Process Management 2

Process Creation, System calls related to the process, cooperation between processes , Message Passing, Interprocess communication

 

fork() system call

A process is created by the fork() system call.

• created a new address space that is a duplicate of the caller

int main(){
	int pid;
    pid = fork();
    if(pid == 0){ /* this is child */			Child process -> pid = 0
    	printf("\n Hello, I am child!\n");
    }else if(pid>0) { /*this is parent */
    	printf("\n Hello, I am parent!\n");		Parent process -> pid > 0
    }
}

exec() system call

A process can execute a diffent program by the exec() system call.

• replaces the memory image of the caller with a new program.

int main(){
	int pid;
    pid = fork();
    if(pid == 0){ /* this is child */			Child process -> pid = 0
    	printf("\n Hello, I am child! Now i'll run date \n");
        execlp("/bin/date", "/bin/date",(char)0);
    }else if(pid>0) { /*this is parent */
    	printf("\n Hello, I am parent!\n");		Parent process -> pid > 0
    }
}

wait() system call

when process 'A' calls wait() system call

• The Kernel puts process A to sleep until the child terminates. (block state)
• once the child process terminates, the kernel wakes up process A (ready state)

int main(){
	int childPid;
    s;
    childPid = fork();
    if(pid == 0){ /* this is child */			Child process -> pid = 0
    	<code for child process>
    }else{
    	wait();
    }
    s;
}

exit() system call

Terminates of process

1. Voluntary termination
   • Through exit() system call after the last statement was performed.
   • The compiler automatically inserts it at the end of the 'main' function, even if not explicitly specified in the program
2. Non-Voluntary termination
   • The parent process forcefully terminates the child process 
     • If the child process requests the resources beyond the maximum limit.
     • the task assigned to the child is no longer required.
   • When the type 'kill', 'break' or similar commands on the keyboard.
   • When it is terminated by the parent
     • The children terminates before the parent process.

Colaboration between processes

Independent process

• Each process has its own address space so, in principle, one process can't affect the execution of another process

Cooperating process

• Through IPC , one process can affect  the execution of another process

IPC(Interprocess Communication)

• The method of message passing.
  • message passing : message passing through the kernel
• The method of sharing the address space
  • shared memory : There is shared memory mechanism that allows certain parts of the address space to be shared even among different processes.
• Thread : thread는 사실상 하나의 프로세스이므로 프로세스 간 협력으로 보기는 어렵지만 동일한 process를 구성하는 thread들 간에는 주소 공간을 공유하므로 협력이 가능

Message Passing

Message system

• 프로세스 사이에 공유 변수(shared variable)를 일체 사용하지 않고 통신하는 시스템

Direct Communication

• 통신하려는 프로세스의 이름을 명시적으로 표시

Indirect Communication

• mailbox(또는 port)를 통해 메시지를 간접 전달