Question

What is the behaviour of a double fork with a signal Handler in the Parent as below?

Asked by: tamg_2009

Hi,

I've been trying to judge the output of the following code. Over a double fork(), I have the signal handler for SIGCHLD initialized. As per my understanding, I believed that the signal SIGCHLD would be handled only once by the parent unless initialized again in the handler.

However, after executing the code handles the Signal only twice (Even if I re-initialized my signal in the handler) and inspite of having 3 childen. The above behaviour is with respect to Darwin OS on Mac. On Ubuntu Linux, the signal is handled by the code based on the timing of the sleep (commented out). Can someone please explain what is happening and How many times the signal for my function would actually be handled??

#include<stdio.h>
#include<unistd.h>
#include<sys/signal.h>
 
int status,pid,cpid;
static void signal_handler(int);
 
int main(void)
{
 pid=fork();
 if(fork()==0)
 {
//  sleep(5);
  printf("Return Status in CHILD : %d\tMy PID : %d\t My PPID : %d\n",status,getpid(),getppid());
 }
 else
 {
  signal(SIGCHLD,signal_handler);
  printf("Return Status in PARENT : %d\t My PID : %d\t My PPID : %d\n",status,getpid(),getppid());
  wait(&status);
 }
}
static void signal_handler(int signo)
{
 printf("SIGCHLD Received : %d\n",signo);
// signal(SIGCHLD,signal_handler);
}

                                  
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:

Select allOpen in new window

This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.

Subscribe now for full access to Experts Exchange and get

Instant Access to this Solution

  • Plus...
  • 30 Day FREE access, no risk, no obligation
  • Collaborate with the world's top tech experts
  • Unlimited access to our exclusive solution database
  • Never be left without tech help again

Subscribe Now

Asked On
2009-03-29 at 12:34:11ID24275620
Tags

Unix fork() and signal()

Topics

Unix Systems Programming

,

Signal Processing

Participating Experts
2
Points
125
Comments
3

Trusted by hundreds of thousands everyday for fast, accurate and reliable tech support.

  • "The time we save is the biggest benefit of Experts Exchange to Warner Bros. What could take multiple guys 2 hours or more each to find is accessed in around 15 minutes on Experts Exchange." Mike Kapnisakis, Warner Bros.
  • "Our team likes having a resource that is more secure than just using Google and most experts using this service really know their stuff. It's nice to look here first versus using Google." Dayna Sellner, Lockheed Martin
  • "Anytime that I've been stumped with a problem, 9 out of 10 times Experts Exchange has either the accepted solution or an open discussion of the potential solution to the problem." Kenny Red, eBay Inc.

See what Experts Exchange can do for you.

Got a question?

We've got the answer.

Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.

Screenshot of Experts Exchange Knowledgebase

Need individual assistance?

Our experts are ready to help.

If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.

Screenshot of Experts Exchange Knowledgebase

Want to learn from the best?

Read articles from industry experts.

Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.

Screenshot of an Article

Working on a long term project?

Store your work and research.

Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.

Screenshot of Experts Exchange Knowledgebase

Access the answers to your technology questions today.

Subscribe Now

30-day free trial. Register in 60 seconds.

What Makes Experts Exchange Unique?

Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.

Trusted by the world's most respected brands.

image of each brand's logo

Faithfully serving IT professionals since 1996.

Experts Exchange Logo

Try it out and discover for yourself.

Subscribe Now

30-day free trial. Register in 60 seconds.

Related Solutions

  1. Pipe & Fork
    Does anybody have a simple example of use of fork and pipe. We are developing a program with a process father and multiple sons, but we don't know how to synchronize them. Other ideas than use of pipe ?! Much Thanks
  2. fork
    I need to write a PERL script that fork a fixed number of child processes ( e.g. 1 or 2 or 3 or ...). The parent process should wait for the child process to finish and each process should generate a random number. The sum of all random numbers generated are calculated and pr...
  3. Clueless with fork....
    Hello, I recently started about fork(), wait(), signal, and so on. I have a program that's supposed to spawn two children. A children is supposed to only add a record to a database (by SIGINT), and the other one is supposed to search for a record (by SIGQUIT). The parent is ...
  4. regarding fork
    i read the following example from this link http://developers.cogentrts.com/cogent/cogentdocs/re-fork.html In this example i have some queries. After fork() is executed if child pid is returned(>0) in if ((childID = fork()) > 0) then main thread(parent) starts executi...
  5. data fork problem
    Hi All, I have an application that locks a file by puting some informations at the beginning of its data fork. so it can,t be open by another application. My question is : I s there any way to prevent the application from doing so? -BT
  6. related to fork
    Hello, We know that setpgrp system call will change the process group id and make the process new group leader. When we fork a child and change the group id the parent will not clean the process table when child dies(i.e, zombie). when ever a signal(termination) is sent...

Free Tech Articles

  1. WARNING: 5 Reasons why you should NEVER fix a computer for free.
    It is in our nature to love the puzzle. We are obsessed. The lot of us. We love puzzles. We love the challenge. We thrive on finding the answer. We hate disarray. It bothers us deep in our soul. W...
  2. SCCM OSD Basic troubleshooting
    SCCM 2007 OSD is a fantastic way to deploy operating systems, however, like most things SCCM issues can sometimes be difficult to resolve due to the sheer volume of logs to sift through and the dispe...
  3. Migrate Small Business Server 2003 to Exchange 2010 and Windows 2008 R2
    This guide is intended to provide step by step instructions on how to migrate from Small Business Server 2003 to Windows 2008 R2 with Exchange 2010. For this migration to work you will need the fo...
  4. Create a Win7 Gadget
    This article shows you how to create a simple "Gadget" -- a sort of mini-application supported by Windows 7 and Vista. Gadgets can be dropped anywhere on the desktop to provide instant information, ...
  5. Outlook continually prompting for username and password
    There have been a lot of questions recently regarding Outlook prompting for a username and password whilst using Exchange 2007. There are a few reasons why this would happen and I will try to cover t...
  6. Backup Exchange 2010 Information Store using Windows Backup
    There seems to be quite a lot of confusion around the ability to backup Exchange 2010 using the built in Windows Backup feature. This stems from the omission of this feature prior to Exchange 2007 s...

Cloud Class Webinars

  1. Avoiding Bugs in Microsoft Access
    Alison Balter takes and in-depth look at avoiding bugs in Access. In this webinar you will learn about using the immediate window to debug your applications, invoking the debugger, using breakpoints to troubleshoot, stepping through code, setting the next statement to execute, ...
  2. Top 10 Best New Features in Visio 2010
    Scott Helmers gives live demonstrations of the top 10 new features in Visio 2010. This webinar will teach you how to create compelling diagrams by adding shapes to the page with a single click, linking the shapes in a diagram to data in Excel (or SQL Server, or SharePoint), ...
  3. IT Consultant Business Secrets Revealed
    Michael Munger, Experts Exchange tech pro and IT consultant, pulls back the curtain on his very successful businesses and answers question on every IT consultant and business owner should know about. He shares secrets on what he did to solve the 5 most common problems in IT, ...
  4. Disaster Recovery and Business Continuity
    Quest CTO, Mike Billon, gives an overview of the steps involved in building a dunamic disaster recovery plan. Through case studies and an examination of software/hardware tooles for monitoring and testing, you'll gain a better understandin of where you are, where you want ...
  5. Organize Your Visio Diagrams with Containers and Lists
    Scott Helmers uses cross functional flowcharts, wireframe diagrams, data graphic legends and seating charts to teach you: how to ustilize all three new structured diagram components in Visio 2010, the best practices for organizeing shapes in previous version of Visio, how to organize ...
  6. How to Us Objects, Properties, Events and Methods in Microsoft Access
    Alison Dalter gives an in-depbth look at objects, properties, events and methods in Microsoft Access. In this webinar you will learn about using the object browser, referring to objects, working with properties and methods, working with object variables, understanding the ...

Join the Community

Give a Little. Get a Lot.

Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.

Join the Community

Answers

 

by: omarfaridPosted on 2009-03-29 at 20:54:38ID: 24015907

According to the code provided, it is the parent that should handle once  (since the 2nd line is commented in the signal handler code) the signal. The child will apply the default action.

 

by: ahoffmannPosted on 2009-04-05 at 11:44:55ID: 24072490

> .. and How many times the signal for my function would actually be handled
it should be handled once as long as not re-initialized again
depending on you os you have a runtime condition if the child gets executed before the parent's signal initialization which then results in on call only

20120131-EE-VQP-002

3 Ways to Join

30-Day Free Trial

The Experts

98% positive feedback on 31,087 answers since March 2000. angeliii is a Microsoft Most Valuable Professional for his work with MS SQL Server & Develoment.

He has also proven his knowledge of Visual Basic Programming, PHP Scripting and Oracle Databases.

The Experts

97% positive feedback on 10,752 answers since July 2000. lrmoore has more than 18 years experience in the networking industry.

The six-time Mircosoft MVPs specialties include firewalls, virtual private networking, and network management.

Testimonials

"...and excellent source for support... Kind of like having your very own IT dept." Electriciansnet

Testimonials

"I was apprehensive at signing up at first. However... it has already made my life as an IT administrator much easier." JaCrews

Testimonials

"WOW! You guys have great, active, and knowledgeable people on here." moore50

Business Clients

Business Clients

In the Press

"If you’ve got a question... Experts Exchange can supply an answer.”

In the Press

"...an invaluable aid for both IT professionals and those who require tech support."

In the Press

"where IT professionals provide quick answers on just about any topic"

Business Account Plans

Loading Advertisement...