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CS 201

Processes

Gerson Robboy

Portland State University

Review

Definition: A Definition: A process process is an instance of a runningis an instance of a running

program. program.

 One of the most fundamental concepts in computer science.

 Not the same as “program” or “processor”

A A programprogram is a set of instructions and initialized data inis a set of instructions and initialized data in

a file, usually found on a disk. a file, usually found on a disk.

A A processprocess is an instance of that program while it isis an instance of that program while it is

running, along with the state of all the CPU registers running, along with the state of all the CPU registers

and the values of data in memory. and the values of data in memory.

A single program can correspond to many processes; A single program can correspond to many processes;

for example, several users can be running a shell. for example, several users can be running a shell.

Logical Control Flows

Time

Process A Process B Process C

Each process has its own logical control flow

Context Switching

Processes are managed by a shared chunk of OS code Processes are managed by a shared chunk of OS code

called the called the kernel kernel

 The kernel is not a separate process, but rather runs as part

of some user process

Control flow passes from one process to another via a Control flow passes from one process to another via a

context switch.context switch.

Process A code Process B code

user code

kernel code

user code

kernel code

user code

Time context switch context switch

How do processes get created?

The The fork() fork() system call creates a new process.system call creates a new process.

Every process is created by another process. Every process is created by another process.

 With one exception, the very first process…

fork() creates a duplicate of the process that called it. fork() creates a duplicate of the process that called it.

fork : Creating new processes

int int fork(void)fork(void)

 creates a new process (child process) that is

identical to the calling process (parent process)

 Fork is called once but returns in two separate

processes.

 The processes are identical except for one detail:

 fork returns 0 to the child process  fork returns the child’s pid to the parent process

Fork

Key Points Key Points

 Parent and child both run the same code

 Distinguish parent from child by return value from fork

 Both processes, after fork, have identical state

 Including shared open file descriptors  Relative ordering of their print statements undefined  The two processes will go their separate ways without synchronizing

 This is important: Separate memory spaces.

Fork Example

void fork1() { int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x); }

What does this program do? What does this program do?

Exercise

void doit()

fork();

fork();

printf(“hello\n");

return;

int main()

doit();

printf(“hello\n”);

exit(0);

What does this program print? What does this program print?

Exercise: What does this program print?

static int j = 0;

do_child(int i)

int pid;

if (i < 2){

pid = fork();

if(pid == -1)

exit(0);

else if(pid == 0){

do_child(i+1);

} else {

j++;

printf("This is process %d, j=%d\n", i, j);

} else {

j++;

printf("This is process %d, j=%d\n", i, j);

main()

do_child(0);

exec : Running new programs

A family of related functions: execv, execp, execl A family of related functions: execv, execp, execl

int int execlexecl(char(char path,path, charchar arg0,arg0, charchar arg1,arg1, ……,, 0)0)

 loads and runs executable at path with args arg0 , arg1 , …

 path is the complete path of an executable  arg0 becomes the name of the process  “real” arguments to the executable start with arg1 , etc.  list of args is terminated by a (char *)0 argument

Here Here’’s what they all do:s what they all do:

 Overwrite the calling process with a new program

 Does not create a new process  Runs a new program

 returns -1 if error, otherwise doesn’t return

 Why doesn’t it return?

Example

A program that creates a child process, the child A program that creates a child process, the child

executes executes /usr/bin/ls /usr/bin/ls, and then the parent prints, and then the parent prints

“ “done.done.””

main() {

if (fork() == 0) {

execl("/usr/bin/ls", “ls”, 0);

wait(0); // This is the parent

printf(“done\n”);

exit(0);

wait : Synchronizing with children

int int wait(wait(intint child_status)child_status)

 suspends current process until one of its children

terminates

 return value is the pid of the child process that terminated

 If the child has already terminated, then wait returns its pid

immediately

 If child_status != NULL , then the object it points to will

be set to a status indicating why the child process

terminated

wait : Synchronizing with children

void fork9() { int child_status; if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit(); }

HP

HC Bye

CT Bye