--author Giovanni Agosta 
--title Processes
--date today
Brief overview of processes and related operation
(ALP chapter 3, GaPiL chapter 3 and 9)
--newpage agenda
--heading Agenda

  * Process Identification

  * Viewing Active Processes

  * Killing a Process

  * Creating Processes

  * Process Scheduling

  * Process Termination

  * Signals
	
--newpage pid
--heading Process Identification

  * pid: process ID (unsigned 16 bit integer
  * low pid values: reserved for kernel processes
  * pid=1: init

--beginoutput
#include <sys/types.h>
#include <unistd.h>

/* pid of the current process */
pid_t getpid(void) 
/* pid of the parent process */
pid_t getppid(void)
--endoutput

--newpage view
--heading Viewing Active Processes
From the shell:

  * ps
	
  * pstree

ps has many options controlling the format and filtering

--newpage kill
--heading  Killing a Process
From the shell:

  * kill <pid> 

kill sends a termination signal to process <pid>

From a program
--beginoutput
#include <sys/types.h>
#include <signal.h>

int kill (pid_t pid, int sig);
--endoutput


--newpage create
--heading  Creating Processes
Using system

--beginoutput
#include <stdlib.h>
int main ()
{
  int return_value;
  return_value = system ("ls -l /");
  return return_value;
}
--endoutput

List contents of the root directory

--newpage fork
--heading  Creating Processes
Using fork 

--beginoutput
#include <sys/types.h>
#include <unistd.h>
pid_t fork(void)
--endoutput

  * fork creates a new process (child)
  * The new process is a copy of the parent
  * The new process shares the text segment of the parent
  * fork returns 0 in the child, child pid in the parent
  * Failure: usually, when too many processes have been created

--newpage fork
--heading  Creating Processes
Using fork and exec 

--beginoutput
#include <unistd.h>

int execve(const char *filename, char *const argv[], char *const envp[])
--endoutput

  * execve changes the program executed by the current process
  * execve returns only on error
  * Many different error conditions
  * Several exec family variants (argv as list or vector; use PATH or not; use environ or explicit environment array)

--newpage scheduling
--heading  Process Scheduling
From shell:

  * nice -n <niceness> <command>
  * renice <niceness> <pid>
  * Only root can decrease niceness!

From a program:
--beginoutput
#include <unistd.h>

int nice(int inc);
--endoutput

--newpage schedstatus
--heading Process Status
--boldon
Status         PS Description
--boldoff
Runnable       R  Running or runnable
Sleep          S  Waiting for system response, can be interrupted
Uninterrutible D  Waiting for system response, cannot be interrupted 
   Sleep
Stopped        T  Stopped with SIGSTOP
Zombie         Z  Terminated, but status is preserved

--newpage term
--heading  Process Termination
Termination modes:
  * exit(status) function call
	* return from main (equivalent to exit)
	* _exit system call (called by _exit)
	* abort() call
	* unhandled signal received

Effects:
  * File descriptors closed
  * Termination status is saved
	* Children are assigned a new parent (init)
	* SIGCHLD signal sent to parent
	* Other cleanup operations for session/group

--newpage signals
--heading  Signals
  * A mechanism for communication between processes
	* Signal: asynchronous message sent to a process
	* Many different signals (/usr/include/bits/signum.h)
  * Signal disposition:
	  * Ignore
    * Terminate
    * Terminate and core dump
    * Use given signal handler function