linux守护进程的几个关键地方

创建一个守护进程，有几个关键的步骤，也有几个地方需要注意，
几个关键的步骤有：
1：清除文件创建权限
2：调用fork，然后使父进程退出
3：调用setsid以创建一个新的会话，有三个目的使调用进程 a:成为新会话的首进程，b:成为新进程的组长进程，c:没有控制终端
4：切换工作目录
5：关闭不需要的文件描述符
 
需要注意的地方
1：因为守护进程没有控制终端，所以不能与标准输入输出出错进行交互，不能使用printf,通常用syslog来解决守护进程的打印信息
 
参考代码如下：

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#include <signal.h>

#include <stdlib.h>

#include <syslog.h>

#include <fcntl.h>

#include <stdio.h>

#include <sys/resource.h>

void daemonize(const char
*cmd)

{

    int                    i, fd0, fd1, fd2;

    pid_t                pid;

    struct rlimit        rl;

    struct sigaction    sa;

    /*

     * Clear file creation mask.

     */

    umask(0);

    /*

     * Get maximum number of file descriptors.

     */

    if (getrlimit(RLIMIT_NOFILE,
&rl)
< 0)

        printf("%s: can't get file limit", cmd);

    /*

     * Become a session leader
to lose controlling TTY.

     */

    if ((pid
= fork())
< 0)

        printf("%s: can't fork", cmd);

    else if
(pid != 0)
/* parent
*/

        exit(0);

    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)

        printf("can't ignore SIGHUP");

    if ((pid
= fork())
< 0)

        printf("%s: can't fork", cmd);

    else if
(pid != 0)
/* parent
*/

        exit(0);

    /*

     * Change the current working directory
to the root so

     * we won't prevent file systems from being unmounted.

     */

    if (chdir("/")
< 0)

        printf("can't change directory to /");

    /*

     * 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);

    /*

     * 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.

     */

    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);

    }

    syslog(LOG_DEBUG,
"daem ok ");

    

}

int main()

{

     daemonize("test");

     while(1);//守护进程所要干的事情

    

}