UNIX Process Management Questions II

Q. What is a Daemon?

A daemon is a process that detaches itself from the terminal and runs, disconnected, in the background, waiting for requests and responding to them. It can also be defined as the background process that does not belong to a terminal session. Many system functions are commonly performed by daemons, including the sendmail daemon, which handles mail, and the NNTP daemon, which handles USENET news. Many other daemons may exist.
Some of the most common daemons are:

1.    init: Takes over the basic running of the system when the kernel has finished the boot process.

2.    inetd: Responsible for starting network services that do not have their own stand-alone daemons. For example, inetd usually takes care of incoming rlogin, telnet, and ftp connections.

3.    cron: Responsible for running repetitive tasks on a regular schedule.

Q. What is "ps" command for?

The "ps" command prints the process status for some or all of the running processes. The information given are the process identification number (PID),the amount of time that the process has taken to execute so far etc.

Q. How would you kill a process?

The "kill" command takes the PID as one argument; this identifies which process to terminate. The PID of a process can be got using "ps" command.

Q. What is an advantage of executing a process in background?

The most common reason to put a process in the background is to allow you to do something else interactively without waiting for the process to complete. At the end of the command you add the special background symbol, &. This symbol tells your shell to execute the given command in the background.
Example: cp *.* ../backup&   (cp is for copy)

Q. How do you execute one program from within another?

The system calls used for low-level process creation are "execlp()" and "execvp()". The "execlp()" call overlays the existing program with the new one, runs that and exits. The original program gets back control only when an error occurs.

    execlp(path,file_name,arguments..); //last argument must be NULL

A variant of "execlp()" called "execvp()" is used when the number of arguments is not known in advance.

    execvp(path,argument_array); //argument array should be terminated by NULL

Q. What is IPC? What are the various schemes available?

The term IPC (Inter-Process Communication) describes various ways by which different process running on some operating system communicate between each other. Various schemes available are as follows:

1.    Pipes: One-way communication scheme through which different process can communicate. The problem is that the two processes should have a common ancestor (parent-child relationship). However this problem was fixed with the introduction of named-pipes (FIFO).

2.    Message Queues : Message queues can be used between related and unrelated processes running on a machine.

3.    Shared Memory: This is the fastest of all IPC schemes. The memory to be shared is mapped into the address space of the processes (that are sharing). The speed achieved is attributed to the fact that there is no kernel involvement. But this scheme needs synchronization.

Various forms of synchronisation are mutexes, condition-variables, read-write locks, record-locks, and semaphores.