Wait ()System Call

Sud_paddy · 08-11-2013, 01:04 AM

Wait system call is where :

The system call wait() is easy. This function blocks the calling process until one of its child processes exits or a signal is received. For our purpose, we shall ignore signals. wait() takes the address of an integer variable and returns the process ID of the completed process. Some flags that indicate the completion status of the child process are passed back with the integer pointer. One of the main purposes of wait() is to wait for completion of child processes.

After Reading above line ,My Question What is critearia of selecting that " One of the child process " .Its is randum ...if so what is use of waiting for a child process to terminate

tronayne · 08-11-2013, 07:29 AM

A different way of saying it: you can control the execution of child processes by call wait() in the parent. wait() forces the parent to suspend execution until the child is finished. wait() returns the process number of the child that finished. If the child finishes before the parent gets around to calling wait(), then when wait() is called by the parent, it will return immediately with the child process number.

Here's a little example that guarantees that the output of the child will precede that of the parent:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <sys/types.h>

void	main	(void)
{
	if (fork () == 0)
		(void) fprintf (stdout, "this is the child\n");
	else {
		(void) wait ((int *) NULL);
		(void) fprintf (stdout, "this is the parent\n");
	}
	exit (EXIT_SUCCESS);
}

Run it:

Code:

prog
this is the child
this is the parent

You can have more than one child process by simply calling fork() more than once.
Hope this helps some.

XavierP · 08-12-2013, 07:47 AM

Moved: This thread is more suitable in Linux-Newbie and has been moved accordingly to help your thread/question get the exposure it deserves.