Copyright ©: Nahrstedt, Angrave, Abdelzaher1 Processes Tarek Abdelzaher Vikram Adve

Copyright ©: Nahrstedt, Angrave, Abdelzaher 1

Processes

Tarek AbdelzaherVikram Adve

Copyright ©: Nahrstedt, Angrave, Abdelzaher

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Users, Programs, Processes

Users have accounts on the system Users launch programs

Many users may launch same program

One user may launch many instances of the same program

Processes: an executing program (the unit of

work of a modern computer)


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Process States

Possible process states Running (occupy CPU) Blocked Ready (does not occupy CPU) Other states: suspended, terminated

Question: in a single processor machine, how many process can be in running state?


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Linux: 5 State Process Model

Add states for creating and deleting process

Add transitions Timeout, Dispatch, Event Occurs


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How to List all Processes? On Windows: run Windows task

manager Hit Control+ALT+delete Click on the “processes” tab

On UNIX $ ps –e Try “man ps” to understand more

about its usage.


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Process Execution and Context Switching


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Process Identification UNIX identifies processes via a unique Process ID

Each process also knows its parent process ID since each process is created from a parent process.

Root process is the ‘init’ process ‘getpid’ and ‘getppid’ – functions to return process ID (PID)

and parent process ID (PPID)

Example 1#include <stdio.h>#include <unistd.h>int main (void) {

printf(“I am process %ld\n”, (long)getpid());printf(“My parent id is %ld\n”, (long)getppid());return 0;

}


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Creating a Process – fork()

fork() creates a duplicate of the calling process: Both processes continue with return from fork() Child gets exact copy of code, stack, file descriptors,

heap, globals, and program counter Child gets new pid, ppid, time; no signals, file locks, …

fork() returns -1 if fork fails 0 in child process child’s PID in parent process


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Creating a Process in Unix

Example 2

#include <stdio.h>#include <unistd.h>

int main(void) { pid_t x; x = fork(); if (x == 0) printf(“In child: fork() returned %ld\n”, (long) x); else printf(“In parent: fork() returned %ld\n", (long) x);}

What does this print?


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Chain and Fan

ChildChildParent

Parent

Child Child… …

pid_t childpid = 0;for (i=1;i<n;i++) if (childpid = fork()) break;

pid_t childpid = 0;for (i=1;i<n;i++) if ((childpid = fork()) <=0) break;

Chain

Fan


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Process Operations: Creation

A new process needs resources such as CPU, memory, file descriptors, I/O devices Child can get resources from OS or from parent Child address space is duplicate of parent process Child process is given a subset of parent

resources Prevents too many processes from overloading system

Execution possibilities are Parent continues concurrently with child Parent waits until child has terminated


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Process Termination

Normal exit (voluntary) End of main() exit(0)

Error exit (voluntary) exit(2) or abort()

Fatal error (involuntary) Divide by 0, seg fault, exceeded resources

Killed by another process (involuntary) Signal: kill(procID)


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Process Operations: Termination

When a process terminates: Open files are flushed and closed Tmp files are deleted Child’s resources are de-allocated

File descriptors, memory, semaphores, file locks, …

Parent process is notified via a signal Exit status is available to parent via wait()


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Process Hierarchies Parent creates a child process, a child

process can create its own processes Forms a hierarchy

UNIX calls this a "process group" Windows has no concept of process

hierarchy all processes are created equal


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wait(), waitpid() System Calls

wait() causes parent process to wait (block) until some child finishes

wait() returns child’s pid and exit status to parent

waitpid() waits for a specific child

errno Cause

ECHILD Caller has no unwaited-for children

EINTR Function was interrupted by signal

EINVAL Options parameter of waitpid was invalid


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Waiting for a child to finish(Try “man –s 2 wait”)

#include <errno.h>#include <sys/wait.h>

pid_t childpid;

childpid = wait(NULL);if (childpid != -1) printf(“waited for child with pid %ld\n”, childpid);

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Copyright ©: Nahrstedt, Angrave, Abdelzaher1 Processes Tarek Abdelzaher Vikram Adve