linux: how to become a real daemon?

When you want to write a daemon - a program that runs in background and serves during all the time that machine run(or almost all the time) you there are few things that's better to remember.

First of all the daemon should release controlling terminal. If daemon holds controlling terminal SIGHUP is sent when controlling terminal is lost. This happens if CLOCAL flag on the terminal is not set. That's not a good idea to handle these signals as a workaround to keep process running if controlling terminal is lost.
In linux(and maybe in some other OSes) lately CLOCAL is usually set but the developer shouldn't rely on this.

There are few ways to detach from the controlling terminal.
The easy way is to request TIOCNOTTY from the tty device if it supports it:

int tty = open("/dev/tty", O_RDWR);
if (tty == -1)
    return -1;

if (ioctl(tty, TIOCNOTTY, (char *)0) == -1)
    return -1;

close(tty);

This might not work for in some OSes. There is another opportunity.
Daemon has to create a new session using setsid. The caller of setsid becomes a process leader without controlling terminal. There is one thing with setsid - it creates a new session if caller is not a process group leader.
To do so first of all the daemon should call setsid from its child. The child will become a process leader and will get closer to be a real daemon and the parent will exit taking controlling tty with itself:

#include <unistd.h>

int main(int argc, char **argv)
{
 if (fork() == 0) {
  setsid();
  while (1) {
   /* Daemons like to sleep */
   sleep(1);
  }
 }

 return 0;
}

Next thing is to release current working directory. It's not good if daemon starts in some directory that is placed on filesystem that might be unmounted during the daemon's work. You won't be able to unmount the filesystem in that case. Simply chdir to '/' is a good choice.
Also it'd be nice to close stdin/stdout/stderr file descriptors since the daemon doesn't need them and some system resources could be saved. If the daemon is going to produce some output that's bad idea to use inherited stdout and stderr since you don't know where they might be redirected. It's better to provide new open descriptors(stdout and/or stderr might point to some log file):

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

int main(int argc, char **argv)
{
 if (fork() == 0) {
  int log;

  setsid();

  log = open("/tmp/test.log", O_CREAT|O_APPEND|O_RDWR, 0755);
  close(0);
  close(2);
  dup2(log, 1);

  printf("Daemon is started\n");
  while (1) {
   /* Daemons like to sleep */
   sleep(1);
   printf("Daemon is sleeping\n");
   fflush(NULL);
  }
 }

 return 0;
}

The output will go to "/tmp/test.log":

*./test 
*tail /tmp/test.log -f
Daemon is started
Daemon is sleeping
Daemon is sleeping
Daemon is sleeping
Daemon is sleeping
Daemon is sleeping

emacs: proper kill-whole-line

I'm using emacs not for a long time but I really liked it. It took not much time to get used to it. Though emacs doesn't have some function you are free to write them in elisp easily!
I used to vim's 'dd' command which kills current line and later you are able to yank it.
emacs has 'kill-line' function to kill line from the current point except newline character or 'kill-whole-line' which kills following newline also. This doesn't fit me entirely. I would like to kill the whole line from the beginning including newline character. Simply you can wrap 'kill-whole-line' with adding an instruction to jump into the beginning of the line and finally kill it.
Looks like everything is done except the situation when you want to yank it back. When you'll try to yank the killed line you'll see that besides wanted line you have an extra newline. It was annoying for me to yank and kill the appendix. Finally I wrote my variant of 'kill-whole-line' - kill-total-line:

(defun kill-total-line ()
  (interactive)
  (let ((kill-whole-line t)) 
        (beginning-of-line)
        (kill-line)
        (setq top (car kill-ring))
        (setq last (substring top -1))
        (if (string-equal last "\n")
                (let ()
                  (setq stripped (substring top 0 -1))
                  (setq kill-ring (cons stripped (cdr kill-ring)))
                  )
          )
        )
  )

This 'kill-total-line' function kills the line from the beginning to the end and cuts newline symbol of the killed line in the kill-ring if any. Now I'm happy to kill lines and yank them back!

c: offset of the member in struct

Sometimes, very rarely, you know the pointer to the member of the structure and the type of the structure it belongs to. And in this rare situation you want to get pointer to the entire structure.

To get the pointer of the original structure you should know the offset of this member in the structure. The hint here is that you can use some base address(0 fits the best here), cast it into the pointer of the structure type, dereference with given member name and voilà - you have the offset. Look at the next code snippet:

struct S {
 int m0;
 char m1;
 short m2;
 long m3;
};
unsigned int m3_offset = (unsigned int)(&((struct S *)0)->m3);

Variable m3_offset holds the offset of the member m3 in structure S. In this case the offset of m3 is equal 8. So the address of the variable of type S is (address of m3) - m3_offset.

struct S *sp = (struct S *)((char *)&s.m3 - m3_offset);

You should cast pointer to member into char * to correctly use pointer arithmetics.
The whole code that shows this trick is below:

#include <stdio.h>

struct S {
 int m0;
 char m1;
 short m2;
 long m3;
};

unsigned int m3_offset = (unsigned int)(&((struct S *)0)->m3);

int main(int argc, char **argv)
{
 struct S s = {1, 2, 3, 4};
 struct S *sp = (struct S *)((char *)&s.m3 - m3_offset);

 printf("m1: %d\n", sp->m1);

 return 0;
}

The output should be

*./test 
m1: 2

This technique could be used to easily construct nested structures:

struct S0 {
 int s0m0;
};

struct S1 {
 struct S0 s1m0;
 int s1m1;
};

/**
 * pointer to member in S structure
 * type of S
 * name of member
 */
#define S(s, st, m)            \
 ({               \
  unsigned int offset = (unsigned int)(&((st *)0)->m); \
  (st *)((char *)s-offset);        \
 })

int main(int argc, char **argv)
{
 struct S1 s = {{1}, 2};

 printf("%d",S(&s.s1m0, struct S1, s1m0)->s1m1);

 return 0;
}

It's easy to build such data types as trees, queues and so on separately defining payload structures(data of the node) and helper structures(pointers to other nodes, etc.)
Doubly ended queue in linux kernel uses this approach and looking into the code you can definitely say - the code looks much clear if this technique is used.