How do I get my program to act like a daemon?

Here are the steps to become a daemon:

1. fork() so the parent can exit, this returns control to the command line or shell invoking your program. This step is required so that the new process is guaranteed not to be a process group leader. The next step, setsid(), fails if you're a process group
leader.

2. setsid() to become a process group and session group leader. Since a controlling terminal is associated with a session, and this new session has not yet acquired a controlling terminal our process now has no controlling terminal, which is a Good Thing for
daemons.

3. fork() again so the parent, (the session group leader), can exit. This means that we, as a non-session group leader, can never regain a controlling terminal.

4. chdir("/") to ensure that our process doesn't keep any directory in use. Failure to do this could make it so that an administrator couldn't unmount a filesystem, because it was our current directory. [Equivalently, we could change to any directory containing
files important to the daemon's operation.]

5. umask(0) so that we have complete control over the permissions of anything we write. We don't know what umask we may have inherited. [This step is optional]

6. close() fds 0, 1, and 2. This releases the standard in, out, and error we inherited from our parent process. We have no way of knowing where these fds might have been redirected to. Note that many daemons use sysconf() to determine the limit _SC_OPEN_MAX.
_SC_OPEN_MAX tells you the maximun open files/process. Then in a loop, the daemon can close all possible file descriptors. You have to decide if you need to do this or not. If you think that there might be file-descriptors open you should close them, since
there's a limit on number of concurrent file descriptors.

7. Establish new open descriptors for stdin, stdout and stderr. Even if you don't plan to use them, it is still a good idea to have them open. The precise handling of these is a matter of taste; if you have a logfile, for example, you might wish to open it
as stdout or stderr, and open `/dev/null' as stdin; alternatively, you could open`/dev/console' as stderr and/or stdout, and `/dev/null' as stdin, or any other combination that makes sense for your particular daemon.

#include <unistd.h>

int daemon()
{
//1. the new process is guaranteed not to be a process group leader
pid_t pid = fork();
if(pid == -1)
{
//errno
return -1;
}
else if(pid == 0)
{
///child process
//2. become a process group and session group leader
pid = setsid();
if(pid == -1)
{
//errno
return -1;
}

//3. as a non-session group leader, can never regain a controlling terminal
pid = fork();
if(pid == -1)
{
//errno
return -1;
}
else if(pid == 0)
{
///child process
//4. our process doesn't keep any directory in use
//  [Equivalently, we could change to any directory containing files important to the daemon's operation.]
int iRet = chdir("/");
if(iRet == -1)
{
//errno
return -1;
}

//5. we have complete control over the permissions of anything we write
//  [This step is optional]
umask(0);

//6. close() fds 0, 1, and 2
//7. Establish new open descriptors for stdin, stdout and stderr
int iMaxfd = open("/dev/null", O_RDWR);
if(iMaxfd == -1)
{
//errno
return -1;
}

//redirect 0, 1, 2
dup2(iMaxfd, 0);
dup2(iMaxfd, 1);
dup2(iMaxfd, 2);
close(iMaxfd);

for(int i = 2; i < iMaxfd - 1; i ++)
{
close(i);
}

return 0;
}
else
{
//parent process
exit(0);
return 0;
}
}
else
{
//parent process
exit(0);
return 0;
}
}

来源于Unix Programming Frequently Asked Questions: 
http://web.archive.org/web/20120418113033/http://www.steve.org.uk/Reference/Unix/faq_toc.html