Process Control

Process Creation

A Process is created in many ways which include

• Creation by User: when we open a app/code
• System Initialization: Some processes may be initialized by sytem or kernal directly. Many processes will automatically start at starting of system.
• BatchJobs: Processes which run as sheduled. (no further user interaction needed)
• By Running Process: A process may be created by some other process

  Process Termination

  A Process can be terminated in many ways which include

• Naturally completed: End of program
• Intrupt by user: probablly with signals
• Any Failure
• Memory scarcity: If process needs much memory
• Need of Other Resource: If a process requests to access resourses other than alloted it will be killed (Ex: Segmentation fault)
• Parent Process: May be terminated by parent process or sometimes if parent process is killed all child processes will be killed

  Parent and Child Processes

  When a process $A$ creates a process $B$ then process $A$ is called Parent process and process $B$ is called child process.

  System Calls

  fork()

  fork creates a copy of all next instructions which need to be run in child process with all current data

  #include <stdio.h>
  #include <sys/types.h>
  #include <unistd.h>
  int main(){
    printf("1");
    fork();
    // At the above point the code is splitted into 2 parts Parent and child 
    // you can assume like a bacteria which is giving birth to a new child
    printf("2");//this will be printed twice. once from parent once from child
    return 0;
  }
  

The return type of fork is a pid_t (process number)

pid_t fork();

Here the pid value is different in parent and child processes

• in parent we will get pid of child process
• in child we get 0 as return By taking advantage of this we can make some changes in our code

  int main(){
    code1();
    pid_t pid=fork();
    code2();
    if(pid==0){
        //child
        code3();
    }else{
        //parent
        code4();
    }
    code5();
  }
  

  In above code The execution process is

we can see code3() is working in only child and code4() is working only in forked child process

exit()

#include <stdlib.h>
int main(){
    exit(EXIT_SUCCESS); /* or return EXIT_SUCCESS */
}

• exit expects a single argument the status 0 for success and other as failure codes
• so you can use as exit(0), exit(3) etc

• EXIT_SUCCESS and EXIT_FAILURE are predefined MACROS for exit function

• when exit is called the process will quiet.
• In case we are using threads the respective thread will be quitted.

  wait()

pid_t wait(int *stat_loc)

• if wait is used the process waits till all child processes completes execution
• it will return the pid of child process whic is terminated
• if there are multiple child processes it will return anyone
• it can also teturns exit codes if child exits

#include<stdio.h>
#include<sys/wait.h>
#include<unistd.h>
 
int main(){
    if (fork()== 0)
        printf("part1");
    else{
        printf("part2\n");
        wait(NULL);
        printf("part3");
    }
    return 0;
}

in this code part1 and part2 may print in any fashion but part3 must be printed after part1 only as wait will wait till completion of child process

kill()

• kill is used to send signals to other processes
• int kill(pid_t pid, int sig);
  • pid=0 sends signal to all processes in current group
  • pid=+ve sends signal to respective process whose wpid matches
  • pid=-1 sends signals to all processes which are permissible from current process (except init)
  • pid=-ve !=-1 send signals to all processes of process group which contains respective -pid
  • sig=0 no signal sent but error checking takes place

    Signal Handling

Process Sheduling Stratagies

FCFS

SPN

SRT

Round Robin

HRRN

Fair Share Sheduling