1

Review

2

Creating a Runnable Program

What is the function of the compiler?

What is the function of the linker?

Java doesn't have a linker.If the "main" class uses other classes, what happens?

does javac combine all classes in one class file? does java load the classes? when?

what does this statement do?

Class.forName("com.mysql.jdbc.Driver");

3

Linker The linker "resolves" external references, it fills in the missing

values in object files. But, it doesn't check that the names match the expected type!

Example: homework 6A problem 1:

/* File 1 */char *foo;extern double bar;...printf("%s", foo);printf("%f", bar);

/* File 2 */extern double foo;char* bar = "foo!";...printf("%d", foo);printf("%s", bar);

4

Homework 4

There are two parts to a language: syntax is the grammar rules for writing (or saying) the

language. It describes what are legal expressions. The grammar has 2 parts.

lexical grammar - rules for tokens (words, punctuation, space)

syntactic grammar - rules for statements What notation is commonly used for these two parts? semantics describes the meaning of the language

usually given in a manual (in human language) formal semantics: axiomatic, denotational, and operational

semantics precisely describe effect of language on the computer's

"state", can be used to prove program correctness.

5

Homework 5

Representation of primitive data types: how big is an int, long, float, double? what's the largest value you can store in a long? what format is used to store signed integers? If you look at a (signed) integer in memory and see:

111111111111111111111111111111111111111111

what value does this represent? If you look in memory using hexadecimal format and see:

7FFFFFFF

is this a positive or negative value? What's the value?

6

Homework 5

Representation of primitive data types: floating point numbers are stored as 3 components. What are the 3

components? the largest float is about 2 x 2126 ( 255 - bias ), so the smallest value

should be 1 x 2-127 ( 0 - bias ), but its not! why is the smallest float 1 x 2-149 ( 1.4E-45 ). Why? what floating point is returned in these cases, using IEEE standard

floating point: float x = 1.0E20; float result1 = x*x*x*x; float result2 = 1.0/(x*x*x*x); double zero = 0.0; double result3 = 1.0/zero; double result4 = zero * result1;

In Java, how can you test if x (say, "double x") is Infinity? Test for x is NaN?

7

Test for Infinity and NaN

In Java...double x = ...;

if ( x == Double.POSITIVE_INFINITY ) ...

if ( Double.isInfinite(x) && x > 0 ) ...

if ( x == Double.NEGATIVE_INFINITY ) ...

if ( Double.isInfinite(x) && x < 0 ) ...

/* for NaN there is no constant. Use function */

if ( Double.isNaN(x) ) ...

In C++ use:

#include <numeric>

numeric_limits<double> dbl;

if ( x == dbl.infinity( ) ) ...

8

3 Memory areas

The memory area allocated for a program's data is divided into 3 areas. What are their names?

9

3 Memory areas

The memory area allocated for a program's data is divided into 3 areas. What are their names?

Static area Stack Heap

How are these three areas used? What sort of values are placed there?

depends on the language: statically typed languages generally use them as discussed in class

dynamic typed languages (Perl, SmallTalk, Scheme) use the heap for most things

10

Weird functions

A programmer writes a timer like this:

time_t tstart = time(0); // starting timedoSomeWork( );time_t tstop = time(0); // stopping time/* now print when the program started/stopped */char* start = ctime( tstart );char* stop = ctime( tstop );printf("Job started: %s\n", start);printf("Job finished: %s\n", stop);

Job started: Wed Dec 14 2005 13:14:39 PMJob finished: Wed Dec 14 2005 13:14:39 PM

but both output values are always the same! Why?

11

Weird functions: static memory ctime uses a static buffer to create a string of the formatted time

("Wed Dec 14 ..."). it returns a pointer to this buffer. it reuses the same buffer each time it is invoked!

char* start = ctime(time1); Tue Dec 13 2005 1:00 AM

char* start Wed Dec 14 2005 13:00 PM

char* stop = ctime(time2);

Static buffer:

12

Weird functions: why static memory? Why use static memory for the string buffer? why not a local (stack automatic) variable?

ctime( time_t time ) {

char buf[80];

...

return buf;

Wed Dec 14 2005 13:00:00 PM\n

char* time = ctime(time1);......x = fun(10,20,30);...

printf("%s\n", time);

000000000000010100000000000001010000000000000011110

stack:

13

Weird functions: why static memory? why not use malloc (heap dynamic) variable?

ctime( time_t time ) {

char* buf = (char *)malloc(80);

...

return buf;

Wed Dec 14 2005 11:00 AMWed Dec 14 2005 12:00 AMWed Dec 14 2005 13:00 PM

char* time;time = ctime(time1);...time = ctime(time2);...time = ctime(time3);

heap:

14

Alignment as described in class, int, float, and double values must usually be aligned

on word boundaries (word = 4 bytes) some CPU require "double" aligned on double word boundaries (8 bytes). how much space is used by these struct variables?

struct one {char c;double d;} x;

/* try changing order */struct two {

double d;char c;} y;

struct three {char c1;double d1;char c2;double d2;char c3;double d3} z;

15

Data type compatibility

each language has rules for compatibility of data types C++, Java, C#: "widening" conversions allowed; others

require a cast and may throw an exception. what about compatibility of user defined types?

double dbl = 3.5;float flt = 1.2F;int nint = 1234567890;long nlng = 12345678901234567890;/* OK */dbl = flt; dbl = nlng; dbl = nint;flt = nlng; flt = nint;nlng = int;/* Error (but OK in C) */flt = dbl; nint = nlng; nlng = float;

is there a problem here?

16

Type Compatibility for user types

struct A {float x;char c;};

struct B {float x;char c;};

struct C {float z;char c;};

struct D {char c;float x;

};

int main( ) {struct A a;struct B b;struct C c;struct D d;a.x = 0.5;a.c = 'a';b = a; // ??c = a; // ??d = a; // ??if (b == a) // true?

Which of these assignments are allowed?

Is the test condition true?

17

Constants

Manifest constant (literal):

const int BUFSIZE = 1024;

char buf[BUFSIZE]; Compile time (everything is known):

const int BUFSIZE = 8 * 128;

Load time (determined when program is loaded):

static final String user = System.getenv("username"); /* java */

Run-time:int fun(const int n) { const size = n*n; ... }

final int width = 400; /* java instance var */

18

Scope of Names

Categories of scope rules: lexical (static) scope - scope known by looking at the code. dynamic scope - follows flow of execution

most languages use lexical scope, but each has its own rules about redefining names in inner scopes.

example of name conflict:

#include <math.h>

/* hypot is an error in C but not in C++ ! */

float hypot(float x, float y) {

return sqrt(x*x + y*y);

}

19

Keeping Track of Names Compiler (or interpreter) uses a symbol table to record

known names and their attributes (bindings). For lexical scope, there are two approaches:

a new symbol table for each scope: a stack of symbol tables

a linked list of definitions for each symbol (name), elements of the linked list have scope number

to keep track of active scopes: scope stack Symbol table is not needed after program is linked, but is usually hidden

in the executable code to aid debugging. To make program smaller or inhibit reverse engineering: strip C#: "Dotfuscation"

pun on "obfuscation" - to make confused or opaque