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Unix-v5 Process Structure. A process is an entity which is created by the operating system and consists of a sequence of bytes which is interpreted by the CPU as Machine instruction. Data Stack.

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process definition

A process is an entity which is created by the operating system and consists of a sequence of bytes which is interpreted by the CPU as

      • Machine instruction.
      • Data
      • Stack.
  • Many processes appear to execute simultaneously as the kernel schedules them for execution and several processes may be an instance of one program. In UNIX fork is used to create a process.

Process Definition

.

process state transition diagram

Interrupt, interrupt return

User running

Sys call, interrupt

return

Return to user

exit

preempt

Zombie

Kernel Running

preempted

Process State Transition Diagram

sleep

Reschedule Process

wakeup

Ready run,

memory

Sleep, memory

Swap In

Enough memory

Swap out

fork

Swap out

created

wakeup

Ready run,

swapped

Sleep, swapped

.

process state transition diagram1

Created : parent execute system call.

  • Ready run, memory: move from created when enough memory.
  • Ready run, no memory: move from created when no memory.
  • Kernel Running: process is syscall or it is interrupted.
  • Sleep, memory: process is waiting for completion of I/O.
  • Sleep, swapped: process is swapped for lack of memory while waiting or I/O completion.
  • User Running: user process is executing user’s code.
  • Ready run, swapped: process is ready to run (e.g. when I/O completes) while it is swapped.
  • Preempted: the process is returning from kernel to user mode, but the kernel preempts it and does a context switching to schedule another process

Process State Transition Diagram

.

process structure

Process Structure

Process consists of 3 regions. Region is a contagious area of the virtual address space

.

user area u

Process A

U Area

Physical memory

User Area - U

Same virtual address

Process B

U Area

Physical memory

  • Each process has a user area.
  • User area (U) has a fixed virtual address; it is mapped to different physical address.
  • Each user area is mapped to a physical memory when process is loaded to memory.

.

data structure for a process

Per process region table

U Area

Region table

Process table

Data structure for a process

text

data

stack

memory

Per process region table allows independent processes to share regions.

.

exec a process

text

text

data

data

stack

stack

Old process deallocate

U Area A

Region table

Per process region table

Process table

exec a process

U Area B

memory

File descriptor table

.

fork a process

text

text

data

data

stack

stack

U Area Parent

Region table

File descriptor table

Process table

fork a process

Per process region table

U Area Child

memory

File descriptor table

.

create a thread

text

text

data

data

stack

stack

U Area A

File descriptor table

create a thread

Process table

Per process region table

Region table

memory

.

paging memory layout

Page 0

Page 1

Page 2

Paging memory layout

Every memory location is addressed as

(page number, byte offset in page)

Memory management hardware divides physical memory into a set of equal sized pages (typical 512-4K bytes). Paging overcomes fragmentation problem.

.

page table

87k

552k

727k

941k

1096k

2001k

Page table

Page table

Page n

Page n+1

Page n+2

Memory

Page table maps virtual address into physical address. Also, it contains access privileges.

.

process virtual space

kernel

Process virtual space

user

  • Process virtual space is divided into two classes
  • Kernel space and associated kernel mode.
  • User space and associated user mode.

.

layout of the kernel

empty

87k

128kk

256k

552k

97k

137k

292k

135k

727k

852k

304k

139k

941k

764k

279k

0k

541k

4k

783k

17k

986k

Page table address Virtual address no of pages

kernel

user

Layout of the kernel

  • Kernel code & data resides in memory permanently, all processes shares it.
  • When executing in user mode you can’t execute kernel code.
  • user accesses kernel mode through interrupts; changes mode from user to kernel.

.

user area

87k

114k

552k

708k

727k

143k

565k

941k

Per process Region table

Page table address Virtual address no of pages

P A

kernel

User Area

user

Page tables

P B

  • Loader assign a U area at fixed virtual location.
  • The proper region user entry contents will be loaded to U area register at context switching. This region is accessed only in kernel mode.

.

region entry structure

The file will be loaded into the region

  • locked
  • In demand
  • In the process of being loaded to memory
  • Loaded in to memory

Region Entry Structure

.

region table

Active list

free list

Region Table

  • Region table is divided into:
  • A linked list of active elements.
  • A linked list of free elements.

.

operations on region

Operations on Region

  • Lock and unlock
  • Allocate region and free region.
  • Attach a region and detach a region.
  • Load a region from a file into memory space of a process
  • Duplicate a region.

.

allocreg allocate region

Allocate a new region during fork, exec, shmget (shared memory) system calls.

  • Remove a region entry from a free list and add it to the active list.
  • Mark the region to be shared (e.g. text) or private (e.g. stack).
  • Set the inode field in the region to the inode of executable file.
  • Increment the inode reference count to prevent other processes from removing its contents when unlinking it.
  • Return a locked region.

allocreg -allocate region

.

allocreg allocate region continue

locked region allocreg (inode pointer, region type) output locked region

{

remove region from linked list of free regions;

assign the free region into active list and lock the region

mark the region type to be shared or private.

if (inode pointer is not null)

increment inode reference count to prevent other processes

from removing the inode when executes unlink.

place region on linked list of active regions.

return locked region

}

allocreg-allocate region (continue)

  • allocreg is called by shmtget, fork and exec.
  • In case of fork and exec the operation is associated with a file. The reference count for inode of this file must be incremented.

.

allocreg allocate region exec example

1

Region table

Active list

Active list

free list

free list

Allocreg -allocate region (exec example)

Allocate first free element in region table

2

Inode table

Active list

free list

Set region to point to file inode

.

attachreg attach region

Attach a region to a process during fork. Exec, and shmat system calls.

  • It connects the region to the process address space.
  • Kernel allocate per process region entry for the new region.
  • Initialize its type to text, data, shared memory or stack.
  • The region’s virtual address space shouldn\'t overlap with other regions.
  • The process virtual address space shouldn’t exceed the limits.
    • Region maximum space is 8M, we can’t attach 1 M region with a process of size 7.5 M.
  • Kernel increase size field in the process table with the region size.
  • Kernel increase the reference count in the region table.

attachreg -attach region

.

attachreg attach region1

attachreg

  • Input: 1 – pointer to locked region to be attached
  • 2- process to which region is being attached
  • 3- virtual address in process where region is attached
  • 4- region type
  • Output : per process region table entry
  • {
  • allocate per process region table entry for process.
  • initialize per process region table entry:
        • Set pointer to region being attached.
        • Set type field.
        • Set virtual address field.
      • Increment region reference count.
      • Increment process size according to growth region
      • Return (per process region table entry)
  • }

attachreg -attach region

.

attachreg attach region exec example

114k

708k

143k

565k

process table

Region table

attachreg -attach region (exec example)

Per process Region table

Allocate new text entry (1)

Allocate new page table

Attach an existing shared text region of size 4 Kbytes to virtual address 0 of the process.

.

growreg grow the region size

Kernel invoke growreg to change the size of the region.

      • Process expands size by executing sbrk system call.
      • Stack expand explicitly according to the depth of nested procedure call.
  • The virtual space of the expanded region shouldn\'t overlap with others.
  • The process size shouldn’t exceed the maximum size.
  • The shared region never increase in size if it is attached to other processes.

growreg – grow the region size

.

growreg grow the region size1

In case of more memory required, kernel allocate new page table or expand existing page table. Allocate physical memory for pages on systems do not support demand pages.

  • If the process contracts the region, the kernel simply release memory assigned to region.
  • Adjust process size, region size, and per process region entry to reflect the new mapping.

growreg – grow the region size

.

growreg grow the region size2

growreg

  • Input: 1 – pointer to per process region table
  • 2- change in size region (+ or -)
  • if (Region size is +)
        • Check legality of new size
          • Allocate page tables
          • If (not system supporting demand paging)
          • transfer pages;
      • else
      • free physical memory for pages.
      • free page table entries
      • Set size field in process table
  • }

growreg – grow the region size

.

growreg grow the region size3

114k

114k

708k

708k

143k

143k

565k

565k

976k

Per process Region table

Per process Region table

stack

stack

growreg – grow the region size

New page

.

freereg freeing a region

freereg – freeing a region

  • The region will be freed when it is not attached to any process (ref count =0).
  • Free the inode associated with region using iput.
  • free the page map table entries and memory pages.

.

freereg free region

freereg

  • Input: 1 – pointer to the locked region
  • if (region reference count > 0)
      • unlock region
        • return
      • release inode if it exists (iput)
      • Free page table entries
      • Free memory associated with pages
      • Place region in region free list
      • Unlock region
  • }

freereg – free region

.

dupreg duplicate a region

dupreg – duplicate a region

  • fork requires the kernel to duplicate data and stack regions.
  • the region reference count is incremented in case of shared text & memory, allowing the parent & child processes to share regions.
  • In case of stack & data regions are copied:
    • Allocate a new region entry.
    • Allocate page map table.
    • Allocate physical memory for the region.

.

dupreg duplicate region

dupreg

  • Input: 1 – pointer to region table entry
  • Output: a region which is identical to input region
  • {
  • if (region type shared)
        • // caller will increment reference count with subsequent attachreg
        • return input region pointer
      • Allocate new region (allocreg)
      • Allocate page map table and physical memory for pages.
      • Copy contents from input region to output region
      • Return pointer to allocated region
  • }

dupreg – duplicate region

.

dupreg duplicate a region1

Per process Region table

Text

Data

Shared text

stack

dupreg – duplicate a region

Process A

Private data

Per process Region table

Private stack

Text

copy

Data

stack

Process B

Private data

Private stack

.

loadreg load region

Allocate memory to load a file (growreg).

  • Load a file on demand if on demand paging is supported.
  • Copy the file into memory if on demand not supported.

loadreg – load region

.

loadreg load region1

loadreg

  • Input: 1- pointer to per process region table entry
  • 2- target virtual address to load region
  • 3- inode pointer of file for loading region
  • 4- byte offset in file for start of region
  • 5- byte count for amount of data to load
  • {
  • increase region size to accommodate file size (growreg)
  • set up u area parameters for reading file
        • Target virtual address where data is read to
        • Start offset value for reading file.
        • Count of bytes to read from file.
      • Read file into region
      • Awaken all processes waiting for region to be loaded
  • }

loadreg – load region

.

loadreg load region2

exec system calls load a text of size 7k into memory with a gap of 1K bytes in the beginning. The page containing address 0 will be protected such that access 0 will incur page fault and abort.

loadreg – load region

.

loadreg load region3

Region table

Per process Region table

loadreg – load region

1

2

  • allocreg: allocate a region for the executable file.
  • attachreg: attach the new region to the process.

.

loadreg load region4

Region table

Per process Region table

loadreg – load region

empty

growreg: Allocate page map table of one empty entry. The size will be increased by one.

.

loadreg load region5

empty

708k

143k

565k

976k

Region table

Per process Region table

loadreg – load region

  • loadreg:
  • growreg: to allocate memory for the file to be loaded.
  • Load the file to be executed.

.

detachreg detach region

The kernel detaches regions in the exec, exit, and shmdt (detach shared memory).

  • Decrement process size.
  • Decrement region reference count..
  • Call free region to-free page map table, physical memory if necessary.

detachreg – detach region

.

detachreg detach region1

detachreg – detach region

detachreg

Input : pointer to per process region table.

Output: none

decrement process size;

Decrement region reference count;

release per process region table;

if (region reference count is 0)

free region (algorithm freereg);

.

fork system call

Process invoke fork() to create a new process.

  • Process invoke fork () is a parent and new process is child process.
  • pid = fork ();
  • pid in the parent process is the child process ID, while pid in the child process is 0.

fork – system call

.

fork system call continue

The kernel does the following:

  • Allocate a new slot in the process table for child process.
  • Assign a unique ID number for the child (Child ID).
  • Make copy for the following:
    • User area (U area), this include copy of file descriptor table, and kernel stack.
    • Increment the reference counts in the file table and inode table for files associated with the process.
    • data area of parent and stack area of parent
  • Return the process id for the child process to parent otherwise return 0.

fork – system call (continue)

.

fork creating a new process context

U Area

U Area

Open Files

Current Directory

Changed Directory

Open Files

Current Directory

Changed Directory

Per process

region table

Per process

region table

Parent data

Child data

Parent user stack

child user stack

text

text

Kernel Stack

Kernel Stack

data

data

stack

stack

Parent Process

Fork Creating a new process context

File Table

Shared Text

Child Process

inode Table

.

process group

pid =777 gid =456

pid =123 gid =456

pid =555 gid =555

pid =666 gid =666

Process group leader

process group

A

pid =456 gid =456

B

C

. The kernel uses a group id to identify the set of processes which receive common signal. If process A sends a signal kill (0,SIGINT), it will be caught by all processes which have the same gid number(e.g. B,C).

.

set group id example

#include <signal.h>

  • main (argc,argv){
    • int i;
    • Setpgrp (); // set group id equal to process id
    • For (i=0;i<4;i++){
    • if (fork() == 0)
    • {
    • // child process
    • if (I & 1) // if process is odd set group id
    • setpgrp ();
    • printf (“pid %d pgrp %d \n”,getpid(),getpgrp ());
    • pause (); //suspend execution until you get signal
    • while (1) printf (“pid %d pgrp %d \n”,getpid(), getpgrp ());
    • }
    • }
    • kill (0,SIGINT); // send a termination signal to all processes in group
  • }

set group id example

Create 4 child processes. Even number processes has the same group id like parent. Processes created during odd iterations of the loop reset their process group number. When kill signal is sent it will terminate the even number while the odd will continue to execute.

.

fork algorithm

fork

Input : none

Output: to parent process, child pid, to child process, 0

allocate process table entry, allocate PID number;

copy data from parent process table slot into new child slot;

copy data region of parent process (dupreg, attachreg);

copy user stack region of parent process, (dupreg, attachreg);

share text for parent process (attachreg);

increment inode count for current directory and changed root;

increment file counts in file table;

if (executing process is parent process)

change the child state in memory into “ready to run”

return (child PID);

else // the child process is executing

return (0)

fork algorithm

.

fork example

#include <fcntl.h>

  • int fdrd, fdwt;
  • char c;
  • main (argc,argv){
    • int argc;
    • char *argv [];
    • fdrd = open (argv[1], O_RDONLY);
    • fdwt = creat (argv[2],0666);
    • fork ();
    • rdwrt ();
    • exit (0);
  • }
  • rdwrt (){
  • for (;;)
  • {
    • if (read (fdrd,&c,1) != 1)
    • return;
    • write (fdwt,&c,1);
  • }
  • }

fork example

.

exit system call

Process is terminated by executing exit system call. An exiting process will enter zombie state, relinquishes its resources, and dismantle its context except for its slot in the process table.

  • terminates the calling process "immediately".
  • Any open file descriptors belonging to the process are closed
  • any children of the process are inherited by process 1, init,
  • the process\'s par ent is sent a SIGCHLD signal. The value status is returned to the parent process as the process\'s exit status, and can be collected using one of the wait family of calls)
  • exit (status)
  • Where the value of status is returned to the parent process for examination. The exit might be called implicitly or explicitly.

exit – system call

.

exit system call1

exit

Input: return code for parent process

Output: none

{

if (process is a group leader)

send an hangup signal to all members of process group

reset process group for all members to 0

close all open files (internal version of close)

release current directory (iput)

release current changed root, if exists (iput);

free region; (freereg)

make process state zombie;

assign parent to all children processes to be init (PPID = 1);

send death of child (SIGCHLD)to parent process

if (child process in zombie state)

// init remove child from process table

send death of child (SIGCHLD) to parent

}

exit – system call

.

exit system call example

main ()

{

int child;

if ((child = fork ()) == 0)

{

printf (“CHILD pid %D \n”, getpid ());

pause (); //suspend execution until signal

}

// parent

printf (“child PID %d\n”, child);

}

exit – system call-example

.

process group1

Processes on UNIX are identified by a unique ID number and by group id number. Both ids are saved in the process table.

  • Kernel uses process group number to identify group of related processes that should receive a common signal.
  • Processes that have a common ancestor process that is a login shell receives a common signal when the user hits control-d character.
  • setpgrp system call sets the group id equal to the process id.
      • grp = setpgrp ();

process group

.

exec system call

exec invoke another program and overlay the memory space of the process with the copy of the executable file.

  • The old user context is no longer accessible except for the exec’s parameters.
  • exec (filename, argv, envp)
      • filename is the name of the file to be executable
      • argv is a pointer to an array of characters that are parameters to the program.
      • envp is a pointer to an array of characters which have the environment of the executable program (e.g. name = value).
      • execl. execv, execle etc are different versions of exec.
  • exec access the file’s inode via algorithmnamei, Determine that it is executable, user has permission to execute it.
  • Since parameters to exec is part of the old memory space about to be freed, the kernel copy argv and envp to holding place such as the kernel stack.

exec – system call

.

exec system call continue

Detach the old region using detachreg.

  • The kernel allocates and attaches regions for text and data, load the contents of the executable file into memory (allocreg, attachreg, and loadreg).
  • The data regions is divided into two parts: initialized at compile time and not initialized data regions. The kernel allocate region (allocreg) for the initialized data region, attach it (attachreg) and initializes the value of memory to 0, for the not initialized data region it increase the size of data region using the growreg.
  • allocate (allocreg) and attach (attachreg) a user stack to the process. Copy the exec parameters into the user stack.
  • Initialize the stack and program counter registers.
  • Release the inode which are allocated by namei in the beginning of exec using iput.
  • The process id stays the same and its position in the process hierarchy stays the same, but only the user contexts change.

exec – system call (continue)

.

exec algorithm

exec

Input : file name

parameter list

environment variable list

{

get file inode (namei)

verify file executable, user has permission to execute

read file headers, check that it is load module

copy exec parameters from old address space to system space

for (every region attached to the process)

detach all regions (detachreg)

for (every region specified in load module)

allocate new region (allocreg)

attach the region (attachreg)

load region to memory if appropriate (loadreg)

copy exec parameters into new user stack region

initialize registers (e.g. program counter & stack register)

release inode of file (iput)

}

exec – algorithm

.

exec system call example

main ()

{

int status;

if (fork () == 0)

exec (“/bin/date”,”date”,0);

wait (&status)

}

exec – system call-example

  • The kernel finds that the /bin/date is an executable file and all users can execute it.
  • The kernel copy “/bin/date”, “date”, into a holding place (e.g. kernel stack).
  • Free text, data, and stack regions occupied by the process.
  • Allocate new text, data, and stack . Copy the instructions of /bin/date/ into the text region, and copy the data area into the data region.
  • The kernel copies the argument “date” into the user stack.
  • After the exec the child process is executing the “date” program.
  • When the date program terminates, the parent process receives its exit status from the wait call.

.

exec executable file structure

Primary Header

Section 1 Header

exec – executable file structure

Section n Header

Section 1

Section n

  • primary header: describe how many sections in the file, the start address for the process execution, the magic number which identifies the executable file.
  • Section Headers: describe the section size, type and virtual address for the section.
  • Data:the section data contains information such as the text that is initially loaded in the process address space. Section data could contains symbol table or debugging information.

.

changing the size of the process s data region

Two functions to change the size of the process

    • brk (ends), ends becomes value of the highest virtual address of the data region of process and is called its break value
    • oldends = sbrk (inc) inc, change the current break value by inc number of bytes, oldends is the break value before the call.
  • Kernel checks if the new process size is less than the system maximum.
  • The new data region doesn’t overlap with other regions.
  • If all checks pass the kernel invoke growreg to allocate auxiliary memory (e.g. page tables) for the data region and increments the process size.
  • It tries to allocate memory for the new space and initialize it to 0. If not able to allocate memory, it swaps the process out until the new space is available.
  • The new increased space is virtually contagious with the old one.

changing the size of the process’s data region

.

brk algorithm

brk

Input : new break value

Output: old break value

lock process data region

if (region size is increasing & new region size is illegal)

unlock data region

return error

change region size (growreg)

zero out addresses in new data space

unlock process data region

brk – algorithm

.

brk example

Main ()

{

char *endpt;

endpt = sbrk(0);

printf (“endpt = %ud \n”, endpt);

while (endpt -- )

{

if (brk (endpt) == -1)

{

printf (“brk of %ud failed\n”,endpt);

exit ();

}

}

}

brk – example

.

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