/*********************************************************************************
 *      Copyright:  (C) 2025 LingYun IoT System Studio
 *                  All rights reserved.
 *
 *       Filename:  mydaemon.c
 *    Description:  This file is daemon check example program
 *
 *        Version:  1.0.0(11/07/2025)
 *         Author:  Guo Wenxue <guowenxue@gmail.com>
 *      ChangeLog:  1, Release initial version on "11/07/2025 10:38:39 AM"
 *
 ********************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>


#define PID_FILE "/tmp/mydaemon.pid"

/* 检查某个 pid 是否存在 */
int is_process_running(pid_t pid)
{
    if (pid <= 0)
        return 0;

    if (kill(pid, 0) == 0)
    {
        return 1; /* 进程存在且可访问 */
    }
    else
    {
        if (errno == ESRCH)
            return 0;   /* 进程不存在 */
        else if (errno == EPERM)
            return 1;   /* 存在但无权限 */
        else
            return 0;   /* 其他错误 */
    }
}

/* 将当前进程 PID 写入文件 */
int write_pid_file(const char *path)
{
    FILE *fp;

    fp = fopen(path, "w");
    if (fp == NULL)
    {
        perror("fopen");
        return -1;
    }

    pid_t pid = getpid();
    fprintf(fp, "%d\n", pid);
    fclose(fp);
    return 0;
}

/* 从文件中读取 PID */
pid_t read_pid_file(const char *path)
{
    FILE *fp;
    pid_t pid = 0;

    fp = fopen(path, "r");
    if (fp == NULL)
        return 0;

    fscanf(fp, "%d", &pid);
    fclose(fp);
    return pid;
}

int main(void)
{
    pid_t old_pid;

    old_pid = read_pid_file(PID_FILE);
    if (is_process_running(old_pid))
    {
        printf("Program is already running with PID %d.\n", old_pid);
        return 0;
    }

    /* 调用 daemon() 放入后台 */
    /*
     * daemon(int nochdir, int noclose)
     * nochdir = 0 表示切换到根目录 /
     * noclose = 0 表示关闭 stdin/stdout/stderr
     */
    if (daemon(0, 0) < 0)
    {
        perror("daemon");
        exit(1);
    }

    /* 写入新的 PID 文件 */
    if (write_pid_file(PID_FILE) < 0)
    {
        fprintf(stderr, "Failed to write PID file.\n");
        exit(1);
    }

    /* 模拟后台工作 */
    while (1)
    {
        sleep(5); /* 这里可以换成自己的业务逻辑 */
    }

    return 0;
}