Vivek Pai

1
OS Structure Processes

• Vivek Pai
• COS 318
• September 20, 2001

2
Foodundgedank

• Youre the host/hostess of a banquet hall that
  has accidentally been double-booked. It has three
  rooms the large dining room, the cocktail room,
  and a small buffet line room.
• If each guest is given a card with the room
  layout with table numbers, and the guests own
  table number, how can you solve this problem?
• What issues are involved regarding the sizes of
  the two groups?

3
Administrative

• First precept happened, more to come
• Room scheduling not finalized, but 103 likely
• Friday may be at 330 instead of 300
• Politely remind me if Im running late
• Ive forgotten about the feedback paper
• Next reading assignment 1.4, 1.5

4
A Friendly Deception

• Hiding information
• Giving out different information while appearing
  to be the same
• Stalling when necessary
• Drawback? Maybe lots of extra work
• Benefit makes life easier for user

5
Host/Hostess Problem

• Each group lt 50
• Have two sets of cards with different numbering
  schemes
• Each group slightly above 50
• Slow down the buffet line entrance
• Each group near 100
• One group in dining room, other in cocktail room

6
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 are instances of a program
• The program can really be a set of processes

7
Programs As Process Collections

• Netscape (output of ps x)
• 7253 p0 S 019.26 /usr/local/lib/netscape/c
  ommunicator-4.75.bin -irix-s
• 7280 p0 I 000.15 (dns helper)
  (communicator-4.7)
• gcc (via gcc pipe v)
• /usr/libexec/cpp
• /usr/libexec/cc1
• /usr/libexec/as, followed by
• /usr/libexec/elf/ld

8
gcc example

• You launch gcc
• It first launches cpp, cc1, as
• It then launches ld
• Each instance is a process, and each program
  actually exists separately
• You could launch cc1 manually if you wanted

9
So What Is A Process?

• Its one instance of a program
• Its separate from other instances
• It can start (launch) other processes
• It can be launched by them

10
What Does This Program Do?

• int myval
• int main(int argc, char argv)
• myval atoi(argv1)
• while (1)
• printf(myval is d, loc 0xlx\n,
• myval, (long) myval)

11
Heres The Output
12
Instances Of Programs

• The address was always the same
• The values were different
• Implies that the programs arent seeing each
  other
• But they think theyre using the same address
• Conclusion addresses are not absolute
• Whats the benefit?

13
So Whats In A Process?

• Information about the hierarchy
• What launched it
• What it has launched
• Information about resources
• Where is it storing data
• What other resources its using
• Various kinds of mappings

14
Consider How To Read a File

• Compare read( ) and fread( )
• read(int d, void buf, size_t nbytes)
• read() attempts to read nbytes of data from the
  object referenced by the descriptor d into the
  buffer pointed to by buf.
• fread(void ptr, size_t size, size_t nmemb, FILE
  stream)
• The function fread() reads nmemb objects, each
  size bytes long, from the stream pointed to by
  stream, storing them at the location given by ptr.

15
Which is a System Call and Why

• read(int d, void buf, size_t nbytes)
• fread(void ptr, size_t size, size_t nmemb, FILE
  stream)
• Both do the same thing, right?
• What gets exposed in each case
• What about a lying programmer
• How hard is it to tell he/she is lying
• What other malicious actions can occur?

16
What State Is Implied?

• Consider the following sequence
• read(int d, void buf, size_t nbytes)
• read(int d, void buf, size_t nbytes)
• read(int d, void buf, size_t nbytes)
• What happens if two programs were doing this?

17
Some Insight Into OS
Array of File Pointers
Process
Actual File Info
What do we gain from this?
18
Some Insight Into OS
Array of File Pointers
Process
Actual File Info
Process
19
Examining Those Other Parameters

• read(int d, void buf, size_t nbytes)
• fread(void ptr, size_t size, size_t nmemb, FILE
  stream)
• Where is buf, bufnbytes)?
• Where is ptr, ptrsizenmemb)?

20
Remember This?
Application
Libraries
User space/level
Kernel space/level
Portable OS Layer
Machine-dependent layer
21
Address Space

• One (common) approach
• Kernel is high memory
• User is low memory
• What restrictions apply?
• read(f, buf, nbytes)

0xffff. Kernel space User space 0x0000
22
Some Definitions

• Kernel heart of the operating system
• Like the program file generated by compiling all
  of the operating system files
• Operating system usually includes more
• Various libraries
• Support programs
• Hard and fast definitions? Rarely

23
More Address Space

• Program segments
• Text
• Data
• Stack
• Heap
• Any obvious choices?

0xffff. Kernel space User space 0x0000
24
One Common Layout

• Lots of flexibility
• Allows stack growth
• Allows heap growth
• No predetermined division
• So what happens when you
• make a system call?

0xffff. Kernel space Stack Heap Code
Data 0x0000
25
Kernel Stack

• Kernel contains functions
• Just like regular functions
• Automatic variables, formal parameters, etc
• They need a stack
• Can we use the users stack?
• Is it possible?
• Is it desirable?

26
What Other Options Possible?

• Remember the host/hostess scenario?
• What would happen if you had a break between
  courses
• Can you apply this logic to the kernel?
• What are the tradeoffs?