Process Creation and Termination in POSIX Systems

Abstract

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• Operation is described in C

Process Creation

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• A 2 steps process - fork(), exec()
• This two-step process gives the child process the flexibility to manipulate its file descriptors (stdin in the example below) after fork() but before execve(), in order to accomplish redirection of standard input, standard output, and standard error
• The code below creates a child process, pipes the input to the child process, then loads and executes the child process

int pid = fork();
if (pid == 0)
{
	dup2(pipe_fds[numProcesses - 2][READ_STREAM], STDIN_FILENO);
	closePipes(numProcesses, pipe_fds);
	execvp(sub_process[0], (char *const *)sub_process);
}

Process Termination

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2 Voluntary Ways

1. Process (进程) ends its job
2. Process hits an error during execution and exits gracefully

2 Involuntary Ways

1. Fatal error - which couldn’t be handled by the Process (进程) itself (eg. Segmentation Fault)
2. Termination by other process (killem all!)

Zombie Process

// In parent process
wait(&status);    // Waits for only one child process to terminate
                  // Other child processes continue running
                  
while (1) {       // Infinite loop
    /* Empty loop body */
}
 
exit(0);          // Unreachable code - program never exits from while loop
                  // WARNING: Any remaining child processes become zombies
                  // since parent never waits for them to terminate

• A Child Process that has completed execution but still has an entry in the process table because its parent has not yet called wait() to retrieve its exit status

A zombie process lives forever

In the code example above, the parent process keeps running and never calls wait().

Why isn’t the child process removed automatically after completing its job?

The OS allows the parent process to check on its child using wait() to retrieve the completion status.

Important

Zombie processes can be removed by terminating their parent process. When the parent process terminates, the OS knows there’s no way for the child processes to be checked, so it cleans them up.

Tool

You can check for zombie processes with the command: ps aux | grep 'Z'.

POSIX

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fork()

/* Fork a new process - returns 0 in child, child's PID in parent */ 
int pid = fork();

• Creates an exact duplicate of the original Process (进程)
• fork() returns -1 if the fork fails

wait()

int status; // Used to store termination information
 
// This only waits for ONE child to finish
wait(&status);
 
// Wait specifically for first child
waitpid(pid1, &status, 0);
 
// Keep waiting until no children are left 
while (wait(NULL) > 0) { 
	// Each iteration handles one child process 
	// Loop continues until all children are done 
}

• Used by a parent process to wait for one of its child processes to terminate

References

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• Understanding Zombie Processes! - YouTube