守护进程

　　守护进程（Daemon）是运行在后台的一种特殊进程，它独立与控制终端并且周期性的执行某种任务，是一个很有用的进程，linux大多数服务器就是用守护进程实现的，例如internet服务器inetd， web服务器httpd等；同时，守护进程完成许多系统任务；比如，作业规划进程crond，打印进程lpd

　　下面代码实现的是：

　　　　创建一个守护进程，该进程的作用是每个5秒往/var/bunfly.log文件中添加当前时间，格式规定为：2015-08-20 16：52：35

　　　　法一：利用 strftime（）函数，具体用法用man手册

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>

int main()
{
    //step 1: parent exit 
    int pid = fork();
    if(pid > 0) {
        exit(EXIT_SUCCESS);
    }

    //step 2: close tty
    int ret = setsid();

    //step 3: close file descriptor
    close(0);
    close(1);
    close(2);

    //step 4: change directory
    chdir("/");

    //step 5: set umask
    umask(0);
    
    //step 6: work    
    while(1) {
        time_t now;
        time(&now);

        char s[32];
         strftime(s, sizeof(s), "%Y-%m-%d %H:%M:%S 
", localtime(&now));

        int fd = open("/var/bunfly.log", O_RDWR | O_CREAT, 0664);
        if(fd < 0) {
            perror("open");
            exit(EXIT_SUCCESS);
        }

        lseek(fd, 0, SEEK_END);
        write(fd, s,  strlen(s));
        memset(s, 0, 32);    
        sleep(5);
    }    
}

　　法二：利用spriintf（）函数

#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>

int main()
{
    pid_t pid = 0;
    //step 1: exit parent
    pid = fork();
    if(pid < 0) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    if(pid > 0) {
        exit(EXIT_SUCCESS);
    }

    //step 2: exit tty
    setsid();
    
    //step 3: close file descriptor
    close(0);
    close(1);
    close(2);

    //step 4: chage directory
    chdir("/");

    //step 5: set umask    
    umask(0);
    
    //step 6: working
    while(1) {
        time_t now;
        time(&now);        
        char nowtime[100] = {0};
        struct tm *p = gmtime(&now);

        sprintf(nowtime, "%d-%0.2d-%0.2d %0.2d:%0.2d:%0.2d
",p->tm_year + 1900, p->tm_mon + 1, p->tm_mday, p->tm_hour + 8, p->tm_min, p->tm_sec);

    int fd = open("/var/dream", O_RDWR|O_CREAT, 0644);
    if(fd < 0) {
        perror("open");
        exit(EXIT_FAILURE);
    }

    lseek(fd, 0, SEEK_END);
    write(fd, nowtime, strlen(nowtime));
    memset(nowtime, 0, 100);
    sleep(5);
    }
    
    return 0;    
}