Nginx-HTTP之listen指令解析

1. ngx_http_core_listen

static char *
ngx_http_core_listen(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    /* conf 即为该 listen 所在的配置信息结构体 */
    ngx_http_core_srv_conf_t *cscf = conf;

    ngx_str_t              *value, size;
    ngx_url_t               u;
    ngx_uint_t              n;
    ngx_http_listen_opt_t   lsopt;

    cscf->listen = 1;
    
    /* 假设当前 listen 指令为： "listen 80;" */

    /* value = "listen" */
    value = cf->args->elts;

    ngx_memzero(&u, sizeof(ngx_url_t));

    /* u.url = "80" */
    u.url = value[1];
    /* 标志位，为 1 表示当前端口有效 */
    u.listen = 1;
    /* 若 listen 没有指定端口，则会使用默认端口 */
    u.default_port = 80;

    /* 解析 listen 的参数 */
    if (ngx_parse_url(cf->pool, &u) != NGX_OK) {
        if (u.err) {
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "%s in \"%V\" of the \"listen\" directive",
                               u.err, &u.url);
        }

        return NGX_CONF_ERROR;
    }

    ngx_memzero(&lsopt, sizeof(ngx_http_listen_opt_t));

    ngx_memcpy(&lsopt.sockaddr.sockaddr, &u.sockaddr, u.socklen);

    lsopt.socklen = u.socklen;
    /* TCP 实现监听时的 backlog 队列，它表示允许正在通过三次握手建立 TCP
     * 连接但没有任何进程开始处理的连接最大个数，linux 下设为 511 */
    lsopt.backlog = NGX_LISTEN_BACKLOG;
    lsopt.rcvbuf = -1;
    lsopt.sndbuf = -1;
#if (NGX_HAVE_SETFIB)
    lsopt.setfib = -1;
#endif
#if (NGX_HAVE_TCP_FASTOPEN)
    lsopt.fastopen = -1;
#endif
    lsopt.wildcard = u.wildcard;
#if (NGX_HAVE_INET6)
    lsopt.ipv6only = 1;
#endif

    /* 将 */
    (void) ngx_sock_ntop(&lsopt.sockaddr.sockaddr, lsopt.socklen, lsopt.addr,
                         NGX_SOCKADDR_STRLEN, 1);

    /* 若当前 listen 指令不止一个参数 */
    for (n = 2; n < cf->args->nelts; n++) {

        /* 若第二个参数为 "default_server" */
        if (ngx_strcmp(value[n].data, "default_server") == 0
            || ngx_strcmp(value[n].data, "default") == 0)
        {
            /* 表示使用默认的服务器 */
            lsopt.default_server = 1;
            continue;
        }

        /* 若listen指定有 bind 参数，则表示需要绑定该端口 */
        if (ngx_strcmp(value[n].data, "bind") == 0) {
            lsopt.set = 1;
            lsopt.bind = 1;
            continue;
        }

#if (NGX_HAVE_SETFIB)
        if (ngx_strncmp(value[n].data, "setfib=", 7) == 0) {
            lsopt.setfib = ngx_atoi(value[n].data + 7, value[n].len - 7);
            lsopt.set = 1;
            lsopt.bind = 1;

            if (lsopt.setfib == NGX_ERROR) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid setfib \"%V\"", &value[n]);
                return NGX_CONF_ERROR;
            }

            continue;
        }
#endif

#if (NGX_HAVE_TCP_FASTOPEN)
        if (ngx_strncmp(value[n].data, "fastopen=", 9) == 0) {
            lsopt.fastopen = ngx_atoi(value[n].data + 9, value[n].len - 9);
            lsopt.set = 1;
            lsopt.bind = 1;

            if (lsopt.fastopen == NGX_ERROR) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid fastopen \"%V\"", &value[n]);
                return NGX_CONF_ERROR;
            }

            continue;
        }
#endif

        if (ngx_strncmp(value[n].data, "backlog=", 8) == 0) {
            lsopt.backlog = ngx_atoi(value[n].data + 8, value[n].len - 8);
            lsopt.set = 1;
            lsopt.bind = 1;

            if (lsopt.backlog == NGX_ERROR || lsopt.backlog == 0) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid backlog \"%V\"", &value[n]);
                return NGX_CONF_ERROR;
            }

            continue;
        }

        if (ngx_strncmp(value[n].data, "rcvbuf=", 7) == 0) {
            size.len = value[n].len - 7;
            size.data = value[n].data + 7;

            lsopt.rcvbuf = ngx_parse_size(&size);
            lsopt.set = 1;
            lsopt.bind = 1;

            if (lsopt.rcvbuf == NGX_ERROR) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid rcvbuf \"%V\"", &value[n]);
                return NGX_CONF_ERROR;
            }

            continue;
        }

        if (ngx_strncmp(value[n].data, "sndbuf=", 7) == 0) {
            size.len = value[n].len - 7;
            size.data = value[n].data + 7;

            lsopt.sndbuf = ngx_parse_size(&size);
            lsopt.set = 1;
            lsopt.bind = 1;

            if (lsopt.sndbuf == NGX_ERROR) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "invalid sndbuf \"%V\"", &value[n]);
                return NGX_CONF_ERROR;
            }

            continue;
        }

        if (ngx_strncmp(value[n].data, "accept_filter=", 14) == 0) {
#if (NGX_HAVE_DEFERRED_ACCEPT && defined SO_ACCEPTFILTER)
            lsopt.accept_filter = (char *) &value[n].data[14];
            lsopt.set = 1;
            lsopt.bind = 1;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "accept filters \"%V\" are not supported "
                               "on this platform, ignored",
                               &value[n]);
#endif
            continue;
        }

        if (ngx_strcmp(value[n].data, "deferred") == 0) {
#if (NGX_HAVE_DEFERRED_ACCEPT && defined TCP_DEFER_ACCEPT)
            lsopt.deferred_accept = 1;
            lsopt.set = 1;
            lsopt.bind = 1;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "the deferred accept is not supported "
                               "on this platform, ignored");
#endif
            continue;
        }

        if (ngx_strncmp(value[n].data, "ipv6only=o", 10) == 0) {
#if (NGX_HAVE_INET6 && defined IPV6_V6ONLY)
            struct sockaddr  *sa;

            sa = &lsopt.sockaddr.sockaddr;

            if (sa->sa_family == AF_INET6) {

                if (ngx_strcmp(&value[n].data[10], "n") == 0) {
                    lsopt.ipv6only = 1;

                } else if (ngx_strcmp(&value[n].data[10], "ff") == 0) {
                    lsopt.ipv6only = 0;

                } else {
                    ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                       "invalid ipv6only flags \"%s\"",
                                       &value[n].data[9]);
                    return NGX_CONF_ERROR;
                }

                lsopt.set = 1;
                lsopt.bind = 1;

            } else {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "ipv6only is not supported "
                                   "on addr \"%s\", ignored", lsopt.addr);
            }

            continue;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "ipv6only is not supported "
                               "on this platform");
            return NGX_CONF_ERROR;
#endif
        }

        if (ngx_strcmp(value[n].data, "reuseport") == 0) {
#if (NGX_HAVE_REUSEPORT)
            lsopt.reuseport = 1;
            lsopt.set = 1;
            lsopt.bind = 1;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "reuseport is not supported "
                               "on this platform, ignored");
#endif
            continue;
        }

        if (ngx_strcmp(value[n].data, "ssl") == 0) {
#if (NGX_HTTP_SSL)
            lsopt.ssl = 1;
            continue;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "the \"ssl\" parameter requires "
                               "ngx_http_ssl_module");
            return NGX_CONF_ERROR;
#endif
        }

        if (ngx_strcmp(value[n].data, "http2") == 0) {
#if (NGX_HTTP_V2)
            lsopt.http2 = 1;
            continue;
#else
            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "the \"http2\" parameter requires "
                               "ngx_http_v2_module");
            return NGX_CONF_ERROR;
#endif
        }

        if (ngx_strcmp(value[n].data, "spdy") == 0) {
            ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
                               "invalid parameter \"spdy\": "
                               "ngx_http_spdy_module was superseded "
                               "by ngx_http_v2_module");
            continue;
        }

        if (ngx_strncmp(value[n].data, "so_keepalive=", 13) == 0) {

            if (ngx_strcmp(&value[n].data[13], "on") == 0) {
                lsopt.so_keepalive = 1;

            } else if (ngx_strcmp(&value[n].data[13], "off") == 0) {
                lsopt.so_keepalive = 2;

            } else {

#if (NGX_HAVE_KEEPALIVE_TUNABLE)
                u_char     *p, *end;
                ngx_str_t   s;

                end = value[n].data + value[n].len;
                s.data = value[n].data + 13;

                p = ngx_strlchr(s.data, end, ':');
                if (p == NULL) {
                    p = end;
                }

                if (p > s.data) {
                    s.len = p - s.data;

                    lsopt.tcp_keepidle = ngx_parse_time(&s, 1);
                    if (lsopt.tcp_keepidle == (time_t) NGX_ERROR) {
                        goto invalid_so_keepalive;
                    }
                }

                s.data = (p < end) ? (p + 1) : end;

                p = ngx_strlchr(s.data, end, ':');
                if (p == NULL) {
                    p = end;
                }

                if (p > s.data) {
                    s.len = p - s.data;

                    lsopt.tcp_keepintvl = ngx_parse_time(&s, 1);
                    if (lsopt.tcp_keepintvl == (time_t) NGX_ERROR) {
                        goto invalid_so_keepalive;
                    }
                }

                s.data = (p < end) ? (p + 1) : end;

                if (s.data < end) {
                    s.len = end - s.data;

                    lsopt.tcp_keepcnt = ngx_atoi(s.data, s.len);
                    if (lsopt.tcp_keepcnt == NGX_ERROR) {
                        goto invalid_so_keepalive;
                    }
                }

                if (lsopt.tcp_keepidle == 0 && lsopt.tcp_keepintvl == 0
                    && lsopt.tcp_keepcnt == 0)
                {
                    goto invalid_so_keepalive;
                }

                lsopt.so_keepalive = 1;

#else

                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "the \"so_keepalive\" parameter accepts "
                                   "only \"on\" or \"off\" on this platform");
                return NGX_CONF_ERROR;

#endif
            }

            lsopt.set = 1;
            lsopt.bind = 1;

            continue;

#if (NGX_HAVE_KEEPALIVE_TUNABLE)
        invalid_so_keepalive:

            ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                               "invalid so_keepalive value: \"%s\"",
                               &value[n].data[13]);
            return NGX_CONF_ERROR;
#endif
        }

        if (ngx_strcmp(value[n].data, "proxy_protocol") == 0) {
            lsopt.proxy_protocol = 1;
            continue;
        }

        ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                           "invalid parameter \"%V\"", &value[n]);
        return NGX_CONF_ERROR;
    }

    /* 将已经初始化好的 lsopt 结构体添加到 */
    if (ngx_http_add_listen(cf, cscf, &lsopt) == NGX_OK) {
        return NGX_CONF_OK;
    }

    return NGX_CONF_ERROR;
}

2. ngx_parse_url

解析 lisetn 指令的参数。

ngx_int_t
ngx_parse_url(ngx_pool_t *pool, ngx_url_t *u)
{
    u_char  *p;
    size_t   len;

    p = u->url.data;
    len = u->url.len;

    /* 检测是否为 unix 域 */
    if (len >= 5 && ngx_strncasecmp(p, (u_char *) "unix:", 5) == 0) {
        return ngx_parse_unix_domain_url(pool, u);
    }

    /* 检测是否为 ip6  */
    if (len && p[0] == '[') {
        return ngx_parse_inet6_url(pool, u);
    }

    /* 否则调用该函数 */
    return ngx_parse_inet_url(pool, u);
}

2.1 ngx_parse_inet_url

static ngx_int_t
ngx_parse_inet_url(ngx_pool_t *pool, ngx_url_t *u)
{
    u_char               *p, *host, *port, *last, *uri, *args;
    size_t                len;
    ngx_int_t             n;
    struct sockaddr_in   *sin;
#if (NGX_HAVE_INET6)
    struct sockaddr_in6  *sin6;
#endif

    u->socklen = sizeof(struct sockaddr_in);
    sin = (struct sockaddr_in *) &u->sockaddr;
    /* 初始化地址组为 AF_INET，即为 ipv4 类型的 */
    sin->sin_family = AF_INET;

    u->family = AF_INET;

    /* 若当前 listen 指令的配置为: listen 80; */

    host = u->url.data;

    last = host + u->url.len;

    port = ngx_strlchr(host, last, ':');

    uri = ngx_strlchr(host, last, '/');

    args = ngx_strlchr(host, last, '?');

    if (args) {
        if (uri == NULL || args < uri) {
            uri = args;
        }
    }

    if (uri) {
        if (u->listen || !u->uri_part) {
            u->err = "invalid host";
            return NGX_ERROR;
        }

        u->uri.len = last - uri;
        u->uri.data = uri;

        last = uri;

        if (uri < port) {
            port = NULL;
        }
    }

    if (port) {
        port++;

        len = last - port;

        n = ngx_atoi(port, len);

        if (n < 1 || n > 65535) {
            u->err = "invalid port";
            return NGX_ERROR;
        }

        u->port = (in_port_t) n;
        sin->sin_port = htons((in_port_t) n);

        u->port_text.len = len;
        u->port_text.data = port;

        last = port - 1;

    } else {
        if (uri == NULL) {

            if (u->listen) {

                /* test value as port only */

                n = ngx_atoi(host, last - host);

                if (n != NGX_ERROR) {

                    if (n < 1 || n > 65535) {
                        u->err = "invalid port";
                        return NGX_ERROR;
                    }

                    /* 当前 listen 指令的参数仅为要监听的端口 */
                    u->port = (in_port_t) n;
                    /* 将该端口转换为网络字节序 */
                    sin->sin_port = htons((in_port_t) n);

                    u->port_text.len = last - host;
                    /* 将该端口以字符串形式保存在该变量中 */
                    u->port_text.data = host;

                    u->wildcard = 1;

                    /* 仅为端口的情况下，这里直接返回了 */
                    return NGX_OK;
                }
            }
        }

        /* no_port 标志位，为 1 表示当前 listen 的参数中没有指定端口 */
        u->no_port = 1;
        /* 因此使用默认端口 80 */
        u->port = u->default_port;
        /* 将主机字节序转换为网络字节序 */
        sin->sin_port = htons(u->default_port);
    }

    len = last - host;

    if (len == 0) {
        u->err = "no host";
        return NGX_ERROR;
    }

    u->host.len = len;
    u->host.data = host;

    if (u->listen && len == 1 && *host == '*') {
        sin->sin_addr.s_addr = INADDR_ANY;
        u->wildcard = 1;
        return NGX_OK;
    }

    sin->sin_addr.s_addr = ngx_inet_addr(host, len);

    if (sin->sin_addr.s_addr != INADDR_NONE) {

        if (sin->sin_addr.s_addr == INADDR_ANY) {
            u->wildcard = 1;
        }

        u->naddrs = 1;

        u->addrs = ngx_pcalloc(pool, sizeof(ngx_addr_t));
        if (u->addrs == NULL) {
            return NGX_ERROR;
        }

        sin = ngx_pcalloc(pool, sizeof(struct sockaddr_in));
        if (sin == NULL) {
            return NGX_ERROR;
        }

        ngx_memcpy(sin, &u->sockaddr, sizeof(struct sockaddr_in));

        u->addrs[0].sockaddr = (struct sockaddr *) sin;
        u->addrs[0].socklen = sizeof(struct sockaddr_in);

        p = ngx_pnalloc(pool, u->host.len + sizeof(":65535") - 1);
        if (p == NULL) {
            return NGX_ERROR;
        }

        u->addrs[0].name.len = ngx_sprintf(p, "%V:%d",
                                           &u->host, u->port) - p;
        u->addrs[0].name.data = p;

        return NGX_OK;
    }

    if (u->no_resolve) {
        return NGX_OK;
    }

    if (ngx_inet_resolve_host(pool, u) != NGX_OK) {
        return NGX_ERROR;
    }

    u->family = u->addrs[0].sockaddr->sa_family;
    u->socklen = u->addrs[0].socklen;
    ngx_memcpy(&u->sockaddr, u->addrs[0].sockaddr, u->addrs[0].socklen);

    switch (u->family) {

#if (NGX_HAVE_INET6)
    case AF_INET6:
        sin6 = (struct sockaddr_in6 *) &u->sockaddr;

        if (IN6_IS_ADDR_UNSPECIFIED(&sin6->sin6_addr)) {
            u->wildcard = 1;
        }

        break;
#endif

    default: /* AF_INET */
        sin = (struct sockaddr_in *) &u->sockaddr;

        if (sin->sin_addr.s_addr == INADDR_ANY) {
            u->wildcard = 1;
        }

        break;
    }

    return NGX_OK;
}

3. ngx_http_add_listen

/* 每监听一个TCP端口，都将使用一个独立的ngx_http_conf_port_t结构体表示。
 * 这个保存着监听端口的ngx_http_conf_port_t将由全局的ngx_http_core_main_conf_t
 * 结构体保存 */
typedef struct {
    /* socket地址家族 */
    ngx_int_t                       family;
    /* 监听端口 */
    in_port_t                       port;
    /* 监听端口下对应着的所有ngx_http_conf_addr_t地址 */
    ngx_array_t                     addrs; /* array of ngx_http_conf_addr_t */
}ngx_http_conf_port_t;

ngx_int_t
ngx_http_add_listen(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_listen_opt_t *lsopt)
{
    in_port_t                   p;
    ngx_uint_t                  i;
    struct sockaddr            *sa;
    ngx_http_conf_port_t       *port;
    ngx_http_core_main_conf_t  *cmcf;

    cmcf = ngx_http_conf_get_module_main_conf(cf, ngx_http_core_module);

    /* 若 ports 数组为空 */
    if (cmcf->ports == NULL) {
        /* posts 存放着该http{}配置块下监听的所有ngx_http_conf_port_t端口 */
        cmcf->ports = ngx_array_create(cf->temp_pool, 2,
                                       sizeof(ngx_http_conf_port_t));
        if (cmcf->ports == NULL) {
            return NGX_ERROR;
        }
    }

    sa = &lsopt->sockaddr.sockaddr;
    /* 获取当前监听的端口，返回值为主机字节序 */
    p = ngx_inet_get_port(sa);

    port = cmcf->ports->elts;
    /* 检测当前所要监听的端口是否已经存在 ports 数组中 */
    for (i = 0; i < cmcf->ports->nelts; i++) {

        if (p != port[i].port || sa->sa_family != port[i].family) {
            continue;
        }

        /* a port is already in the port list */

        return ngx_http_add_addresses(cf, cscf, &port[i], lsopt);
    }

    /* add a port to the port list */
    
    /* 若当前所要监听的端口不存在 ports 数组中，则
     * 将该端口添加到 ports 数组中 */
    port = ngx_array_push(cmcf->ports);
    if (port == NULL) {
        return NGX_ERROR;
    }
    
    /* 端口的地址族 */
    port->family = sa->sa_family;
    /* 监听的端口 */
    port->port = p;
    port->addrs.elts = NULL;

    /* 为该端口添加地址 */
    return ngx_http_add_address(cf, cscf, port, lsopt);
}

对ngx_http_conf_port_t的addrs动态数组的理解：

• 在nginx.conf配置文件中，对于同一个端口8000，我们可以同时监听127.0.0.1:8000、192.168.1.82:8000这两个地址，当一台物理主机具备多个IP地址时这是很有用的。具体到HTTP框架的实现上，Nginx是使用ngx_http_conf_addr_t结构体来表示一个对应着具体地址的监听端口的，因此，一个ngx_http_conf_port_t将会对应多个ngx_http_conf_addr_t，而ngx_http_conf_addr_t就是以动态数组的形式保存在addrs成员中的.

3.1 ngx_http_add_address

typedef struct {
    /* 监听套接字的各种属性 */
    ngx_http_listen_opt_t           opt;

    /* 以下3个散列表用于加速寻找对应监听端口上的新连接，确定到底使用哪个server{}
     * 虚拟主机下的配置来处理它。所以，散列表的值就是ngx_http_core_srv_conf_t
     * 结构体的地址 */

    /* 完全匹配server name的散列表 */
    ngx_hash_t                      hash;
    /* 通配符前置的散列表 */
    ngx_hash_wildcard_t            *wc_head;
    /* 通配符后置的散列表 */
    ngx_hash_wildcard_t            *wc_tail;

#if (NGX_PCRE)
    /* 下面的regex数组中元素的个数 */
    ngx_uint_t                      nregex;
    /* regex指向静态数组，其数组成员就是ngx_http_server_name_t结构体，
     * 表示正则表达式及其匹配的server{}虚拟主机 */
    ngx_http_server_name_t         *regex;
#endif

    /* 该监听端口下对应的默认server{}虚拟主机 */
    /* the default server configuration for this address:port */
    ngx_http_core_srv_conf_t       *default_server;
    /* servers动态数组中的成员将指向ngx_http_core_srv_conf_t结构体 */
    ngx_array_t                     servers; /* array of ngx_http_core_srv_conf_t */
}ngx_http_conf_addr_t;

/*
 * add the server address, the server names and the server core module
 * configurations to the port list.
 */
static ngx_int_t
ngx_http_add_address(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_conf_port_t *port, ngx_http_listen_opt_t *lsopt)
{
    ngx_http_conf_addr_t  *addr;

    /* 一个端口可能对应几个主机地址，具体看当前主机有多少个 ip 地址 */
    if (port->addrs.elts == NULL) {
        if (ngx_array_init(&port->addrs, cf->temp_pool, 4,
                           sizeof(ngx_http_conf_addr_t))
            != NGX_OK)
        {
            return NGX_ERROR;
        }
    }

#if (NGX_HTTP_V2 && NGX_HTTP_SSL                                              \
     && !defined TLSEXT_TYPE_application_layer_protocol_negotiation           \
     && !defined TLSEXT_TYPE_next_proto_neg)

    if (lsopt->http2 && lsopt->ssl) {
        ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
                           "nginx was built with OpenSSL that lacks ALPN "
                           "and NPN support, HTTP/2 is not enabled for %s",
                           lsopt->addr);
    }

#endif

    /* 向该 port->addrs 数组中添加一个地址 */
    addr = ngx_array_push(&port->addrs);
    if (addr == NULL) {
        return NGX_ERROR;
    }

    addr->opt = *lsopt;
    addr->hash.buckets = NULL;
    addr->hash.size = 0;
    addr->wc_head = NULL;
    addr->wc_tail = NULL;
#if (NGX_PCRE)
    addr->nregex = 0;
    addr->regex = NULL;
#endif
    addr->default_server = cscf;
    addr->servers.elts = NULL;

    return ngx_http_add_server(cf, cscf, addr);
}

3.2 ngx_http_add_server

/* 
 * add the server core module configuration to the address:port 
 */
static ngx_int_t
ngx_http_add_server(ngx_conf_t *cf, ngx_http_core_srv_conf_t *cscf,
    ngx_http_conf_addr_t *addr)
{
    ngx_uint_t                  i;
    ngx_http_core_srv_conf_t  **server;

    /* servers动态数组中的成员将指向ngx_http_core_srv_conf_t结构体 */
    if (addr->servers.elts == NULL) {
        if (ngx_array_init(&addr->servers, cf->temp_pool, 4,
                           sizeof(ngx_http_core_srv_conf_t *))
            != NGX_OK)
        {
            return NGX_ERROR;
        }

    } else {
        server = addr->servers.elts;
        for (i = 0; i < addr->servers.nelts; i++) {
            if (server[i] == cscf) {
                ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                                   "a duplicate listen %s", addr->opt.addr);
                return NGX_ERROR;
            }
        }
    }

    /* 向该数组中添加一个 ngx_http_core_srv_conf_t 结构体 */
    server = ngx_array_push(&addr->servers);
    if (server == NULL) {
        return NGX_ERROR;
    }

    /* 将当前 listen 指令所在的 server{} 下 srv 相关配置项的
     * ngx_http_core_srv_conf_t 添加到 servers 数组中 */
    *server = cscf;

    return NGX_OK;
}