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Full Discussion: parent called more times - fork - C language
Top Forums Programming parent called more times - fork - C language Post 302498869 by wakatana on Tuesday 22nd of February 2011 05:36:09 PM
Old 02-22-2011
parent called more times - fork - C language
Hi gurus can you explain following lines of code ?

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

int main(void)
{
    pid_t pid;
    int rv;

    switch(pid = fork()) {
    case -1:
        perror("fork");  /* something went wrong */
        exit(1);         /* parent exits */

    case 0:
        printf(" CHILD: This is the child process!\n");
        printf(" CHILD: My PID is %d\n", getpid());
        printf(" CHILD: My parent's PID is %d\n", getppid());
    printf(" CHILD: I am sleeping for 15 seconds\n");
        sleep(15);           // exits as a second
    printf(" CHILD: Sleeping is over\n");
    


    default:
        printf("PARENT: This is the parent process!\n");
        printf("PARENT: My PID is %d\n", getpid());
        printf("PARENT: My child's PID is %d\n", pid);
    printf("PARENT: I am sleeping for 10 seconds\n");
        sleep(10);             // exits first
    printf("PARENT: Sleeping is over\n");
    }

    return 0;
}

As far as I know:
- if fork() function is called - code is executing in child and parent section (if fork() did not exits with -1 of course)
- after calling fork() function all code after fork() is executed sequentially (but due to sheduller is unable to predict if will be executed parent or child part of code)
- if parent code exits before child code (without waiting or hadnling signal), childs new parent will be init proces (PPID 1)

I get the following output


Code:
bash-3.2$ ./a2.out
PARENT: This is the parent process!
 CHILD: This is the child process!
PARENT: My PID is 15852
PARENT: My child's PID is 15853
PARENT: I am sleeping for 10 seconds
 CHILD: My PID is 15853
 CHILD: My parent's PID is 15852
 CHILD: I am sleeping for 15 seconds
PARENT: Sleeping is over
bash-3.2$  CHILD: Sleeping is over
PARENT: This is the parent process!
PARENT: My PID is 15853
PARENT: My child's PID is 0
PARENT: I am sleeping for 10 seconds
PARENT: Sleeping is over

So my question is:
- why is not child mark as defunct when parent exits first ? (I tried ps -elf | grep defunct during execution of code)
- Why is parent code executed second time (but now with the pid of child ?)

Thanks a lot
 

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LEARN ABOUT V7

preap

preap(1)							   User Commands							  preap(1)

NAME

       preap - force a defunct process to be reaped by its parent

SYNOPSIS

       preap [-F] pid...

DESCRIPTION

       A  defunct  (or	zombie)  process  is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
       waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
       happen  if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
       to PID 1. See init(1M), which periodically reaps such processes.

       An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
       nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
       do consume a small amount of system memory.

       preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.

       preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

	 o  The process is a child of init(1M).

	 o  The parent process is stopped and might wait on the child when it is again allowed to run.

	 o  The process has been defunct for less than one minute.

OPTIONS

       The following option is supported:

       -F	Forces the parent to reap the child, overriding safety checks.

OPERANDS

       The following operand is supported:

       pid	Process ID list.

EXIT STATUS

       The following exit values are returned by preap, which prints the exit status of each target process reaped:

       0	Successfully operation.

       non-zero Failure, such as no such process, permission denied, or invalid option.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWesu (32-bit)		   |
       +-----------------------------+-----------------------------+
       |			     |SUNWesxu (64-bit) 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)

WARNINGS

       preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes  will
       not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.

SunOS 5.10							    26 Mar 2001 							  preap(1)
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