1. CGS 3763 Operating Systems Concepts Spring 2013
Dan C. Marinescu
Office: HEC 304
Office hours: M-Wd 11: :30 AM

2. 11/28/2018

3. Lecture 12 – Monday, February 4, 2013
Last time:
Answers to student questions
Today:
Creation and termination of processes
Inter-process communication
Message passing
Sockets
Next time
Client-server systems
Remote procedure call
Reading assignments
Chapters 3 and 4 of the textbook
Chapters 3 and 4 textbook slides
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4. Process Creation
A parent process create children processes, which, in turn create other processes, forming a tree of processes
A process is identified by a process identifier (pid)
Possible resource sharing strategies between parent and children
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Execution
Parent and children execute concurrently
Parent waits until children terminate

5. Process Creation (Cont’d)
Address space
Child duplicate of parent
Child has a program loaded into it
UNIX examples
fork system call creates new process
exec system call used after a fork to load in the address space of the child process a new program

6. Process Creation – the parent waits until the child process terminates

7. Pid – the process ID In UNIX-like operating systems
The current process ID is provided by a getpid() system call.
The process ID of a parent process can be obtained with the getppid() system call.
In Windows one can get
the ID of the current process using GetCurrentProcessId() 
the ID of other processes using  GetProcessId()
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8. C Program Forking Separate Process
int main() {
pid_t pid;
/* fork another process */
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
exit(-1); }
else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL);
else { /* parent process */
/* parent will wait for the child to complete */
wait (NULL);
printf ("Child Complete");
exit(0);

9. System processes
swapper or sched has PID=0 and is responsible for paging, and is a kernel process (daemon)
init – in Unix-based computer systems, init (short for initialization) is a daemon, the direct/indirect ancestor of all other processes. Init is the first process started during booting, and is typically assigned PID =1.
inetd (internet service daemon) is a daemon on Unix systems that manages Internet services. It  listens on designated ports used by Internet services such as FTP, POP3, and telnet. When a TCP packet or an UDP packet arrives with a particular destination port number, inetd launches the appropriate server program to handle the connection e.g., telnetd.
Telnetd – Telnet daemon process handling communication using the Telnet protocol
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10. OS shells
Operating system shell  a software that presents a user interface to various OS functions (e.g., process and file management) and services.
C shell (csh) is a Unix shell   a command processor typically run in a text window, allowing the user to type commands. The C shell can also read commands from a file, called a script.
On Mac OS X and Red Hat Linux, csh is actually tcsh, an improved version of csh.
On Debian, Ubuntu, and their derivatives, there are two different packages: csh and tcsh.
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11. A tree of processes on a typical Solaris

12. Process Termination
Process executes last statement and asks the operating system to delete it (exit)
Output data from child to parent (via wait)
Process’ resources are deallocated by operating system
Parent may terminate execution of children processes (abort)
Child has exceeded allocated resources
Task assigned to child is no longer required
If parent is exiting
Some operating system do not allow child to continue if its parent terminates
All children terminated - cascading termination

13. Interprocess communication
Processes within a system may be
independent
cooperating
A process can affect or be affected by other processes, including sharing data
Reasons for cooperating processes:
Information sharing
Computation speedup
Modularity
Convenience
IPC - interprocess communication. Two models:
Shared memory
Message passing

14. Communications models
Message passing Shared memory

15. Asynchronicity
Asynchronous events occur independently of the main program flow.
Asynchronous actions  executed in a non-blocking scheme, allowing the main program flow to continue processing.
Asynchronous I/O   input/output processing that permits a processing to continue before the I/O operations has finished.
Asynchronous processes do not share a clock
Asynchronous communication  the sender does not wait to receive an acknowledgment from the receiver.
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