Daemons and Processes

Hey what is daemon and processes…


Daemons and Processes:


Q1. What are applications daemons?

Application Daemon are those which can be killed & will have no effect to the system

$ kill -15 <appd-pid>

For eg. – firefox, openoffice, X server, etc…


Q2. What are system daemons?

System Daemons are those which can be killed & will affect the system.


$ kill -9 <sysd-pid>

For eg – init, kerneld, ksoftirqd, khelper, kthread, kblockd


Q3. What is a Process?

Processes carry out tasks within the operating system. A process has many components and properties.

->exec thread



->memory context


->file descriptors

->security credentials


Q4. What are signals in linux?

Signals are a software mechanism that are similar to a message of some

sort. They can be trapped and handled or ignored


Signals operate through two different system calls

1) The kill system call

2) The signal system call


1) The kill System Call

The kill system call sends a signal to a process kill is generally used to

terminate a process. It requires the PID of the process to be terminated

and the signal number to send as arguments.


2) The Signal System Call

The signal system call is much more diverse. When a signal occurs, the

kernel checks to see if the user had executed a signal system call and was

therefor expecting a signal. If the call was to ignore the signal, the

kernel returns

Otherwise, it checks to see if it was a trap or kill signal If not, it

processes the signal If it was a trap or kill signal, the kernel checks to

see if core should be dumped and then calls the exit routine to terminate

the user process.


Common Unix Signals

$kill -l

SIGHUP    Hang-up

SIGINT     Interrupt

SIGQIT    Quit

SIGINS     Illegal Instruction

SIGTRAP Trace Trap


SIGSYS   Bad argument to system call

SIGPIPE  Write on pipe with no one to read it

SIGTERM Software termination signal from kill

SIGSTOP  Stop signal




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