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Need help getting the correct signal for child process?

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Join Date: Jul 2009

Posts: 6

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guyod

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Newbie Poster

Need help getting the correct signal for child process?

Aug 8th, 2009

Hello, I am learning C, POSIX, and Unix.

I am trying to write a code that wait until the child process is done then terminate, however, I do not know what to put in the while loop.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
Parent function:
 signal(SIGCHLD, handler)
 while(child process is running)
print the number of seconds
 
Child function:
does something for some random amount of seconds
 
{
Parent function:
 signal(SIGCHLD, handler)
 while(child process is running)
print the number of seconds

Child function:
does something for some random amount of seconds

{

The teacher advised me to use signal(SIGCHLD, handler) part. I do not complete know what it does, all i know is that it does not ignore signal from the child. How do i know the child has ended? Basically, what do i put in the "child process is running" of my while loop. It can also be something like, while child process is not exited.

Thanks in advance guys, I am not sure if i posted in the correct thread. So sorry if i got the incorrect one.

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Join Date: Jun 2008

Posts: 255

Reputation: Hiroshe is a jewel in the rough

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Hiroshe

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Posting Whiz in Training

Re: Need help getting the correct signal for child process?

-1

Aug 8th, 2009

What do you want to do exactly? It sounds like your talking about functions.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
void child(void);
 
int main(void) {
  child();  /* Call child process */
 
  return 0;
}
 
void child(void) {
  /* Random stuff */
}
void child(void);

int main(void) {
  child();  /* Call child process */

  return 0;
}

void child(void) {
  /* Random stuff */
}

Last edited by Hiroshe; Aug 8th, 2009 at 12:26 pm.

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Join Date: Oct 2007

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Duoas

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Posting Virtuoso

Re: Need help getting the correct signal for child process?

Aug 8th, 2009

If you have actually fork()ed (or spawn()ed) a child process, and you just want to check whether or not it has terminated, you want the waitpid() function.

You can also use the usleep() function to cause the parent to spin its wheels for a short bit. There are more modern and/or better ways to do this, but this is short and simple...

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
...
int status;
int seconds = 0;
do {
  /* Pause for a second */
  usleep( 1000000 );
  /* Tell the user the number of elapsed seconds */
  printf( "\r%d seconds...", ++seconds );
  /* Check the child's current status */
  waitpid( (pid_t)(-1), &status, WNOHANG );
  /* Only break if it has terminated */
} while (!WIFEXITED( status ));
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
...
int status;
int seconds = 0;
do {
  /* Pause for a second */
  usleep( 1000000 );
  /* Tell the user the number of elapsed seconds */
  printf( "\r%d seconds...", ++seconds );
  /* Check the child's current status */
  waitpid( (pid_t)(-1), &status, WNOHANG );
  /* Only break if it has terminated */
} while (!WIFEXITED( status ));

Hope this helps.

Last edited by Duoas; Aug 8th, 2009 at 11:59 pm.

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Join Date: Jul 2009

Posts: 6

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guyod

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Newbie Poster

Re: Need help getting the correct signal for child process?

Aug 9th, 2009

Thanks guys, what i really wanted is exactly what Duoas's answer gave. I was having a hard time being able to check for the child status. I wanted the child to run, however, I do not want the parent to exit and terminate the child's time is up. I will try out the code below and let you guys know how it goes.

I am very thankful for all the help guys.

Last edited by guyod; Aug 9th, 2009 at 2:27 pm.