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上图

此图着重描述的是子线程，一个heap（由heap_info结构体描述）用完，需要另一个的情况。

子线程内存特点

1. 第一个heap物理内存上从低地址到高地址依次是：heap_info+malloc_state(arena)+chunks

/*
arena.c
#0  new_heap (size=size@entry=6328, top_pad=131072) at arena.c:528
#1  0x00007ffff7895c2a in _int_new_arena (size=4096) at arena.c:720
#2  arena_get2 (a_tsd=a_tsd@entry=0x0, size=size@entry=4096, avoid_arena=avoid_arena@entry=0x0) at arena.c:871
#3  0x00007ffff78963b6 in __GI___libc_malloc (bytes=4096) at malloc.c:2856
*//* Create a new arena with initial size "size".  */static mstate
_int_new_arena(size_t size)
{mstate a;heap_info *h;char *ptr;unsigned long misalign;h = new_heap(size + (sizeof(*h) + sizeof(*a) + MALLOC_ALIGNMENT),mp_.top_pad);if(!h) {/* Maybe size is too large to fit in a single heap.  So, just tryto create a minimally-sized arena and let _int_malloc() attemptto deal with the large request via mmap_chunk().  */h = new_heap(sizeof(*h) + sizeof(*a) + MALLOC_ALIGNMENT, mp_.top_pad);if(!h)return 0;}a = h->ar_ptr = (mstate)(h+1);

2. 第二个heap没有malloc_state(arena)，其heap_info.ar_ptr指向第一个heap里的arena

/*
#0  new_heap (size=size@entry=4176, top_pad=131072) at arena.c:528
#1  0x00007ffff7894ad1 in sysmalloc (av=0x7ffff0000020, nb=4112) at malloc.c:2390
#2  _int_malloc (av=av@entry=0x7ffff0000020, bytes=bytes@entry=4096) at malloc.c:3718
#3  0x00007ffff78963d2 in __GI___libc_malloc (bytes=4096) at malloc.c:2859
*/
static void *sysmalloc(INTERNAL_SIZE_T nb, mstate av)
...
else if ((heap = new_heap(nb + (MINSIZE + sizeof(*heap)), mp_.top_pad))){/* Use a newly allocated heap.  */heap->ar_ptr = av;heap->prev = old_heap;av->system_mem += heap->size;arena_mem += heap->size;/* Set up the new top.  */top(av) = chunk_at_offset(heap, sizeof(*heap));set_head(top(av), (heap->size - sizeof(*heap)) | PREV_INUSE);

3. 每个heap都是调用mmap分配的内存，大小为HEAP_MAX_SIZE。使用mprotect使得只有几百KB可读可写，以后不够用时再割一块使得更多内存对用户可用（grow_heap）。

//new_heap(size_t size, size_t top_pad)if(aligned_heap_area) {p2 = (char *)MMAP(aligned_heap_area, HEAP_MAX_SIZE, PROT_NONE,MAP_NORESERVE);aligned_heap_area = NULL;if (p2 != MAP_FAILED && ((unsigned long)p2 & (HEAP_MAX_SIZE-1))) {__munmap(p2, HEAP_MAX_SIZE);p2 = MAP_FAILED;}}if(p2 == MAP_FAILED) {p1 = (char *)MMAP(0, HEAP_MAX_SIZE<<1, PROT_NONE, MAP_NORESERVE);...if(__mprotect(p2, size, PROT_READ|PROT_WRITE) != 0) 

4. 每个heap的起始地址与HEAP_MAX_SIZE对齐（64M）

#  define HEAP_MAX_SIZE (2 * DEFAULT_MMAP_THRESHOLD_MAX)  //64M
#define DEFAULT_MMAP_THRESHOLD_MAX (4 * 1024 * 1024 * sizeof(long)) 

两个问题

1. 如何由任意要free的地址找到其对应的arena？

arena是线程相关的，可以通过线程找到arena。但是malloc可能发生在线程A，而free不一定非要发生在同一个线程A。那free时如何找到对应的arena哪？

ar_ptr = arena_for_chunk(ptr);free(ptr) 
-> get chunk pointer chunkptr by ptr-0x10 
-> get heap_info by chunkptr & ~(HEAP_MAX_SIZE-1) 
-> get arena by heap_info.ar_ptr

2. 如何确保“每个heap的起始地址与HEAP_MAX_SIZE对齐”？

向mmap申请分配两倍的HEAP_MAX_SIZE，只要中间部分，两头unmap回系统。

arena.cp1 = (char *)MMAP(0, HEAP_MAX_SIZE<<1, PROT_NONE, MAP_NORESERVE);if(p1 != MAP_FAILED) {p2 = (char *)(((unsigned long)p1 + (HEAP_MAX_SIZE-1))& ~(HEAP_MAX_SIZE-1));ul = p2 - p1;if (ul)__munmap(p1, ul);elsealigned_heap_area = p2 + HEAP_MAX_SIZE;__munmap(p2 + HEAP_MAX_SIZE, HEAP_MAX_SIZE - ul);}

读者可通过cat /proc/[pid]/maps查看内存的变化。

一个例子

提供一个c程序例子，帮助读者调试。你可以给new_heap, grow_heap, 或者free下断点。

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>// Function executed by the sub-thread
void* thread_function(void* arg) {// Allocate memory for an integervoid* ptr;int i=0;while(i++<16*1024){ //HEAP_MAX_SIZE/4096=64M/4096ptr = malloc(4096);if (ptr == NULL) {perror("Memory allocation failed");pthread_exit(NULL);}}free(ptr);return NULL;
}int main() {pthread_t tid;int ret;// Create a sub-threadret = pthread_create(&tid, NULL, thread_function, NULL);if (ret != 0) {perror("pthread_create failed");return 1;}// Wait for the sub-thread to finishret = pthread_join(tid, NULL);if (ret != 0) {perror("pthread_join failed");return 1;}void* main = malloc(20);return 0;
}