LINUX Daemon程序設計

引言

Daemon程序，又稱爲守護進程，一般在系統後臺長時間運行，因爲沒有控制終端而沒法與前臺交互。Daemon程序通常做爲系統服務使用，Linux系統中運行着不少這樣的守護進程，如iptables，nfs，ypbind，dhcpd等。

Daemon設計原則

Daemon程序設計主要原則包括：

（1）       程序運行後調用fork，並讓父進程退出。子進程得到一個新的進程ID，但繼承了父進程的進程組ID。

（2）       調用setsid建立一個新的session，使本身成爲新session和新進程組的leader，並使進程沒有控制終端(tty)。

（3）       設置文件建立mask爲0，避免建立文件時權限的影響。

（4）       關閉不須要的打開文件描述符。由於Daemon程序在後臺執行，不須要於終端交互，一般就關閉STDIN、STDOUT和STDERR。其它根據實際狀況處理。

（5）       Daemon沒法輸出信息，能夠使用SYSLOG或本身的日誌系統進行日誌處理。（可選）

（6）       編寫管理Daemon的SHELL腳本，使用service對Daemon進行管理和監控。（可選）

Daemon程序框架

int init_daemon(void)

{

  pid_t pid;

  int i;

 

  /* parent exits , child continues */

  if((pid = fork()) < 0)

    return -1;

  else if(pid != 0)

    exit(0);

 

  setsid(); /* become session leader */

 

  for(i=0;i< NOFILE ;++i) /* close STDOUT, STDIN, STDERR, */

    close(i);

 

  umask(0); /* clear file mode creation mask */

  return 0;

}

 

void sig_term(int signo)

{

  if(signo == SIGTERM)  /* catched signal sent by kill(1) command */

  {

     wsio_logit("", "wsiod stopped\n");

     exit(0);

}

}

 

/* main program of daemon */

int main(void)

{

if(init_daemon() == -1){

printf("can't fork self\n");

exit(0);

  }

  wsio_logit("", "wsiod started\n");

  signal(SIGTERM, sig_term); /* arrange to catch the signal */

 

  while (1) {

    // Do what you want here

    … …

  }

  exit(0);

}

Daemon日誌

這裏使用本身的日誌系統，固然也能夠使用SYSLOG。

#define LOGBUFSZ 256     /*log buffer size*/

#define LOGFILE  "/var/log/wsiod.log"  /*log filename*/

int wsio_logit(char * func, char *msg, ...)

{

        va_list args;

        char prtbuf[LOGBUFSZ];

        int save_errno;

        struct tm *tm;

        time_t current_time;

        int fd_log;

 

        save_errno = errno;

        va_start (args, msg);

        (void) time (&current_time);            /* Get current time */

        tm = localtime (&current_time);

        sprintf (prtbuf, "%02d/%02d %02d:%02d:%02d %s ", tm->tm_mon+1,

                    tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec, func);

        vsprintf (prtbuf+strlen(prtbuf), msg, args);

        va_end (args);

        fd_log = open (LOGFILE, O_WRONLY | O_CREAT | O_APPEND, 0664);

        write (fd_log, prtbuf, strlen(prtbuf));

        close (fd_log);

        errno = save_errno;

        return 0;

}

Daemon管理

Daemon程序能夠使用service工具進行管理，包括啓動、中止、查看狀態等，但前題是須要編寫一個以下的簡單SHELL腳本。

# /etc/rc.d/init.d/wsiod

#!/bin/sh

#

# wsiod         This shell script takes care of starting and stopping wsiod.

#

# chkconfig: 35 65 35

# description: wsiod is web servce I/O server, which is used to access files on remote hosts.

 

# Source function library.

. /etc/rc.d/init.d/functions

 

# Source networking configuration.

. /etc/sysconfig/network

 

# Check that networking is up.

[ ${NETWORKING} = "no" ] && exit 0

 

RETVAL=0

prog="wsiod"

WSIOARGS="-h $HOSTNAME -p 80 -t STANDALONE -k -c -d"

start() {

        # Start daemons.

        echo -n $"Starting $prog: "

        daemon /usr/local/bin/wsiod ${WSIOARGS}

        RETVAL=$?

        echo

        [ $RETVAL -eq 0 ] && touch /var/lock/subsys/wsiod

        return $RETVAL

}

stop() {

        # Stop daemons.

        echo -n $"Shutting down $prog: "

        killproc wsiod

        RETVAL=$?

        echo

        [ $RETVAL -eq 0 ] && rm -f /var/lock/subsys/wsiod

        return $RETVAL

}

 

# See how we were called.

case "$1" in

  start)

        start

        ;;

  stop)

        stop

        ;;

  restart|reload)

        stop

        start

        RETVAL=$?

        ;;

  status)

        status wsiod

        RETVAL=$?

        ;;

  *)

        echo $"Usage: $0 {start|stop|restart|status}"

        exit 1

esac

 

exit $RETVAL

 

到這兒爲止，一個完整的Linux Daemon程序就完成了。

//////////////////////////////////

#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <syslog.h>
#include "become_daemon.h"
#include "log.h" 


int              /* Returns 0 on success, -1 on error */
becomeDaemon(int flags)
{
    int maxfd, fd;
    pid_t pid = fork();
    log_sys("[cksettling>> becomeDaemon begin...]");
    if(pid<0)/* while fail  */
    {
    	log_sys("[cksettling>> fork fail");
    	return(-1);
    }
	else if(pid!=0)  /* while parent  */
	{
		log_sys("[cksettling>> while parent]");
		_exit(0);

	}
    /* Child falls through... */


    if (setsid() == -1)                 /* Become leader of new session */
    {
    	log_sys("[cksettling>> setsid() fail]");
    	return -1;
    }

    /* Ensure we are not session leader */
    pid=fork();
	if(pid<0)/* while fail  */
	{
		log_sys("[cksettling>> fork(2) fail]");
		return(-1);
	}
	else if(pid!=0)  /* while parent  */
	{
		log_sys("[cksettling>> while parent2]");
		_exit(0);
	}
	/* Child falls through... */

    if (!(flags & BD_NO_UMASK0))
        umask(0);                       /* Clear file mode creation mask */

    if (!(flags & BD_NO_CHDIR))
        chdir("/");                     /* Change to root directory */

    if (!(flags & BD_NO_CLOSE_FILES)) { /* Close all open files */
        maxfd = sysconf(_SC_OPEN_MAX);
        if (maxfd == -1)                /* Limit is indeterminate... */
            maxfd = BD_MAX_CLOSE;       /* so take a guess */

        for (fd = 0; fd < maxfd; fd++)
            close(fd);
    }

    if (!(flags & BD_NO_REOPEN_STD_FDS)) {
        close(STDIN_FILENO);            /* Reopen standard fd's to /dev/null */

        fd = open("/dev/null", O_RDWR);

        if (fd != STDIN_FILENO)         /* 'fd' should be 0 */
        {
        	log_sys("[cksettling>> fd != STDIN_FILENO]");
        	return -1;
        }
        if (dup2(STDIN_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
        {
        	log_sys("[cksettling>> dup2 >>!= STDIN_FILENO]");
            return -1;
        }
        if (dup2(STDIN_FILENO, STDERR_FILENO) != STDERR_FILENO)
        {
        	log_sys("[cksettling>> dup2 2 >>!= STDIN_FILENO]");
        	return -1;
        }
    }
    log_sys("[cksettling>> becomeDaemon ok]");
    return 0;
}