTERS被定義了的話，則被分配的記憶體增加了一個額外的資訊用來做校驗，這些資料在一次記憶體分配後，下一次記憶體分配前不會修訂。如果在釋放的時候發現這個資料被改了，則拒絕釋放，否則會破壞堆記憶體。

預設情況下，如果malloc發現記憶體被篡改，則會推出程序(abort）。使用者也可以自定義處理方式，讓程式繼續執行而不推出，但這樣做，會導致後期記憶體逐漸耗盡，因為被篡改的記憶體不會回收。空閒記憶體會逐漸減少。

最後最好限制一下可以申請的記憶體空間的總大小。

Thread-safety: NOT thread-safe unless USE_LOCKS defined non-zero
       When USE_LOCKS is defined, each public call to malloc, free,
       etc is surrounded with a lock. By default, this uses a plain
       pthread mutex, win32 critical section, or a spin-lock if if
       available for the platform and not disabled by setting
       USE_SPIN_LOCKS=0.  However, if USE_RECURSIVE_LOCKS is defined,
       recursive versions are used instead (which are not required for
       base functionality but may be needed in layered extensions).
       Using a global lock is not especially fast, and can be a major
       bottleneck.  It is designed only to provide minimal protection
       in concurrent environments, and to provide a basis for
       extensions.  If you are using malloc in a concurrent program,
       consider instead using nedmalloc
       (http://www.nedprod.com/programs/portable/nedmalloc/) or
       ptmalloc (See http://www.malloc.de), which are derived from
       versions of this malloc.

在高併發環境下，考慮使用其他的malloc庫而不是當前的這個庫，因為這個庫的鎖的粒度比較粗，效能較低。

System requirements: Any combination of MORECORE and/or MMAP/MUNMAP
       This malloc can use unix sbrk or any emulation (invoked using
       the CALL_MORECORE macro) and/or mmap/munmap or any emulation
       (invoked using CALL_MMAP/CALL_MUNMAP) to get and release system
       memory.  On most unix systems, it tends to work best if both
       MORECORE and MMAP are enabled.  On Win32, it uses emulations
       based on VirtualAlloc. It also uses common C library functions
       like memset.

  Compliance: I believe it is compliant with the Single Unix Specification
       (See http://www.unix.org). Also SVID/XPG, ANSI C, and probably
       others as well.

系統需求和相容性。僅依賴於2到3個系統呼叫或者等價體。相容C標準和單一UNIX規範。

* Overview of algorithms

  This is not the fastest, most space-conserving, most portable, or
  most tunable malloc ever written. However it is among the fastest
  while also being among the most space-conserving, portable and
  tunable.  Consistent balance across these factors results in a good
  general-purpose allocator for malloc-intensive programs.

  In most ways, this malloc is a best-fit allocator. Generally, it
  chooses the best-fitting existing chunk for a request, with ties
  broken in approximately least-recently-used order. (This strategy
  normally maintains low fragmentation.) However, for requests less
  than 256bytes, it deviates from best-fit when there is not an
  exactly fitting available chunk by preferring to use space adjacent
  to that used for the previous small request, as well as by breaking
  ties in approximately most-recently-used order. (These enhance
  locality of series of small allocations.)  And for very large requests
  (>= 256Kb by default), it relies on system memory mapping
  facilities, if supported.  (This helps avoid carrying around and
  possibly fragmenting memory used only for large chunks.)

  All operations (except malloc_stats and mallinfo) have execution
  times that are bounded by a constant factor of the number of bits in
  a size_t, not counting any clearing in calloc or copying in realloc,
  or actions surrounding MORECORE and MMAP that have times
  proportional to the number of non-contiguous regions returned by
  system allocation routines, which is often just 1. In real-time
  applications, you can optionally suppress segment traversals using
  NO_SEGMENT_TRAVERSAL, which assures bounded execution even when
  system allocators return non-contiguous spaces, at the typical
  expense of carrying around more memory and increased fragmentation.

  The implementation is not very modular and seriously overuses
  macros. Perhaps someday all C compilers will do as good a job
  inlining modular code as can now be done by brute-force expansion,
  but now, enough of them seem not to.

  Some compilers issue a lot of warnings about code that is
  dead/unreachable only on some platforms, and also about intentional
  uses of negation on unsigned types. All known cases of each can be
  ignored.

  For a longer but out of date high-level description, see
     http://gee.cs.oswego.edu/dl/html/malloc.html

這是一個性價比較高的記憶體分配演算法。

它儘量從最近剛釋放的記憶體中分配，增加cache命中率

大多數函式的執行時間是常量時間。

* MSPACES
  If MSPACES is defined, then in addition to malloc, free, etc.,
  this file also defines mspace_malloc, mspace_free, etc. These
  are versions of malloc routines that take an "mspace" argument
  obtained using create_mspace, to control all internal bookkeeping.
  If ONLY_MSPACES is defined, only these versions are compiled.
  So if you would like to use this allocator for only some allocations,
  and your system malloc for others, you can compile with
  ONLY_MSPACES and then do something like...
    static mspace mymspace = create_mspace(0,0); // for example
    #define mymalloc(bytes)  mspace_malloc(mymspace, bytes)

  (Note: If you only need one instance of an mspace, you can instead
  use "USE_DL_PREFIX" to relabel the global malloc.)

  You can similarly create thread-local allocators by storing
  mspaces as thread-locals. For example:
    static __thread mspace tlms = 0;
    void*  tlmalloc(size_t bytes) {
      if (tlms == 0) tlms = create_mspace(0, 0);
      return mspace_malloc(tlms, bytes);
    }
    void  tlfree(void* mem) { mspace_free(tlms, mem); }

  Unless FOOTERS is defined, each mspace is completely independent.
  You cannot allocate from one and free to another (although
  conformance is only weakly checked, so usage errors are not always
  caught). If FOOTERS is defined, then each chunk carries around a tag
  indicating its originating mspace, and frees are directed to their
  originating spaces. Normally, this requires use of locks.

記憶體的申請和釋放可以由多個mspace來進行管理

/* --- begin vpp customizations --- */

#if CLIB_DEBUG > 0
#define FOOTERS 1  /* extra debugging */
#define DLM_MAGIC_CONSTANT 0xdeaddabe
#endif
#define USE_LOCKS 1
#define DLM_ABORT {extern void os_panic(void); os_panic(); abort();}
#define ONLY_MSPACES 1

/* --- end vpp customizations --- */

在開啟debug的情況下，每個記憶體塊記錄魔鬼數字，用於檢查記憶體是否被寫越界。

預設的錯誤處理方法是使程序退出

/*
  malloc(size_t n)
  Returns a pointer to a newly allocated chunk of at least n bytes, or
  null if no space is available, in which case errno is set to ENOMEM
  on ANSI C systems.

  If n is zero, malloc returns a minimum-sized chunk. (The minimum
  size is 16 bytes on most 32bit systems, and 32 bytes on 64bit
  systems.)  Note that size_t is an unsigned type, so calls with
  arguments that would be negative if signed are interpreted as
  requests for huge amounts of space, which will often fail. The
  maximum supported value of n differs across systems, but is in all
  cases less than the maximum representable value of a size_t.
*/
DLMALLOC_EXPORT void* dlmalloc(size_t);

申請給定大小的記憶體空間，返回記憶體空間的首地址。大小引數不要填成了負數，否則會當成正數解釋，而去獲取一個很大的記憶體，導致失敗。

/*
  free(void* p)
  Releases the chunk of memory pointed to by p, that had been previously
  allocated using malloc or a related routine such as realloc.
  It has no effect if p is null. If p was not malloced or already
  freed, free(p) will by default cause the current program to abort.
*/
DLMALLOC_EXPORT void  dlfree(void*);

釋放之前申請的記憶體塊。如果重複釋放或者釋放不存在的記憶體塊，則程序一般情況下會退出。

/*
  calloc(size_t n_elements, size_t element_size);
  Returns a pointer to n_elements * element_size bytes, with all locations
  set to zero.
*/
DLMALLOC_EXPORT void* dlcalloc(size_t, size_t);

申請1個數組的記憶體，並初始化所有元素為0

/*
  realloc(void* p, size_t n)
  Returns a pointer to a chunk of size n that contains the same data
  as does chunk p up to the minimum of (n, p's size) bytes, or null
  if no space is available.

  The returned pointer may or may not be the same as p. The algorithm
  prefers extending p in most cases when possible, otherwise it
  employs the equivalent of a malloc-copy-free sequence.

  If p is null, realloc is equivalent to malloc.

  If space is not available, realloc returns null, errno is set (if on
  ANSI) and p is NOT freed.

  if n is for fewer bytes than already held by p, the newly unused
  space is lopped off and freed if possible.  realloc with a size
  argument of zero (re)allocates a minimum-sized chunk.

  The old unix realloc convention of allowing the last-free'd chunk
  to be used as an argument to realloc is not supported.
*/
DLMALLOC_EXPORT void* dlrealloc(void*, size_t);

重新申請記憶體大小，即記憶體伸縮。可以簡單理解成申請一塊新記憶體，將舊記憶體的資料拷貝到新記憶體，釋放舊記憶體。

/*
  realloc_in_place(void* p, size_t n)
  Resizes the space allocated for p to size n, only if this can be
  done without moving p (i.e., only if there is adjacent space
  available if n is greater than p's current allocated size, or n is
  less than or equal to p's size). This may be used instead of plain
  realloc if an alternative allocation strategy is needed upon failure
  to expand space; for example, reallocation of a buffer that must be
  memory-aligned or cleared. You can use realloc_in_place to trigger
  these alternatives only when needed.

  Returns p if successful; otherwise null.
*/
DLMALLOC_EXPORT void* dlrealloc_in_place(void*, size_t);

與上一個函式語義相同，不同的地方是，直接在當前記憶體上進行伸縮，而不是重新申請記憶體，效率較高。但這個容易失敗，特別是記憶體擴充套件的時候，如果旁邊的記憶體塊不是空閒的話，想一想？

/*
  memalign(size_t alignment, size_t n);
  Returns a pointer to a newly allocated chunk of n bytes, aligned
  in accord with the alignment argument.

  The alignment argument should be a power of two. If the argument is
  not a power of two, the nearest greater power is used.
  8-byte alignment is guaranteed by normal malloc calls, so don't
  bother calling memalign with an argument of 8 or less.

  Overreliance on memalign is a sure way to fragment space.
*/
DLMALLOC_EXPORT void* dlmemalign(size_t, size_t);

申請一個有位元組對齊要求的記憶體塊。

/*
  int posix_memalign(void** pp, size_t alignment, size_t n);
  Allocates a chunk of n bytes, aligned in accord with the alignment
  argument. Differs from memalign only in that it (1) assigns the
  allocated memory to *pp rather than returning it, (2) fails and
  returns EINVAL if the alignment is not a power of two (3) fails and
  returns ENOMEM if memory cannot be allocated.
*/
DLMALLOC_EXPORT int dlposix_memalign(void**, size_t, size_t);

與上一個函式類似，記憶體塊由輸出引數返回，函式本身返回錯誤程式碼。

/*
  valloc(size_t n);
  Equivalent to memalign(pagesize, n), where pagesize is the page
  size of the system. If the pagesize is unknown, 4096 is used.
*/
DLMALLOC_EXPORT void* dlvalloc(size_t);

同memalign，  pagesize引數取系統預設值。

/*
  mallopt(int parameter_number, int parameter_value)
  Sets tunable parameters The format is to provide a
  (parameter-number, parameter-value) pair.  mallopt then sets the
  corresponding parameter to the argument value if it can (i.e., so
  long as the value is meaningful), and returns 1 if successful else
  0.  To workaround the fact that mallopt is specified to use int,
  not size_t parameters, the value -1 is specially treated as the
  maximum unsigned size_t value.

  SVID/XPG/ANSI defines four standard param numbers for mallopt,
  normally defined in malloc.h.  None of these are use in this malloc,
  so setting them has no effect. But this malloc also supports other
  options in mallopt. See below for details.  Briefly, supported
  parameters are as follows (listed defaults are for "typical"
  configurations).

  Symbol            param #  default    allowed param values
  M_TRIM_THRESHOLD     -1   2*1024*1024   any   (-1 disables)
  M_GRANULARITY        -2     page size   any power of 2 >= page size
  M_MMAP_THRESHOLD     -3      256*1024   any   (or 0 if no MMAP support)
*/
DLMALLOC_EXPORT int dlmallopt(int, int);

設定malloc的全域性引數，會影響malloc的行為。

/*
  malloc_footprint();
  Returns the number of bytes obtained from the system.  The total
  number of bytes allocated by malloc, realloc etc., is less than this
  value. Unlike mallinfo, this function returns only a precomputed
  result, so can be called frequently to monitor memory consumption.
  Even if locks are otherwise defined, this function does not use them,
  so results might not be up to date.
*/
DLMALLOC_EXPORT size_t dlmalloc_footprint(void);

當前malloc記憶體用量，這個統計不是實時的，有點延遲，大致準確。但是多執行緒安全。

/*
  malloc_max_footprint();
  Returns the maximum number of bytes obtained from the system. This
  value will be greater than current footprint if deallocated space
  has been reclaimed by the system. The peak number of bytes allocated
  by malloc, realloc etc., is less than this value. Unlike mallinfo,
  this function returns only a precomputed result, so can be called
  frequently to monitor memory consumption.  Even if locks are
  otherwise defined, this function does not use them, so results might
  not be up to date.
*/
DLMALLOC_EXPORT size_t dlmalloc_max_footprint(void);

malloc用量的歷史最大值

/*
  malloc_footprint_limit();
  Returns the number of bytes that the heap is allowed to obtain from
  the system, returning the last value returned by
  malloc_set_footprint_limit, or the maximum size_t value if
  never set. The returned value reflects a permission. There is no
  guarantee that this number of bytes can actually be obtained from
  the system.
*/
DLMALLOC_EXPORT size_t dlmalloc_footprint_limit();

malloc用量的理論極限值。

/*
  malloc_set_footprint_limit();
  Sets the maximum number of bytes to obtain from the system, causing
  failure returns from malloc and related functions upon attempts to
  exceed this value. The argument value may be subject to page
  rounding to an enforceable limit; this actual value is returned.
  Using an argument of the maximum possible size_t effectively
  disables checks. If the argument is less than or equal to the
  current malloc_footprint, then all future allocations that require
  additional system memory will fail. However, invocation cannot
  retroactively deallocate existing used memory.
*/
DLMALLOC_EXPORT size_t dlmalloc_set_footprint_limit(size_t bytes);

設定malloc用量的理論極限值

/*
  malloc_inspect_all(void(*handler)(void *start,
                                    void *end,
                                    size_t used_bytes,
                                    void* callback_arg),
                      void* arg);
  Traverses the heap and calls the given handler for each managed
  region, skipping all bytes that are (or may be) used for bookkeeping
  purposes.  Traversal does not include include chunks that have been
  directly memory mapped. Each reported region begins at the start
  address, and continues up to but not including the end address.  The
  first used_bytes of the region contain allocated data. If
  used_bytes is zero, the region is unallocated. The handler is
  invoked with the given callback argument. If locks are defined, they
  are held during the entire traversal. It is a bad idea to invoke
  other malloc functions from within the handler.

  For example, to count the number of in-use chunks with size greater
  than 1000, you could write:
  static int count = 0;
  void count_chunks(void* start, void* end, size_t used, void* arg) {
    if (used >= 1000) ++count;
  }
  then:
    malloc_inspect_all(count_chunks, NULL);

  malloc_inspect_all is compiled only if MALLOC_INSPECT_ALL is defined.
*/

遍歷所有的malloc記憶體塊，執行指定的函式。

/*
  mallinfo()
  Returns (by copy) a struct containing various summary statistics:

  arena:     current total non-mmapped bytes allocated from system
  ordblks:   the number of free chunks
  smblks:    always zero.
  hblks:     current number of mmapped regions
  hblkhd:    total bytes held in mmapped regions
  usmblks:   the maximum total allocated space. This will be greater
                than current total if trimming has occurred.
  fsmblks:   always zero
  uordblks:  current total allocated space (normal or mmapped)
  fordblks:  total free space
  keepcost:  the maximum number of bytes that could ideally be released
               back to system via malloc_trim. ("ideally" means that
               it ignores page restrictions etc.)

  Because these fields are ints, but internal bookkeeping may
  be kept as longs, the reported values may wrap around zero and
  thus be inaccurate.
*/
DLMALLOC_EXPORT struct dlmallinfo dlmallinfo(void);

獲取malloc相關的統計資訊

/*
  independent_calloc(size_t n_elements, size_t element_size, void* chunks[]);

  independent_calloc is similar to calloc, but instead of returning a
  single cleared space, it returns an array of pointers to n_elements
  independent elements that can hold contents of size elem_size, each
  of which starts out cleared, and can be independently freed,
  realloc'ed etc. The elements are guaranteed to be adjacently
  allocated (this is not guaranteed to occur with multiple callocs or
  mallocs), which may also improve cache locality in some
  applications.

  The "chunks" argument is optional (i.e., may be null, which is
  probably the most typical usage). If it is null, the returned array
  is itself dynamically allocated and should also be freed when it is
  no longer needed. Otherwise, the chunks array must be of at least
  n_elements in length. It is filled in with the pointers to the
  chunks.

  In either case, independent_calloc returns this pointer array, or
  null if the allocation failed.  If n_elements is zero and "chunks"
  is null, it returns a chunk representing an array with zero elements
  (which should be freed if not wanted).

  Each element must be freed when it is no longer needed. This can be
  done all at once using bulk_free.

  independent_calloc simplifies and speeds up implementations of many
  kinds of pools.  It may also be useful when constructing large data
  structures that initially have a fixed number of fixed-sized nodes,
  but the number is not known at compile time, and some of the nodes
  may later need to be freed. For example:

  struct Node { int item; struct Node* next; };

  struct Node* build_list() {
    struct Node** pool;
    int n = read_number_of_nodes_needed();
    if (n <= 0) return 0;
    pool = (struct Node**)(independent_calloc(n, sizeof(struct Node), 0);
    if (pool == 0) die();
    // organize into a linked list...
    struct Node* first = pool[0];
    for (i = 0; i < n-1; ++i)
      pool[i]->next = pool[i+1];
    free(pool);     // Can now free the array (or not, if it is needed later)
    return first;
  }
*/
DLMALLOC_EXPORT void** dlindependent_calloc(size_t, size_t, void**);

申請陣列記憶體，以指標陣列的形式返回。實際上申請了2級記憶體。

/*
  independent_comalloc(size_t n_elements, size_t sizes[], void* chunks[]);

  independent_comalloc allocates, all at once, a set of n_elements
  chunks with sizes indicated in the "sizes" array.    It returns
  an array of pointers to these elements, each of which can be
  independently freed, realloc'ed etc. The elements are guaranteed to
  be adjacently allocated (this is not guaranteed to occur with
  multiple callocs or mallocs), which may also improve cache locality
  in some applications.

  The "chunks" argument is optional (i.e., may be null). If it is null
  the returned array is itself dynamically allocated and should also
  be freed when it is no longer needed. Otherwise, the chunks array
  must be of at least n_elements in length. It is filled in with the
  pointers to the chunks.

  In either case, independent_comalloc returns this pointer array, or
  null if the allocation failed.  If n_elements is zero and chunks is
  null, it returns a chunk representing an array with zero elements
  (which should be freed if not wanted).

  Each element must be freed when it is no longer needed. This can be
  done all at once using bulk_free.

  independent_comallac differs from independent_calloc in that each
  element may have a different size, and also that it does not
  automatically clear elements.

  independent_comalloc can be used to speed up allocation in cases
  where several structs or objects must always be allocated at the
  same time.  For example:

  struct Head { ... }
  struct Foot { ... }

  void send_message(char* msg) {
    int msglen = strlen(msg);
    size_t sizes[3] = { sizeof(struct Head), msglen, sizeof(struct Foot) };
    void* chunks[3];
    if (independent_comalloc(3, sizes, chunks) == 0)
      die();
    struct Head* head = (struct Head*)(chunks[0]);
    char*        body = (char*)(chunks[1]);
    struct Foot* foot = (struct Foot*)(chunks[2]);
    // ...
  }

  In general though, independent_comalloc is worth using only for
  larger values of n_elements. For small values, you probably won't
  detect enough difference from series of malloc calls to bother.

  Overuse of independent_comalloc can increase overall memory usage,
  since it cannot reuse existing noncontiguous small chunks that
  might be available for some of the elements.
*/
DLMALLOC_EXPORT void** dlindependent_comalloc(size_t, size_t*, void**);

獨立的通用記憶體申請。比獨立的指標陣列二級記憶體申請更加靈活。

相當於指標陣列中，每一個指標元素指向的記憶體大小可以不同。

/*
  bulk_free(void* array[], size_t n_elements)
  Frees and clears (sets to null) each non-null pointer in the given
  array.  This is likely to be faster than freeing them one-by-one.
  If footers are used, pointers that have been allocated in different
  mspaces are not freed or cleared, and the count of all such pointers
  is returned.  For large arrays of pointers with poor locality, it
  may be worthwhile to sort this array before calling bulk_free.
*/
DLMALLOC_EXPORT size_t  dlbulk_free(void**, size_t n_elements);

釋放一組記憶體，這些記憶體由指標陣列記錄。