写一个C语言用select实现udp自定义IP，端口的服务器端例子

可以参考以下代码实现：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <sys/select.h>

#define BUF_SIZE 1024
#define IP_ADDR "127.0.0.1"
#define PORT 8888

int main()
{
    struct sockaddr_in serv_addr, cli_addr;
    int serv_sock, cli_sock;
    socklen_t addr_len;
    char buf[BUF_SIZE];
    int i, n;

    serv_sock = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr(IP_ADDR);
    serv_addr.sin_port = htons(PORT);

    bind(serv_sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr));

    printf("UDP server is running on %s:%d\n", IP_ADDR, PORT);

    fd_set read_fds, all_fds;
    int max_fd = serv_sock;

    FD_ZERO(&all_fds);
    FD_SET(serv_sock, &all_fds);

    while (1) {
        read_fds = all_fds;

        if (select(max_fd + 1, &read_fds, NULL, NULL, NULL) < 0) {
            perror("select");
            exit(1);
        }

        for (i = 0; i <= max_fd; ++i) {
            if (FD_ISSET(i, &read_fds)) {
                if (i == serv_sock) {
                    addr_len = sizeof(cli_addr);
                    n = recvfrom(serv_sock, buf, BUF_SIZE, 0,
                                 (struct sockaddr *)&cli_addr, &addr_len);

                    printf("Received packet from %s:%d\n",
                           inet_ntoa(cli_addr.sin_addr), ntohs(cli_addr.sin_port));

                    if (n < 0) {
                        perror("recvfrom");
                        exit(1);
                    }

                    FD_SET(cli_sock, &all_fds);
                    if (cli_sock > max_fd) {
                        max_fd = cli_sock;
                    }
                } else {
                    n = sendto(i, buf, n, 0, (struct sockaddr *)&cli_addr, addr_len);

                    if (n < 0) {
                        perror("sendto");
                        exit(1);
                    }
                }
            }
        }
    }

    return 0;
}

当然，以上代码只是一个简单的例子，实际应用中可能还需要增加更多的错误处理和逻辑。