C语言实现局域网服务发现（基于mDNS的零配置网络编程教程）

准备工作

要实现 C语言mDNS实现，我们需要以下工具：

• Linux 或 macOS 系统（Windows 需要额外配置）
• GCC 编译器
• Avahi 开发库（用于 mDNS 支持）

安装 Avahi 开发包（以 Ubuntu 为例）：

sudo apt-get install libavahi-client-dev libavahi-common-dev

编写服务注册程序

首先，我们创建一个程序，将自己的服务（例如 HTTP 服务）注册到局域网中。

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <avahi-client/client.h>#include <avahi-client/publish.h>#include <avahi-common/alternative.h>#include <avahi-common/simple-watch.h>#include <avahi-common/malloc.h>#include <avahi-common/error.h>static AvahiEntryGroup *group = NULL;static AvahiSimplePoll *simple_poll = NULL;static void create_services(AvahiClient *client) {    int ret;    char *name = "My Web Service";    if (!group) {        group = avahi_entry_group_new(client, NULL, NULL);        if (!group) {            fprintf(stderr, "avahi_entry_group_new() failed: %s\n",                    avahi_strerror(avahi_client_errno(client)));            goto fail;        }    }    if (!avahi_entry_group_is_empty(group))        return;    // 注册 _http._tcp 服务    ret = avahi_entry_group_add_service(        group, AVAHI_IF_UNSPEC, AVAHI_PROTO_UNSPEC,        0, name, "_http._tcp", NULL, NULL, 8080, NULL    );    if (ret < 0) {        if (ret == AVAHI_ERR_COLLISION)            goto fail;        fprintf(stderr, "Failed to add _http._tcp service: %s\n",                avahi_strerror(ret));        goto fail;    }    ret = avahi_entry_group_commit(group);    if (ret < 0) {        fprintf(stderr, "Failed to commit entry group: %s\n",                avahi_strerror(ret));        goto fail;    }    printf("Service '%s' successfully established.\n", name);    return;fail:    avahi_simple_poll_quit(simple_poll);}static void client_callback(AvahiClient *client, AvahiClientState state, void *userdata) {    switch (state) {        case AVAHI_CLIENT_S_RUNNING:            create_services(client);            break;        case AVAHI_CLIENT_FAILURE:            fprintf(stderr, "Client failure: %s\n",                    avahi_strerror(avahi_client_errno(client)));            avahi_simple_poll_quit(simple_poll);            break;        default:            ;    }}int main(int argc, char* argv[]) {    AvahiClient *client = NULL;    int error;    simple_poll = avahi_simple_poll_new();    if (!simple_poll) {        fprintf(stderr, "Failed to create simple poll object.\n");        goto cleanup;    }    client = avahi_client_new(        avahi_simple_poll_get(simple_poll),        0,        client_callback,        NULL,        &error    );    if (!client) {        fprintf(stderr, "Failed to create client: %s\n", avahi_strerror(error));        goto cleanup;    }    printf("Registering service... Press Ctrl+C to exit.\n");    avahi_simple_poll_loop(simple_poll);cleanup:    if (client)        avahi_client_free(client);    if (simple_poll)        avahi_simple_poll_free(simple_poll);    return 0;}

编译与运行

将上述代码保存为 publish.c，然后使用以下命令编译：

gcc -o publish publish.c `pkg-config --cflags --libs avahi-client`

运行程序：

./publish

此时，你的设备已在局域网中广播一个名为 “My Web Service” 的 HTTP 服务，端口为 8080。

服务发现（客户端）

接下来，我们编写一个简单的发现程序，用于查找局域网中的 HTTP 服务。

#include <stdio.h>#include <avahi-client/lookup.h>#include <avahi-common/simple-watch.h>#include <avahi-common/malloc.h>static AvahiSimplePoll *simple_poll = NULL;static void resolve_callback(    AvahiServiceResolver *r,    AvahiIfIndex interface,    AvahiProtocol protocol,    AvahiResolverEvent event,    const char *name,    const char *type,    const char *domain,    const char *host_name,    const AvahiAddress *address,    uint16_t port,    AvahiStringList *txt,    AvahiLookupResultFlags flags,    void* userdata) {    if (event == AVAHI_RESOLVER_FOUND) {        char a[AVAHI_ADDRESS_STR_MAX];        avahi_address_snprint(a, sizeof(a), address);        printf("Found service '%s' of type '%s' in domain '%s':\n",               name, type, domain);        printf("  Host: %s, Port: %u, IP: %s\n", host_name, port, a);    }    avahi_service_resolver_free(r);}static void browse_callback(    AvahiServiceBrowser *b,    AvahiIfIndex interface,    AvahiProtocol protocol,    AvahiBrowserEvent event,    const char *name,    const char *type,    const char *domain,    AvahiLookupResultFlags flags,    void* userdata) {    AvahiClient *c = userdata;    switch (event) {        case AVAHI_BROWSER_NEW:            avahi_service_resolver_new(                c, interface, protocol, name, type, domain,                AVAHI_PROTO_UNSPEC, 0, resolve_callback, c            );            break;        case AVAHI_BROWSER_REMOVE:            printf("Service removed: %s\n", name);            break;        case AVAHI_BROWSER_FAILURE:            fprintf(stderr, "Browse error: %s\n",                    avahi_strerror(avahi_client_errno(avahi_service_browser_get_client(b))));            avahi_simple_poll_quit(simple_poll);            break;    }}static void client_callback(AvahiClient *client, AvahiClientState state, void *userdata) {    if (state == AVAHI_CLIENT_S_RUNNING) {        AvahiServiceBrowser *sb;        sb = avahi_service_browser_new(            client, AVAHI_IF_UNSPEC, AVAHI_PROTO_UNSPEC,            "_http._tcp", NULL, 0, browse_callback, client        );        if (!sb) {            fprintf(stderr, "Failed to create service browser: %s\n",                    avahi_strerror(avahi_client_errno(client)));            avahi_simple_poll_quit(simple_poll);        }    }}int main(int argc, char* argv[]) {    AvahiClient *client = NULL;    int error;    simple_poll = avahi_simple_poll_new();    if (!simple_poll) {        fprintf(stderr, "Failed to create simple poll object.\n");        return 1;    }    client = avahi_client_new(        avahi_simple_poll_get(simple_poll),        0,        client_callback,        NULL,        &error    );    if (!client) {        fprintf(stderr, "Failed to create client: %s\n", avahi_strerror(error));        avahi_simple_poll_free(simple_poll);        return 1;    }    printf("Browsing for _http._tcp services... Press Ctrl+C to exit.\n");    avahi_simple_poll_loop(simple_poll);    avahi_client_free(client);    avahi_simple_poll_free(simple_poll);    return 0;}

这个程序会持续监听局域网中所有 _http._tcp 类型的服务，并打印其主机名、IP 和端口。

总结

通过本教程，你已经掌握了如何使用 C语言服务发现 技术实现局域网内的服务注册与发现。这种 零配置网络编程 方法极大简化了设备互联的复杂度，是构建智能设备生态的关键技术之一。

记住，实际项目中可能需要处理更多边界情况（如服务冲突、网络中断等），但 Avahi 库已经为你封装了大部分底层细节。希望你能在此基础上开发出更强大的应用！

关键词回顾：C语言服务发现、零配置网络编程、C语言mDNS实现、局域网服务自动发现。