在Nginx里，一块完整的共享内存以数据结构ngx_shm_zone_t来封装表示。
<code><span>typedef</span><span>struct</span> {
u_char *addr; <span>// 分配的共享内存的实际地址（这里实际共享内存的分配，根据当前系统可提供的接口，可以调用mmap或者shmget来进行分配，具体的用法，自己man吧）</span>
size_t size; <span>// 共享内存的大小</span>
ngx_str_t name; <span>// 该字段用作共享内存的唯一标识，能让Nginx知道想使用哪个共享内存</span>
ngx_log_t *<span>log</span>;
ngx_uint_t exists; <span>/* unsigned exists:1; */</span>
} ngx_shm_t;
<span>typedef</span><span>struct</span> ngx_shm_zone_s ngx_shm_zone_t;
<span>typedef</span> ngx_int_t (*ngx_shm_zone_init_pt) (ngx_shm_zone_t *zone, <span>void</span> *data);
<span>struct</span> ngx_shm_zone_s {
<span>void</span> *data;
ngx_shm_t shm;
ngx_shm_zone_init_pt init; <span>// 这里有一个钩子函数，用于实际共享内存进行分配后的初始化</span><span>void</span> *tag; <span>// 区别于shm.name，shm.name没法让Nginx区分到底是想新创建一个共享内存，还是使用已存在的旧的共享内存</span><span>// 因此这里引入tag字段来解决该问题，tag一般指向当前模块的ngx_module_t变量，见：...</span>
};</code>要在Nginx里使用一个共享内存，需要在配置文件里加上该共享内存的相关信息（添加一条指令），如：共享内存的名称，共享内存的大小等。因此在配置解析阶段，解析到相应的指令时，会创建对应的共享内存（此时创建的仅仅是代表共享内存的结构体：ngx_shm_zone_t，真实共享内存的分配在ngx_init_cycle(&init_cycle)解析完配置后，进行实际共享内存的分配并初始化）。见：
<code><span>int</span> ngx_cdecl
main(<span>int</span> argc, <span>char</span> *<span>const</span> *argv) <span>// 在master进程中</span>
{
cycle = ngx_init_cycle(&init_cycle);
{
<span>if</span> (ngx_conf_parse(&conf, &cycle->conf_file) != NGX_CONF_OK) { <span>// 解析配置</span>
{
解析到http指令（进入ngx_http_block()）
{
<span>// 会依次执行</span><span>typedef</span><span>struct</span> {
ngx_int_t (*preconfiguration)(ngx_conf_t *cf); <span>/* 执行顺序4 */</span>
ngx_int_t (*postconfiguration)(ngx_conf_t *cf); <span>/* 执行顺序8 */</span><span>void</span> *(*create_main_conf)(ngx_conf_t *cf); <span>/* 执行顺序1 */</span><span>char</span> *(*init_main_conf)(ngx_conf_t *cf, <span>void</span> *conf); <span>/* 执行顺序5 */</span><span>void</span> *(*create_srv_conf)(ngx_conf_t *cf); <span>/* 执行顺序2 */</span><span>char</span> *(*merge_srv_conf)(ngx_conf_t *cf, <span>void</span> *prev, <span>void</span> *conf); <span>/* 执行顺序6 */</span><span>void</span> *(*create_loc_conf)(ngx_conf_t *cf); <span>/* 执行顺序3 */</span><span>char</span> *(*merge_loc_conf)(ngx_conf_t *cf, <span>void</span> *prev, <span>void</span> *conf); <span>/* 执行顺序7 */</span>
} ngx_http_module_t;
同时，还有个执行顺序<span>4.5</span>:
<span>struct</span> ngx_command_s { <span>/* 执行顺序4.5 */</span>
ngx_str_t name;
ngx_uint_t type;
<span>char</span> *(*<span>set</span>)(ngx_conf_t *cf, ngx_command_t *cmd, <span>void</span> *conf);
ngx_uint_t conf;
ngx_uint_t offset;
<span>void</span> *post;
};
<span>for</span> (m = <span>0</span>; ngx_modules[m]; m++) {
<span>if</span> (module->create_main_conf) {ctx->main_conf[mi] = module->create_main_conf(cf);}
<span>if</span> (module->create_srv_conf) {ctx->srv_conf[mi] = module->create_srv_conf(cf);}
<span>if</span> (module->create_loc_conf) {ctx->loc_conf[mi] = module->create_loc_conf(cf);}
}
<span>for</span> (m = <span>0</span>; ngx_modules[m]; m++) {
<span>if</span> (module->preconfiguration) {<span>if</span> (module->preconfiguration(cf) != NGX_OK) {}
}
rv = ngx_conf_parse(cf, NULL);
{
<span>/*
* 指令的解析
* 共享内存配置相关的指令也在这里进行解析
* 详细见：
* ngx_shm_zone_t *
* ngx_shared_memory_add(ngx_conf_t *cf, ngx_str_t *name, size_t size, void *tag)
*/</span> }
<span>for</span> (m = <span>0</span>; ngx_modules[m]; m++) {
<span>if</span> (module->init_main_conf) {rv = module->init_main_conf(cf, ctx->main_conf[mi]);}
rv = ngx_http_merge_servers(cf, cmcf, module, mi);
}
<span>for</span> (m = <span>0</span>; ngx_modules[m]; m++) {
<span>if</span> (module->postconfiguration) {<span>if</span> (module->postconfiguration(cf) != NGX_OK)}
}
}
}
<span>// in ngx_init_cycle(&init_cycle)</span>
line: <span>462</span><span>if</span> (ngx_shm_alloc(&shm_zone[i].shm) != NGX_OK) <span>/* 实际共享内存分配的地方 */</span>
line: <span>466</span><span>if</span> (ngx_init_zone_pool(cycle, &shm_zone[i]) != NGX_OK)
<span>/* 共享内存管理机制的初始化
* 共享内存的使用涉及另外两个主题：
* 1、多进程共同使用时之间的互斥问题
* 2、引入特定的使用方式（slab机制，这在下一个主题：“Nginx源码分析（2）之——共享内存管理之slab机制”中进行介绍），以提高性能
*/</span>
line: <span>470</span><span>if</span> (shm_zone[i].init(&shm_zone[i], NULL) != NGX_OK) <span>/* 分配之后的初始化 */</span>
}
｝
ngx_shm_zone_t *
ngx_shared_memory_add(ngx_conf_t *cf, ngx_str_t *name, size_t size, <span>void</span> *tag)
{
ngx_uint_t i;
ngx_shm_zone_t *shm_zone;
ngx_list_part_t *part;
<span>/*
* Nginx中所有的共享内存都以list链表的形式组织在全局变量cf->cycle->shared_memory中
* 在创建新的共享内存之前，会对该链表进行遍历查找以及冲突检测，
* 对于已经存在且不存在冲突时，对共享内存直接进行返回并引用
* 存在且不存在冲突：共享内存的名称相同，大小相同，且tag指向的是同一个模块
* 有冲突，则报错
* 否则，重新分配ngx_shm_zone_t,并挂到全局链表cf->cycle->shared_memory中，最后进行结构初始化
* shm_zone = ngx_list_push(&cf->cycle->shared_memory);
* 至此：
* 仅仅是创建了共享内存的结构体：ngx_shm_zone_t，ngx_shm_zone_t.shm.addr指向的真实共享内存并没有进行实际的分配
*/</span>
part = &cf->cycle->shared_memory.part;
shm_zone = part->elts;
<span>for</span> (i = <span>0</span>; <span>/* void */</span> ; i++) {
<span>if</span> (i >= part->nelts) {
<span>if</span> (part->next == NULL) {
<span>break</span>;
}
part = part->next;
shm_zone = part->elts;
i = <span>0</span>;
}
<span>if</span> (name->len != shm_zone[i].shm.name.len) {
<span>continue</span>;
}
<span>if</span> (ngx_strncmp(name->data, shm_zone[i].shm.name.data, name->len)
!= <span>0</span>)
{
<span>continue</span>;
}
<span>if</span> (tag != shm_zone[i].tag) {
ngx_conf_log_error(NGX_LOG_EMERG, cf, <span>0</span>,
<span>"the shared memory zone \"%V\" is "</span><span>"already declared for a different use"</span>,
&shm_zone[i].shm.name);
<span>return</span> NULL;
}
<span>if</span> (size && size != shm_zone[i].shm.size) {
ngx_conf_log_error(NGX_LOG_EMERG, cf, <span>0</span>,
<span>"the size %uz of shared memory zone \"%V\" "</span><span>"conflicts with already declared size %uz"</span>,
size, &shm_zone[i].shm.name, shm_zone[i].shm.size);
<span>return</span> NULL;
}
<span>return</span> &shm_zone[i];
}
shm_zone = ngx_list_push(&cf->cycle->shared_memory);
<span>if</span> (shm_zone == NULL) {
<span>return</span> NULL;
}
shm_zone->data = NULL;
shm_zone->shm.<span>log</span> = cf->cycle-><span>log</span>;
shm_zone->shm.size = size;
shm_zone->shm.name = *name;
shm_zone->shm.exists = <span>0</span>;
shm_zone->init = NULL;
shm_zone->tag = tag;
<span>return</span> shm_zone;
}</code>最后，推荐两本书：
《深入剖析Nginx》 by 高群凯
《深入理解Nginx ——模块开发与架构解析》 by 陶辉
以上就介绍了Nginx源码分析（1）之——共享内存的配置、分配及初始化，包括了方面的内容，希望对PHP教程有兴趣的朋友有所帮助。
