Message-ID: <4D162DB6.1000808@aknet.ru>
Date: Sat, 25 Dec 2010 20:45:26 +0300
From: Stas Sergeev <stsp@aknet.ru>
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To: linux-kernel@vger.kernel.org
Subject: [RFC][patch] PR_DETACH: detach from parent
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Hi.

I was trying to use daemon() library call to daemonize the process, and 
found out that it is not suitable
for the process that have threads: all the threads are simply being killed.
The reason is that the daemon() function does fork(), then exit() in 
parent and setsid() in child, which is
a bit ugly, and doesn't work with threads (fork() doesn't fork threads).
I've found no solution to that, and just hacked up one instead.
The attached patch adds the PR_DETACH prctl command, which detaches the 
process from its parent.
With that, the daemon() function can be implemented without the 
fork/exit/setsid hacks, and here is an
example of it:

---
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/prctl.h>

#ifndef PR_DETACH
#define PR_DETACH 35
#endif

int daemon2(int nochdir, int noclose)
{
      int err, fd;

      err = prctl(PR_DETACH);
      if (err == -1) {
          if (errno == EINVAL)
              errno = ENOTSUP;
          return -1;
      }

      fd = open("/dev/tty", O_RDWR);
      if (fd != -1) {
          ioctl(fd, TIOCNOTTY, NULL);
          close(fd);
      }

      if (!nochdir && chdir("/") == -1)
          return -1;

      if (!noclose) {
          fd = open("/dev/null", O_RDWR, 0);
          if (fd == -1)
              return -1;
          if (dup2(fd, STDIN_FILENO) == -1)
              return -1;
          if (dup2(fd, STDOUT_FILENO) == -1)
              return -1;
          if (dup2(fd, STDERR_FILENO) == -1)
              return -1;
          if (fd > 2)
              close(fd);
      }
      return 0;
}
---

With this implementation, all threads survive the daemonization.
Thoughts?


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