This document is a presentation from Emery Berger at the University of Massachusetts Amherst on operating systems and C/C++. It discusses why C and C++ are commonly used for operating systems and low-level programming due to their direct memory access and efficiency. It also outlines the key topics to be covered in the presentation, including basics of C/C++, pointers, functions, classes, and differences between C/C++ and Java.

1. Operating Systems
CMPSCI 377
C/C++
Emery Berger
University of Massachusetts Amherst
UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science

2. Why C?
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3. Why C?
Low-level
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Direct access to memory
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WYSIWYG (more or less)
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Effectively no runtime system
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No garbage collector
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No other threads
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No “read” or “write barriers”
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Efficient
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Space & time
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C: effectively portable assembly code
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4. OK, Why C++?
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5. OK, Why C++?
C++: extends C
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Upwardly-compatible
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Adds significant software engineering
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benefits
Classes
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Encapsulation (private)
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Templates (“generics”)
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Other modularity advantages
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Inlining instead of macros
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6. Outline, part I
Basics – compiling & running
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Intrinsic types, conditionals, etc.
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Pointers + Reference variables
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Assignment
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Objects
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&, *, ->
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Stack vs. heap
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7. Outline, part II
Functions
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Parameter passing
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Structs & classes
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Overloading & inheritance
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Stack vs. heap
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I/O, command-line
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STL
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8. Basics
Main & compilation
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9. Intrinsic Types
Essentially identical
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10. Conditionals
Mostly the same
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C/C++: nonzero int same as true
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11. File I/O
Simple stream-based I/O
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print foo
cout << “foo”
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read x from the console
cin >> x
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12. Command-line Arguments
Again, similar to Java
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13. Key Differences
Differences between C/C++ and Java
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Assignment
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Pointers
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Parameter passing
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Heap & Stack
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Arrays
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14. Assignment
Java assignment: makes reference
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C++ assignment: makes copy
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15. Pointers & Friends
Pointers are like jumps, leading wildly from
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one part of the data structure to another.
Their introduction into high-level languages
has been a step backwards from which we
may never recover.
C.A.R. Hoare
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16. Pointers & Friends
Concept not in Java: address manipulation
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17. Functions & Parameter Passing
C/C++ – all parameters copied by default
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18. Parameter Passing
To change input, pass pointer
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or call by reference
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19. Pass by Reference
Syntactic sugar:
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foo (int &i) = pass by reference
Secretly does pointer stuff for you
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20. Stack & Heap
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21. Stack & Heap
In C/C++ as in Java, objects can live on
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stack or on heap
Stack = region of memory for temporaries
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Stack pointer pushed on function entry
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Popped on function exit
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Heap = distinct region of memory for
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persistent objects
C/C++ – explicitly managed
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Pointers introduce problems!
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22. The Stack
Stack data: new every time
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23. Big Stack Mistake
Never return pointers to the stack!
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24. The Heap
Allocate persistent data on heap with new
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25. Explicit Memory Management
Java heap – garbage collected
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C/C++ – explicit memory management
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You must delete items (or memory leak)
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Delete them too soon (still in use) – crash
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“Dangling pointer” error
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Delete something twice – crash
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“Double-free” error
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26. Classes & Objects
No “top” object (as in Java Object)
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Also: C++ has no interfaces but has multiple
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inheritance – stay far away
Key difference for you – not all methods
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dynamically-dispatched
Methods associated with declared type
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rather than dynamic type unless labeled
virtual
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27. Struct Member Access
struct = class with everything public
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Use these sparingly
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28. Class Declaration
Pretty similar
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29. Arrays
Numerous differences
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Arrays do not have to be allocated with new
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Array bounds not checked
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Item[0] = pointer to start of array
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Arrays just syntactic sugar
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for pointer arithmetic! (scary! avoid!)
v = 12; *(Item + v) = 1;
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Same as Item[12] = 1;
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Note: sizeof(x) = number of bytes to hold x
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Multi-dimensional arrays (matrices)
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just arrays of pointers to arrays
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30. Other Features
Operator overloading
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New meanings to existing operators
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int operator+(MyType& a, MyType& b);
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Controversial, but useful for things like
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complex math, matrix operations
int& operator()(int x, int y);
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Templates
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A.k.a. generics in Java
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template <class X> void foo (X arg);
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