linux select 可写

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

int main() {
    int sockfd;
    struct sockaddr_in servaddr, cliaddr;
    socklen_t len;
    char buffer[1024];
    fd_set writefds;

    // 创建套接字
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    memset(&servaddr, 0, sizeof(servaddr));
    memset(&cliaddr, 0, sizeof(cliaddr));

    // 配置服务器地址
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(8080);

    // 绑定套接字
    if (bind(sockfd, (const struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    while (1) {
        FD_ZERO(&writefds);
        FD_SET(sockfd, &writefds);

        // 使用 select 监视套接字的可写状态
        int activity = select(sockfd + 1, NULL, &writefds, NULL, NULL);
        if ((activity < 0) && (errno != EINTR)) {
            perror("select error");
        }

        if (FD_ISSET(sockfd, &writefds)) {
            len = sizeof(cliaddr);
            int n = recvfrom(sockfd, (char *)buffer, 1024, MSG_WAITALL, (struct sockaddr *)&cliaddr, &len);
            buffer[n] = '\0';
            printf("Received message: %s\n", buffer);

            // 发送响应
            sendto(sockfd, (const char *)buffer, strlen(buffer), MSG_CONFIRM, (const struct sockaddr *)&cliaddr, len);
        }
    }

    close(sockfd);
    return 0;
}