Using pidfd instead of pid in WClient::pidFD

[zccrs]

This is an example:

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/socket.h>  // for SO_PEERPIDFD
#include <signal.h>

int main() {
    int sv[2]; // Socket pair: sv[0] and sv[1]
    if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == -1) {
        perror("socketpair");
        exit(EXIT_FAILURE);
    }

    pid_t child = fork();
    if (child == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if (child == 0) {
        // 子进程
        close(sv[0]); // 关闭父端
        pause();      // 保持进程存在
        exit(0);
    } else {
        // 父进程
        close(sv[1]); // 关闭子端

        int pidfd;
        socklen_t len = sizeof(pidfd);
        if (getsockopt(sv[0], SOL_SOCKET, SO_PEERPIDFD, &pidfd, &len) == -1) {
            perror("getsockopt SO_PEERPIDFD");
            exit(EXIT_FAILURE);
        }

        printf("Received pidfd = %d\n", pidfd);

        // 验证 pidfd 是否可用，例如：使用 pidfd_send_signal 测试
        if (pidfd_send_signal(pidfd, 0, NULL, 0) == 0) {
            printf("Process is alive (via pidfd_send_signal).\n");
        } else {
            perror("pidfd_send_signal");
        }

        close(pidfd);
        close(sv[0]);
        wait(NULL);
    }

    return 0;
}