nginx 源码学习笔记（七）——内存分配相关源码分析
内存分配相关1.系统功能封装
内存相关的操作主要在os/unix/ngx_alloc.{h,c} 和 core/ngx_palloc.{h,c}中。
其中os/unix/ngx_alloc.{h,c}封装了最基本的内存分配函数，是对c原有的malloc/free/memalign等函数的封装，对应函数为：
a.ngx_alloc：对malloc进行了简单的封装；
void *ngx_alloc(size_t size, ngx_log_t *log){void*p;p = malloc(size);if (p == NULL) {ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,"malloc(%uz) failed", size); }ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, log, 0, "malloc: %p:%uz", p, size);return p;}
b.ngx_calloc：使用ngx_alloc分配内存，并且把内存赋值0：
void *ngx_calloc(size_t size, ngx_log_t *log){void*p;p = ngx_alloc(size, log);if (p) {ngx_memzero(p, size); }在core/ngx_string.h中定义// #define ngx_memzero(buf, n) (void) memset(buf, 0, n) 初始化为0return p;}
c. ngx_memalign 返回基于一个指定的alignment大小的数值为对齐基数的空间
d.ngx_free 内存释放操作
2. nginx内存池
为了方便系统模块对内存的使用，方便内存的管理，nginx自己是信了进程池机制来进行内存的分配和释放，首先nginx会在特定的生命周期帮你统一建立内存池，当需要进行内存分配的时候同一通过内存池中的内存进行分配，最后nginx会在适当的时候释放内存吃的资源，开发者只要在需要的时候对内存进行申请即可，不用过多考虑释放的问题，这也就是在os/unix/ngx_alloc.c文件中没有看到free操作的原因吧。
下面来看一下内存池的主要结构：
ngx_palloc.hstruct ngx_pool_s {ngx_pool_data_t d;size_t max;ngx_pool_t *current;ngx_chain_t *chain;ngx_pool_large_t *large;ngx_pool_cleanup_t *cleanup;ngx_log_t *log;};typedef struct {u_char *last;u_char *end;ngx_pool_t *next;ngx_uint_t failed;} ngx_pool_data_t;ngx_core.htypedef struct ngx_pool_s ngx_pool_t;typedef struct ngx_chain_s ngx_chain_t;
下面是几个比较重要的操作
src/core/ngx_palloc.c//创建内存池ngx_pool_t *ngx_create_pool(size_t size, ngx_log_t *log){ngx_pool_t*p;p = ngx_memalign(NGX_POOL_ALIGNMENT, size, log);//创建对其空间if (p == NULL) {return NULL;}p->d.last = (u_char *) p + sizeof(ngx_pool_t); //初始指向ngx_pool_t结构体后面p->d.end = (u_char *) p + size; //整个结构体的结尾p->d.next = NULL; //没有nextp->d.failed = 0;size = size - sizeof(ngx_pool_t); //剩余大小p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;//最大不超过NGX_MAX_ALLOC_FROM_POOL//#define NGX_MAX_ALLOC_FROM_POOL(ngx_pagesize - 1)p->current = p;p->chain = NULL;p->large = NULL;p->cleanup = NULL;p->log = log;return p;}//销毁内存池voidngx_destroy_pool(ngx_pool_t *pool){ngx_pool_t *p, *n;ngx_pool_large_t *l;ngx_pool_cleanup_t*c;for (c = pool->cleanup; c; c = c->next) {//如果注册了clenup(一种链表结构)，会依次调用clenup的handler进行清理if (c->handler) {ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,"run cleanup: %p", c);c->handler(c->data);}}for (l = pool->large; l; l = l->next) { //遍历链表，释放所有large内存ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, "free: %p", l->alloc);if (l->alloc) {ngx_free(l->alloc);}}#if (NGX_DEBUG)//等译debug级别，如果为true，会打印日志/** we could allocate the pool->log from this pool* so we cannot use this log while free()ing the pool*/for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, pool->log, 0,"free: %p, unused: %uz", p, p->d.end - p->d.last);if (n == NULL) {break;}}#endiffor (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {//遍历链表 ，释放内存空间ngx_free(p);if (n == NULL) {break;}}}//重置内存池voidngx_reset_pool(ngx_pool_t *pool){ngx_pool_t *p;ngx_pool_large_t*l;for (l = pool->large; l; l = l->next) { //释放掉所有large段内存if (l->alloc) {ngx_free(l->alloc);}}pool->large = NULL;for (p = pool; p; p = p->d.next) {p->d.last = (u_char *) p + sizeof(ngx_pool_t);将指针重新指向ngx_pool_t(和创建时一样)}}//从内存池里分配内存void * ngx_palloc(ngx_pool_t *pool, size_t size)void * ngx_pnalloc(ngx_pool_t *pool, size_t size)void * ngx_pmemalign(ngx_pool_t *pool, size_t size, size_t alignment)void * ngx_pcalloc(ngx_pool_t *pool, size_t size)这里以ngx_palloc为例讲解，其他大同小异：void *ngx_palloc(ngx_pool_t *pool, size_t size){u_char *m;ngx_pool_t*p;if (size <= pool->max) { //判断分配内存是否大于pool->max，如果小于等于p = pool->current; //尝试从链表的current开始遍历，do {m = ngx_align_ptr(p->d.last, NGX_ALIGNMENT);//#define ngx_align_ptr(p,a)//(u_char *) (((uintptr_t) (p) + ((uintptr_t) a - 1)) & ~((uintptr_t) a - 1))if ((size_t) (p->d.end - m) >= size) { //当找到可以分配的空间时p->d.last = m + size;return m; //分配内存后返回}p = p->d.next;} while (p);return ngx_palloc_block(pool, size);//如果无法分配内存，就生成一个新的节点，同时pool->current指针指向新的位置}return ngx_palloc_large(pool, size);//如果分配的内存大于pool->max则在large链表分配一段内存}//释放指定的内存ngx_int_tngx_pfree(ngx_pool_t *pool, void *p){ngx_pool_large_t*l;for (l = pool->large; l; l = l->next) {if (p == l->alloc) { //存在alloc注册ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,"free: %p", l->alloc);ngx_free(l->alloc);l->alloc = NULL;return NGX_OK;}}return NGX_DECLINED;}//由代码可以看出，这个操作只有在内存large链表里面注册内存才会真正释放，如果分配的是普通的内存，则会在destory_pool的时候统一释放。//注册cleanup回调函数ngx_pool_cleanup_t *ngx_pool_cleanup_add(ngx_pool_t *p, size_t size){ngx_pool_cleanup_t*c;c = ngx_palloc(p, sizeof(ngx_pool_cleanup_t));//分配cleanup空间if (c == NULL) {return NULL;}if (size) {c->data = ngx_palloc(p, size); //为cleanup结构体分配data空间if (c->data == NULL) {return NULL;}} else {c->data = NULL;}c->handler = NULL;c->next = p->cleanup;p->cleanup = c; // 增加cleanupngx_log_debug1(NGX_LOG_DEBUG_ALLOC, p->log, 0, "add cleanup: %p", c);return c; //返回结构体分配的空间}
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