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：

#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

}