linux 使用PF_NETLINK协议用户进程间通信, 服务端收不到请求是什么问题? 正确的服务端客户端代码应该怎么写

如果服务端收不到请求，可能有以下几个问题：

1. 服务端没有正确地绑定监听的 netlink socket，需要使用 `bind()` 函数将其绑定到正确的 netlink family 和 multicast group 上；
2. 服务端没有正确地设置 netlink socket 的接收缓冲区大小，可以使用 `setsockopt()` 函数来设置；
3. 客户端没有正确地发送请求，需要使用 `sendto()` 函数发送请求，并且请求结构体的长度需要和发送的字节数相等。

下面是一个简单的服务端和客户端代码示例：

服务端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>

#define NETLINK_USER 31

struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;

int main() {
    // 创建 netlink socket
    sock_fd = socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sock_fd < 0) {
        printf("Error creating socket.\n");
        return -1;
    }

    // 绑定 netlink socket
    memset(&src_addr, 0, sizeof(src_addr));
    src_addr.nl_family = AF_NETLINK;
    src_addr.nl_pid = getpid();
    src_addr.nl_groups = 0;
    if (bind(sock_fd, (struct sockaddr*)&src_addr, sizeof(src_addr)) < 0) {
        printf("Error binding socket.\n");
        return -1;
    }

    // 设置 netlink socket 接收缓冲区大小
    int rcvbuf_size = 1024 * 1024;
    setsockopt(sock_fd, SOL_SOCKET, SO_RCVBUF, &rcvbuf_size, sizeof(rcvbuf_size));

    // 接收客户端请求
    while (1) {
        // 准备接收缓冲区
        char recv_buf[1024];
        memset(recv_buf, 0, sizeof(recv_buf));
        iov.iov_base = recv_buf;
        iov.iov_len = sizeof(recv_buf);

        // 接收请求
        memset(&dest_addr, 0, sizeof(dest_addr));
        nlh = (struct nlmsghdr*)recv_buf;
        nlh->nlmsg_len = NLMSG_SPACE(sizeof(int));
        nlh->nlmsg_pid = 0;
        nlh->nlmsg_flags = 0;
        iov.iov_len = nlh->nlmsg_len;
        struct msghdr msg;
        memset(&msg, 0, sizeof(msg));
        msg.msg_name = &dest_addr;
        msg.msg_namelen = sizeof(dest_addr);
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        recvmsg(sock_fd, &msg, 0);

        // 解析请求数据
        int* data = (int*)NLMSG_DATA(nlh);
        printf("Received request: %d\n", *data);

        // 构造响应数据
        char resp_buf[1024];
        memset(resp_buf, 0, sizeof(resp_buf));
        nlh = (struct nlmsghdr*)resp_buf;
        nlh->nlmsg_len = NLMSG_SPACE(sizeof(int));
        nlh->nlmsg_pid = getpid();
        nlh->nlmsg_flags = 0;
        data = (int*)NLMSG_DATA(nlh);
        *data = *data * 2;

        // 发送响应
        iov.iov_base = (void*)NLMSG_DATA(nlh);
        iov.iov_len = sizeof(int);
        iov.iov_len = nlh->nlmsg_len;
        memset(&dest_addr, 0, sizeof(dest_addr));
        dest_addr.nl_family = AF_NETLINK;
        dest_addr.nl_pid = 0;
        dest_addr.nl_groups = 0;
        msg.msg_name = &dest_addr;
        msg.msg_namelen = sizeof(dest_addr);
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        sendmsg(sock_fd, &msg, 0);
    }

    return 0;
}

客户端：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>

#define NETLINK_USER 31

struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;

int main() {
    // 创建 netlink socket
    sock_fd = socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sock_fd < 0) {
        printf("Error creating socket.\n");
        return -1;
    }

    // 准备请求数据
    int data = 10;
    char req_buf[1024];
    memset(req_buf, 0, sizeof(req_buf));
    nlh = (struct nlmsghdr*)req_buf;
    nlh->nlmsg_len = NLMSG_SPACE(sizeof(int));
    nlh->nlmsg_pid = getpid();
    nlh->nlmsg_flags = 0;
    struct iovec iov_req;
    iov_req.iov_base = &data;
    iov_req.iov_len = sizeof(data);
    iov.iov_len = nlh->nlmsg_len;
    NLMSG_DATA(nlh) = &data;

    // 发送请求
    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.nl_family = AF_NETLINK;
    dest_addr.nl_pid = 0;
    dest_addr.nl_groups = 0;
    struct msghdr msg;
    memset(&msg, 0, sizeof(msg));
    msg.msg_name = &dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = &iov_req;
    msg.msg_iovlen = 1;
    sendmsg(sock_fd, &msg, 0);

    // 接收响应
    char resp_buf[1024];
    memset(resp_buf, 0, sizeof(resp_buf));
    iov.iov_base = resp_buf;
    iov.iov_len = sizeof(resp_buf);
    nlh = (struct nlmsghdr*)resp_buf;
    memset(&dest_addr, 0, sizeof(dest_addr));
    memset(&msg, 0, sizeof(msg));
    msg.msg_name = &dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    recvmsg(sock_fd, &msg, 0);

    // 解析响应数据
    int* resp_data = (int*)NLMSG_DATA(nlh);
    printf("Received response: %d\n", *resp_data);

    return 0;
}