/**
 * @FileName    daemon_process.c
 * @Describe    A simple example for creating a single object of daemon process in linux.
 * @Author      vfhky 2016-03-14 17:52 https://typecodes.com/cseries/apuesingledaemonprocess.html
 * @Compile     gcc daemon_process.c -o daemon_process
 * @Reference   program list 13-1 in APUE.
 */
#include <stdio.h>
#include <unistd.h>
#include <stdarg.h>
#include <string.h>
#include <stdlib.h>
#include <signal.h>
#include <syslog.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/resource.h>
#include <errno.h>

#define PRINT_PID() printf( "Row[%d]: getpid=[%d].\n", __LINE__, getpid() )
//守护进程对应的用户必须对该文件具有访问权限
#define LOCK_FILE "/home/vfhky/daemon_process.pid"
#define MAXLINE 1024


/**
 * Print a message and return to caller.
 * Caller specifies "errnoflag".
 */
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
{
    char    buf[MAXLINE];

    vsnprintf(buf, MAXLINE, fmt, ap);
    if (errnoflag)
        snprintf(buf+strlen(buf), MAXLINE-strlen(buf), ": %s",
                 strerror(error));
    strcat(buf, "\n");
    fflush(stdout);     /* in case stdout and stderr are the same */
    fputs(buf, stderr);
    fflush(NULL);       /* flushes all stdio output streams */
}

/**
 * Fatal error unrelated to a system call.
 * Print a message and terminate.
 */
void err_quit(const char *fmt, ...)
{
    va_list     ap;

    va_start(ap, fmt);
    err_doit(0, 0, fmt, ap);
    va_end(ap);
    exit(1);
}


void daemonize(const char *cmd)
{
    int                 i, fd0, fd1, fd2;
    pid_t               pid;
    struct rlimit       rl;
    struct sigaction    sa;

    /**
     * 第一步：设置文件模式屏蔽字为0
     * Clear file creation mask.
     */
    umask(0);

    /**
     * 获取最大的文件描述符数目
     * Get maximum number of file descriptors.
     */
    if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
        err_quit("%s: can't get file limit", cmd);

    /**
     * 第二步：创建一个子进程，使父进程退出
     * Become a session leader to lose controlling TTY.
     */
    if ((pid = fork()) < 0)
        err_quit("%s: can't fork", cmd);
    else if (pid != 0) /* parent */
        exit(0);

    PRINT_PID();

    /**
     * 第三步：创建一个新的会话ID
     */
    setsid();

    /**
     * Ensure future opens won't allocate controlling TTYs.
     */
    sa.sa_handler = SIG_IGN;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    if ( sigaction(SIGHUP, &sa, NULL) < 0 )
        err_quit( "%s: can't ignore SIGHUP" );
    if( ( pid = fork() ) < 0 )                  //再次创建一个子进程，同样使父进程退出
        err_quit("%s: can't fork", cmd);
    else if( pid != 0 ) /* parent */
        exit(0);

    PRINT_PID();

    /**
     * 第四步：切换当前工作目录到根目录
     * Change the current working directory to the root so
     * we won't prevent file systems from being unmounted.
     */
    if (chdir("/") < 0)
        err_quit("%s: can't change directory to /");

    /**
     * 第五步：关闭所有打开的文件描述符
     * Close all open file descriptors.
     */
    if (rl.rlim_max == RLIM_INFINITY)
        rl.rlim_max = 1024;
    for (i = 0; i < rl.rlim_max; i++)
        close(i);

    /**
     * 第六步：使/dev/null具有文件描述符0，1，2.
     * Attach file descriptors 0, 1, and 2 to /dev/null.
     */
    fd0 = open("/dev/null", O_RDWR);
    fd1 = dup(0);
    fd2 = dup(0);

    /**
     * Initialize the log file.
     * @para-in:    cmd: the identifier in the log.
     */
    openlog( cmd, LOG_CONS, LOG_DAEMON );
    if( fd0 != 0 || fd1 != 1 || fd2 != 2 )
    {
        syslog( LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2 );
        exit(1);
    }

    /**
     * 第七步：通过文件锁避免重复运行多个守护进程
     */
    int lockfd = open( LOCK_FILE, O_RDWR );
    if( lockfd < 0 )
    {
        syslog( LOG_ERR, "Cannot lock file[%s], aborting[%s].\n", LOCK_FILE, strerror(errno) );
        //下面这一行无法打印到控制台，项目上应该打印到日志文件中
        printf( "Cannot lock file[%s], aborting[%s].\n", LOCK_FILE, strerror(errno) );
        exit(-1);
    }
    if( lockf(lockfd,F_TLOCK,0) < 0 )
    {
        syslog( LOG_ERR, "Daemon process is already running[%s].\n", strerror(errno) );
        //下面这一行无法打印到控制台，项目上应该打印到日志文件中
        printf( "Daemon process is already running[%s].\n", strerror(errno) );
        exit(-2);
    }
}

int main( int argc, char **argv )
{
    PRINT_PID();
    daemonize( "Daemon test." );
    //由于父进程退出，所有只有最后一个子进程执行下面的语句（休眠）
    printf( "This line will not be print for the daemon process has no terminate.\n" );
    while(1)
        sleep(120);
    return 0;
}