1. Unix-v5 Process Structure
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2. 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.
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3. Process State Transition Diagram
Interrupt, interrupt return
User running
Sys call, interrupt
return
Return to user
Zombie
exit
Kernel Running
preempt
preempted
sleep
Reschedule Process
wakeup
Sleep, memory
Ready run,
memory
Swap In
Enough memory
Swap out
fork
Swap out
created
wakeup
Sleep, swapped
Ready run,
swapped
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4. Process State Transition Diagram
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
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5. text Data Stack Process Structure
Process consists of 3 regions. Region is a contagious area of the virtual address space
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6. User Area - U text Data Stack text Data Stack
Process A
text
Data
Stack
U Area
Physical memory
Same virtual address
Process B
text
Data
Stack
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.
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7. Data structure for a process
Per process region table
U Area
Region table
Process table
text
data
stack
memory
Per process region table allows independent processes to share regions.
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8. exec a process memory U Area A U Area B Old process deallocate
Region table
Per process region table
Process table
text
data
stack
text
data
stack
U Area B
memory
File descriptor table
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9. Per process region table
fork a process
U Area Parent
Region table
File descriptor table
Process table
text
data
stack
Per process region table
text
data
stack
U Area Child
memory
File descriptor table
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10. Per process region table
create a thread
U Area A
File descriptor table
text
data
stack
text
data
stack
Process table
Per process region table
Region table
memory
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11. (page number, byte offset in page)
Paging memory layout
Page 0
Page 1
Page 2
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.
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12. Page table
87k
1096k
941k
552k
2001k
727k
Page table
Page n
Page n+1
Page n+2
Memory
Page table maps virtual address into physical address. Also, it contains access privileges.
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13. Process virtual space is divided into two classes
text
data 1M
stack 2M 4M
kernel
user
Process virtual space is divided into two classes
Kernel space and associated kernel mode.
User space and associated user mode.
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14. Layout of the kernel
Page table address Virtual address no of pages
text
data 1M
stack 2M 4M
kernel
user
541k
783k
986k
87k
941k
552k
727k
empty
764k
137k
852k 0k 4k
17k
128kk
139k
97k
135k
256k
279k
292k
304k
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.
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15. Per process Region table
User Area
Per process Region table
text
data 1M
stack 2M
U area 4M
Page table address Virtual address no of pages
text
data 1M
stack 2M
U area 4K 4M
P A
kernel
user
Page tables
text
data 1M
stack 2M
U area 4M
114k
565k
708k
143k
87k
941k
552k
727k
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.
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16. 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
Inode pointer to the file
Size of the region
status
Count number of processes that reference the region
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17. Region Table Region table is divided into:
Active list
free list
Region table is divided into:
A linked list of active elements.
A linked list of free elements.
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18. 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.
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19. 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.
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20. 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 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.
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21. Allocreg -allocate region (exec example)1
Region table
Active list
Active list
free list
free list
Allocate first free element in region table 2
Inode table
Active list
free list
Set region to point to file inode
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22. 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.
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23. attachreg -attach region
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) }
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24. attachreg -attach region (exec example)
process table
Region table
Per process Region table 4
Allocate new text entry (1)
114k
565k
708k
143k
Allocate new page table
Attach an existing shared text region of size 4 Kbytes to virtual address 0 of the process.
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25. 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.
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26. growreg – grow the region size
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.
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27. growreg – grow the region size
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 }
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28. growreg – grow the region size
Per process Region table
Per process Region table
128 4
128 5
stack
stack
114k
565k
708k
143k
114k
565k
708k
143k
976k
New page
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29. 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.
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30. 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 }
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31. 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.
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32. dupreg – duplicate region
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 }
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33. dupreg – duplicate a region
Per process Region table
Text
Data
Shared text
stack
Process A
Private data
Per process Region table
Private stack
Text
copy
Data
stack
Process B
Private data
Private stack
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34. 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.
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35. loadreg – load region 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 }
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36. loadreg – load region
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.
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37. Per process Region table
loadreg – load region
Region table
Per process Region table
Page table addr
Proc virtual address
Size and protect 1 2
allocreg: allocate a region for the executable file.
attachreg: attach the new region to the process.
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38. Per process Region table
loadreg – load region
Region table
Per process Region table
Page table addr
Proc virtual address
Size and protect 1
empty
growreg: Allocate page map table of one empty entry. The size will be increased by one.
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39. Per process Region table
loadreg – load region
Region table
Per process Region table
Page table addr
Proc virtual address
Size and protect 8
empty
565k
708k
143k
976k
loadreg:
growreg: to allocate memory for the file to be loaded.
Load the file to be executed.
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40. 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.
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41. detachreg – detach region
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);
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42. Process Control
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43. 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.
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44. 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.
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45. Fork Creating a new process context
data
stack
Per process
region table
Parent data
Parent user stack
Open Files
Current Directory
Changed Directory
U Area
Kernel Stack
Parent Process
File Table
Shared Text
text
data
stack
Per process
region table
Child data
child user stack
Open Files
Current Directory
Changed Directory
U Area
Kernel Stack
Child Process
inode Table
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46. process group
Process group leader A
pid =456 gid =456
pid =555 gid =555
pid =666 gid =666
pid =123 gid =456
pid =777 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).
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47. 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
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.
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48. 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)
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49. 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);
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50. fork example (continue)
input : abcde
Output : abcde
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51. exit – system call exit (status)
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.
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52. exit – system call if (child process in zombie state)
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 }
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53. 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);
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54. process group grp = setpgrp ();
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 ();
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55. 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 algorithm namei, 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.
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56. 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.
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57. 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) }
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58. exec – system call-example
main () {
int status;
if (fork () == 0)
exec (“/bin/date”,”date”,0);
wait (&status) }
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.
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59. exec – executable file structure
Magic Number
Number Of Sections
Initial Register Values
Section Type
Section Size
Virtual Address
Data (e.g. text)
Primary Header
Section 1 Header
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.
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60. 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.
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61. 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
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62. 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 (); }
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