Why we use setsid() while daemonizing a process

daemon

IN order to create a service(daemon) we fork the parent and make it to exit while making the child to be the session leader by calling setsid().
Moreover why do we use setsid()?Our orphan process is taken care of by init(though in not all cases) process.

Why do we use stsid()?
Is there any relation between setsid() and handling SIGHUP signal.

Best Answer

We use setsid() because if we just kill the parent the child will be killed too, the setsid()

creates a new session if the calling process is not a process group leader. The calling process is the leader of the new session, the process group leader of the new process group, and has no controlling terminal.

The parent is the first process group leader, killing it - or killing the session/terminal - kills the group, which is why we switch the leader (and create a new session while we're at it).

Related Solutions

How to use daemonize to always keep the tomcat process running

If you don't want to use the tomcat package of your distribution, you can use systemd and define your own unit file such as:

# Systemd unit file for tomcat
[Unit]
Description=Apache Tomcat Web Application Container
After=syslog.target network.target

[Service]
Type=forking

Environment=JAVA_HOME=/usr/lib/jvm/jre
Environment=CATALINA_PID=/opt/tomcat/temp/tomcat.pid
Environment=CATALINA_HOME=/opt/tomcat
Environment=CATALINA_BASE=/opt/tomcat
Environment='CATALINA_OPTS=-Xms512M -Xmx1024M -server -XX:+UseParallelGC'
Environment='JAVA_OPTS=-Djava.awt.headless=true -Djava.security.egd=file:/dev/./urandom'

ExecStart=/opt/tomcat/bin/startup.sh
ExecStop=/bin/kill -15 $MAINPID

User=tomcat
Group=tomcat
UMask=0007
RestartSec=10
Restart=always

[Install]
WantedBy=multi-user.target

Read the tutorial How To Install Apache Tomcat 8 on CentOS 7 for full details.

Why do daemons store their PID (process id) in a file

For many daemons, only one instance of the daemon should be running on a system at any one time. In this use case, the daemon typically stores its PID in a well known directory (on Linux, currently /run, previously /var/run) to indicate that an instance of the daemon is running.

If you attempt to invoke a second instance of such a daemon, the newly invoked daemon checks for an existing entry (think of it as a lock file) under /run and exits if found.

If the daemon is restarted then the PID of the new instance is written to the file. The new instance gets its own PID, there's no way to launch a process with a given PID.

The PID file is also used to determine what process to kill to stop the daemon.

Best Answer

Related Solutions

How to use daemonize to always keep the tomcat process running

Why do daemons store their PID (process id) in a file

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