Daemon templates for Linux

Description

A daemon is a type of program on Unix-like operating systems that runs unobtrusively in the background, rather than under the direct control of a user, waiting to be activated by the occurance of a specific event or condition.

The standard procedure for daemonize a process has a few steps:

• Fork, allowing the parent process to terminate.
• Set the umask to zero (reset umask).
• Open any logs for writing (optional).
• Start a new session for the daemon by calling setsid().
• Change the current working directory to a safe location (default: "/").
• Redirect stdin, stdout and stderr to /dev/null (not redirect for DEBUG mode).

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Daemon templates for Linux

I wrote various versions of demons for C/C++. From the simplest (which uses the standard daemon() function) to the daemon, with support for command-line options and CMD_PIPE for controlling the daemon from the command line.

Feature/template #	1	2	3	4	5
no_chdir	+	+	+	+	+
no_close	+	+	+	+	+
LOG file	+	+	+	+	+
PID file	-	+	+	+	+
init_signals	-	-	+	+	+
support cmd	-	-	-	+	+
CMD Pipe	-	-	-	-	+

Note:

template_1 uses daemon() function from unistd.h see man-pages for daemon() function: man 3 daemon()

Example implementing function daemon() in uClibc: daemon.c

CMD PIPE - Management daemon via a control pipe using the function getopt_long

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Usage

To start working, perform the following steps:

• Choose the appropriate template.
• Correct the DAEMON_XXX variables in the Makefile.
• Choose your compiler (or toolchain) in the Makefile (see variable $GCC).
• Add your source files to the list (see SOURCES variable in Makefile)
• Add your code to main loop.
• Add your initialization code in the function init (optional)
• Achieve a clean build for the project (the complete absence of errors and warnings when you build the project).
• Perform testing demon.
• If you are using embedded Linux you can use the template start script

Testing

Check that the daemon has no controlling terminal and is not a session leader

The controlling terminal and session ID of a process can be inspected using the ps command:

ps -o ppid,pid,sid,tty,cmd -C name_daemon

This should output a table of the form:

PPID   PID    SID    TT       CMD
2173   29964  29961  ?        ./name_daemon

Note:

Why has my process PPID is not 1? How can I execute a new process from GNOME Terminal so that the child process's parent PID becomes 1 and not the PID of the ubuntu session init process?

Answers:

This is intentionally hard. Service managers want to keep track of orphaned child processes. They want not to lose them to process #1. Stop trying to do that. If you are asking solely because you think that your process ought to have a parent process ID of 1, then wean yourself off this idea. see more

There is no way to discern a regular process from a daemon process. A daemon can be started from anywhere, not just init. It is not necessary to double fork and detach from the terminal to be considered a daemon. On GNOME systems, gnome-settings-daemon keeps its parent, logs to the parent's terminal, and does not have PPID == 1 as its parent PID, yet it is still considered a daemon. A daemon is simply a continuously running process. They are, therefore, impossible to identify with one command. see more

Check the current working directory

Obtain the working directory pathname from /proc:

ls -l /proc/PID_DAEMON/cwd

The working directory of the process is the target of the softlink:

lrwxrwxrwx 1 root root 0 Feb  6 15:57 cwd -> /

Check file descriptors

The file descriptors of a running process can be inspected by looking in /proc:

ls -l /proc/PID_DAEMON/fd

Each open descriptor is presented as a softlink. If a descriptor is associated with a filesystem object then the target of the softlink is the pathname of the object. For a daemon the output would typically be similar to:

lrwx------ 1 root root 64 2011-02-08 06:40 0 -> /dev/null
lrwx------ 1 root root 64 2011-02-08 06:40 1 -> /dev/null
lrwx------ 1 root root 64 2011-02-08 06:40 2 -> /dev/null

License

BSD-3-Clause.

Copyright

Copyright (C) 2015 Koynov Stas - skojnov@yandex.ru