ptmalloc heap_free

arena.c

static int heap_trim(heap_info *heap, size_t pad);函数有一段没读懂

prev_size = prev_heap->size - (MINSIZE - 2 * SIZE_SZ);

后面才看《Glibc 内存管理Ptmalloc2 源代码分析》才发现没理解top_chunk

-------------------------------------------------------------------------

fencepost chunk1 size:(MINSIZE - 2 * SIZE_SZ)

-------------------------------------------------------------------------

fencepost chunk2 size:2 * SIZE_SZ

---------------------------------------------------------------------------

mem chunk

----------------------------------------------------------------------------

top chunk最后两个chunk为 fencepost，总大小是MINSIZE。

#define NBINS 128

#define NSMALLBINS        64

#define SMALLBIN_WIDTH    MALLOC_ALIGNMENT

#define SMALLBIN_CORRECTION (MALLOC_ALIGNMENT > 2 * SIZE_SZ)

#define MIN_LARGE_SIZE    ((NSMALLBINS - SMALLBIN_CORRECTION) * SMALLBIN_WIDTH)

第一反应 SMALLBIN_CORRECTION为true/false，用在min_large_size:NSMALLBINS - SMALLBIN_CORRECTION开始没看懂，后面才反应过来true/false可以用1/0表示。

MIN_LARGE_SIZE简化为NSMALLBINS*SMALLBIN_WIDTH，64位系统为64*0x10，值为0x400。（SMALLBIN_CORRECTION通常为0）