<< Chapter < Page Chapter >> Page >

Multiprocessing software

In this section, we explore how programs access multiprocessing features. These examples are written in C using the POSIX 1003.1 application programming interface. This example runs on most UNIX systems and on other POSIX-compliant systems including OpenNT, Open- VMS, and many others. In this example, the program creates a new process using the fork( ) function. The new process (child) prints some messages and then changes its identity using exec( ) by loading a new program. The original process (parent) prints some messages and then waits for the child process to complete:

int globvar; /* A global variable */ main () {int pid,status,retval; int stackvar; /* A stack variable */globvar = 1; stackvar = 1;printf("Main - calling fork globvar=%d stackvar=%d\n",globvar,stackvar); pid = fork();printf("Main - fork returned pid=%d\n",pid); if ( pid == 0 ) {printf("Child - globvar=%d stackvar=%d\n",globvar,stackvar); sleep(1);printf("Child - woke up globvar=%d stackvar=%d\n",globvar,stackvar); globvar = 100;stackvar = 100; printf("Child - modified globvar=%d stackvar=%d\n",globvar,stackvar);retval = execl("/bin/date", (char *) 0 ); printf("Child - WHY ARE WE HERE retval=%d\n",retval);} else { printf("Parent - globvar=%d stackvar=%d\n",globvar,stackvar);globvar = 5; stackvar = 5;printf("Parent - sleeping globvar=%d stackvar=%d\n",globvar,stackvar); sleep(2);printf("Parent - woke up globvar=%d stackvar=%d\n",globvar,stackvar); printf("Parent - waiting for pid=%d\n",pid);retval = wait(&status); status = status>>8; /* Return code in bits 15-8 */ printf("Parent - status=%d retval=%d\n",status,retval);} }

The key to understanding this code is to understand how the fork( ) function operates. The simple summary is that the fork( ) function is called once in a process and returns twice, once in the original process and once in a newly created process. The newly created process is an identical copy of the original process. All the variables (local and global) have been duplicated. Both processes have access to all of the open files of the original process. [link] shows how the fork operation creates a new process.

The only difference between the processes is that the return value from the fork( ) function call is 0 in the new (child) process and the process identifier (shown by the ps command) in the original (parent) process. This is the program output:

recs % cc -o fork fork.crecs % fork Main - calling fork globvar=1 stackvar=1Main - fork returned pid=19336 Main - fork returned pid=0Parent - globvar=1 stackvar=1 Parent - sleeping globvar=5 stackvar=5Child - globvar=1 stackvar=1 Child - woke up globvar=1 stackvar=1Child - modified globvar=100 stackvar=100 Thu Nov 6 22:40:33Parent - woke up globvar=5 stackvar=5 Parent - waiting for pid=19336Parent - status=0 retval=19336 recs %

Tracing this through, first the program sets the global and stack variable to one and then calls fork( ) . During the fork( ) call, the operating system suspends the process, makes an exact duplicate of the process, and then restarts both processes. You can see two messages from the statement immediately after the fork. The first line is coming from the original process, and the second line is coming from the new process. If you were to execute a ps command at this moment in time, you would see two processes running called “fork.” One would have a process identifier of 19336.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'High performance computing' conversation and receive update notifications?