glibc ptmalloc中的宏操作

glibc ptmalloc很有意思的宏操作：
1.MORECORE

/* Definition for getting more memory from the OS.  */#define MORECORE         (*__morecore)#define MORECORE_FAILURE 0void * __default_morecore (ptrdiff_t);void *(*__morecore)(ptrdiff_t) = __default_morecore;

__default_morecore是一个函数声明。morecore是一个函数类型定义。
MORECORE是一个对函数类型的宏定义。
例如MORECORE(1000);就等于调用函数__default_morecore(1000);

#ifndef MORECORE#define MORECORE sbrk#endif

如果没有定义MORECORE，那么那就是sbrk函数的宏定义。

2. brk & sbrk

int__brk (void *addr){  void *__unbounded newbrk;  INTERNAL_SYSCALL_DECL (err);  newbrk = (void *__unbounded) INTERNAL_SYSCALL (brk, err, 1,                              __ptrvalue (addr));  __curbrk = newbrk;  if (newbrk < addr)    {      __set_errno (ENOMEM);      return -1;    }  return 0;}void *__sbrk (intptr_t increment){  void *oldbrk;  /* If this is not part of the dynamic library or the library is used     via dynamic loading in a statically linked program update     __curbrk from the kernel's brk value.  That way two separate     instances of __brk and __sbrk can share the heap, returning     interleaved pieces of it.  */  if (__curbrk == NULL || __libc_multiple_libcs)    if (__brk (0) < 0)          /* Initialize the break.  */      return (void *) -1;  if (increment == 0)    return __curbrk;  oldbrk = __curbrk;  if ((increment > 0       ? ((uintptr_t) oldbrk + (uintptr_t) increment < (uintptr_t) oldbrk)       : ((uintptr_t) oldbrk < (uintptr_t) -increment))      || __brk (oldbrk + increment) < 0)    return (void *) -1;  return oldbrk;}

3. malloc_chunk结构体

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;  /* Only used for large blocks: pointer to next larger size.  */  struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */  struct malloc_chunk* bk_nextsize;};

chunk <=> mem的转换

#define chunk2mem(p)   ((void*)((char*)(p) + 2*SIZE_SZ))#define mem2chunk(mem) ((mchunkptr)((char*)(mem) - 2*SIZE_SZ))

4. 求结构体某成员的偏移量

#define offsetof(Type, Member) ((size_t) &((Type *) NULL)->Member)#define MIN_CHUNK_SIZE        (offsetof(struct malloc_chunk, fd_nextsize))

其实它求得的就是16，因为fd_nextsize变量的偏移量是16.


5.  将请求的内存大小转成chunk对其的内存量

#define request2size(req)                                         \  (((req) + SIZE_SZ + MALLOC_ALIGN_MASK < MINSIZE)  ?             \   MINSIZE :                                                      \   ((req) + SIZE_SZ + MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK)

注意，这里只是多申请了一个size_sz，就是chunk size的4个字节。
MALLOC_ALIGN_MASK = 8 - 1 = 7，后面计算时将(req+size_sz) up align to 8的倍数。


6. 最低三位的设置与判断
4个字节的size域，共可以存储最大2^32的chunk size内存量，最低3位可以用来存储下面三个域，于是chunk size总是8的倍数。

/* size field is or'ed with PREV_INUSE when previous adjacent chunk in use */#define PREV_INUSE 0x1/* extract inuse bit of previous chunk */#define prev_inuse(p)       ((p)->size & PREV_INUSE)/* size field is or'ed with IS_MMAPPED if the chunk was obtained with mmap() */#define IS_MMAPPED 0x2/* check for mmap()'ed chunk */#define chunk_is_mmapped(p) ((p)->size & IS_MMAPPED)/* size field is or'ed with NON_MAIN_ARENA if the chunk was obtained   from a non-main arena.  This is only set immediately before handing   the chunk to the user, if necessary.  */#define NON_MAIN_ARENA 0x4/* check for chunk from non-main arena */#define chunk_non_main_arena(p) ((p)->size & NON_MAIN_ARENA)

所以如果要获取chunk size，只需要将这个4字节的数作如下操作就可以了：

#define SIZE_BITS (PREV_INUSE|IS_MMAPPED|NON_MAIN_ARENA)/* Get size, ignoring use bits */#define chunksize(p)         ((p)->size & ~(SIZE_BITS))

以及获取它前后2个chunk的头指针

/* Ptr to next physical malloc_chunk. */#define next_chunk(p) ((mchunkptr)( ((char*)(p)) + ((p)->size & ~SIZE_BITS) ))/* Ptr to previous physical malloc_chunk */#define prev_chunk(p) ((mchunkptr)( ((char*)(p)) - ((p)->prev_size) ))

下面的宏操作才是获取当前chunk的是否是free状态

/* extract p's inuse bit */#define inuse(p)\((((mchunkptr)(((char*)(p))+((p)->size & ~SIZE_BITS)))->size) & PREV_INUSE)/* set/clear chunk as being inuse without otherwise disturbing */#define set_inuse(p)\((mchunkptr)(((char*)(p)) + ((p)->size & ~SIZE_BITS)))->size |= PREV_INUSE#define clear_inuse(p)\((mchunkptr)(((char*)(p)) + ((p)->size & ~SIZE_BITS)))->size &= ~(PREV_INUSE)

由chunk头地址来得到这个chunk来自于哪个分配区：

#define heap_for_ptr(ptr) \ ((heap_info *)((unsigned long)(ptr) & ~(HEAP_MAX_SIZE-1)))#define arena_for_chunk(ptr) \ (chunk_non_main_arena(ptr) ? heap_for_ptr(ptr)->ar_ptr : &main_arena)