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Zombie and Orphan Process in C ++
2023-03-16 21:16:12

Zombie and Orphan Process in C ++

Zombie and Orphan Process

In computer science, a process is an instance of a program in execution. When a process ends its execution, it can be said to be dead. However, if the process has a parent process and the parent process has not yet read the status information of the child process, the child process remains in a state known as a zombie process. In this state, the child process still exists in the system table, but it has no function and its resources have been freed. This article explains the concept of zombie and orphan processes in C++.

Zombie Processes:

A zombie process occurs when a child process terminates but its parent process does not read its exit status. The child process remains in the system table as a zombie process until the parent process reads its exit status. The exit status is usually read by the parent process through the wait() system call.

In a zombie process, the process ID (PID) and other process attributes, such as user ID and group ID, are still present in the system table. However, the process cannot execute any tasks and its resources have been freed. The presence of zombie processes in the system table is an indication of a failure in the communication between parent and child processes.

#include <iostream>
#include <unistd.h>


#include <sys/wait.h>


using namespace std;


int main()
{
pid_t child_pid = fork();


if (child_pid == 0)
{
cout << "I am the child process with PID " << getpid() << endl;
cout << "Exiting now..." << endl;
exit(0);
}
else
{
cout << "I am the parent process with PID " << getpid() << endl;
cout << "Sleeping for 20 seconds..." << endl;
sleep(20);
}


return 0;
}

Explanation:

This program demonstrates the creation of a child process using the fork() system call in C++. The main process creates a child process using the fork() call and the child process is given a separate copy of the program code and data in memory.

Here's what the program does:

  • The parent process is created first when the program starts. The parent process calls the fork() function, which creates a child process.
  • The value returned by the fork() function is checked. If the value is 0, it means the current process is the child process. If the value is greater than 0, it means the current process is the parent process.
  • If the current process is the child process, it prints "I am the child process with PID [child process ID]." The child process then exits immediately using the exit() function.
  • If the current process is the parent process, it prints "I am the parent process with PID [parent process ID]." The parent process then waits for 20 seconds using the sleep() function.
  • After the 20-second wait, the parent process terminates and the program ends.

Because fork() is disabled in online compilers, it should be noted that the code above might not run.

Note: The child process becomes a zombie process when it exits before the parent process calls the wait() or waitpid() function to retrieve its exit status. This causes the child process to remain in the system's process table, consuming system resources, until the parent process retrieves its exit status.

Orphan Processes

An orphan process occurs when the parent process terminates before its child process. In this case, the child process becomes an orphan and its parent process ID is set to 1, which is the process ID of the init process. The init process is the first process that is started when the system boots, and it acts as a parent for all orphan processes.

An orphan process continues to execute normally and can continue to create child processes of its own. However, its exit status is not read by any process, and it becomes a zombie process if it terminates before its exit status is read.

Here is a simple C++ program that demonstrates the creation of an orphan process:

#include <iostream>
#include <unistd.h>
#include <sys/wait.h>




using namespace std;


int main()
{
pid_t child_pid = fork();


if (child_pid == 0)
{
cout << "I am the child process with PID " << getpid() << endl;

Explanation:

This program demonstrates the creation of a child process using the fork() system call and the retrieval of its exit status using the waitpid() function in C++. The main process creates a child process using the fork() call and the child process is given a separate copy of the program code and data in memory.

Here's what the program does:

  • The parent process is created first when the program starts. The parent process calls the fork() function, which creates a child process.
  • The value returned by the fork() function is checked. If the value is 0, it means the current process is the child process. If the value is greater than 0, it means the current process is the parent process.
  • If the current process is the child process, it prints "I am the child process with PID [child process ID]." The child process then exits immediately using the exit() function.
  • If the current process is the parent process, it prints "I am the parent process with PID [parent process ID]."
  • The parent process then calls the waitpid() function to wait for the child process to terminate and retrieve its exit status.
  • The second argument of the waitpid() function is a pointer to an int variable that will hold the child process's exit status. The third argument of the waitpid() function specifies options for the waitpid() call.
  • The parent process then prints "Child process has terminated with exit status: [child process exit status]." The exit status is stored in the variable passed as the second argument to the waitpid() function.

By using the waitpid() function, the parent process can retrieve the exit status of the child process and avoid creating a zombie process.

Because fork() is disabled in online compilers, it should be noted that the code above might not run.

Conclusion:

In conclusion, zombie and orphan processes are important concepts in C++ programming and systems programming.

A zombie process is a process that has completed execution but still exists in the system's process table, consuming system resources. An orphan process is a process whose parent has terminated, leaving it without a parent process.

The use of the wait() or waitpid() function by the parent process can avoid the creation of zombie processes by retrieving the exit status of the child process. Proper use of these functions is important for the efficient use of system resources and avoiding potential performance issues.

In C++, the fork() system call is used to create a child process, and the exit() and waitpid() functions are used to exit a process and retrieve the exit status of a child process, respectively. Understanding and correctly using these functions is crucial for creating reliable and efficient systems in C++.