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Unix Daemon Server Programming
Unix processes works either in foreground or background. A process running in foreground interacts with the user in front of the terminal (makes I/O), whereas a background process runs by itself. The user can check its status but he doesn't (need to) know what it is doing. The term 'daemon' is used for processes that performs service in background. A server is a process that begins execution at startup (not neccessarily), runs forever, usually do not die or get restarted, operates in background, waits for requests to arrive and respond to them and frequently spawn other processes to handle these requests.
Readers are suppossed to know Unix fundamentals and C language. For further description on any topic use "man" command (I write useful keywords in brackets), it has always been very useful, trust me :)) Keep in mind that this document does not contain everything, it is just a guide.
1) Daemonizing (programming to operate in background) [fork]
First the fork() system call will be used to create a copy of our process(child), then let parent exit. Orphaned child will become a child of init process (this is the initial system process, in other words the parent of all processes). As a result our process will be completely detached from its parent and start operating in background.
i=fork();
if (i<0) exit(1); /* fork error */
if (i>0) exit(0); /* parent exits */
/* child (daemon) continues */
2) Process Independency [setsid]
A process receives signals from the terminal that it is connected to, and each process inherits its parent's controlling tty. A server should not receive signals from the process that started it, so it must detach itself from its controlling tty.
In Unix systems, processes operates within a process group, so that all processes within a group is treated as a single entity. Process group or session is also inherited. A server should operate independently from other processes.
setsid() /* obtain a new process group */
This call will place the server in a new process group and session and detach its controlling terminal. (setpgrp() is an alternative for this)
3) Inherited Descriptors and Standart I/0 Descriptors [gettablesize,fork,open,close,dup,stdio.h]
Open descriptors are inherited to child process, this may cause the use of resources unneccessarily. Unneccesarry descriptors should be closed before fork() system call (so that they are not inherited) or close all open descriptors as soon as the child process starts running.
for (i=getdtablesize();i>=0;--i) close(i); /* close all descriptors */
There are three standart I/O descriptors: standart input 'stdin' (0), standart output 'stdout' (1), standart error 'stderr' (2). A standard library routine may read or write to standart I/O and it may occur to a terminal or file. For safety, these descriptors should be opened and connectedthem to a harmless I/O device (such as /dev/null).
i=open("/dev/null",O_RDWR); /* open stdin */
dup(i); /* stdout */
dup(i); /* stderr */
As Unix assigns descriptors sequentially, fopen call will open stdin and dup calls will provide a copy for stdout and stderr.
