Review

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 xxxx
• float result2 1.0/(xxxx)
• 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 gt 0 ) ...
• if ( x Double.NEGATIVE_INFINITY ) ...
• if ( Double.isInfinite(x) x lt 0 ) ...
• / for NaN there is no constant. Use function
  /
• if ( Double.isNaN(x) ) ...
• In C use
• include ltnumericgt
• numeric_limitsltdoublegt 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
time doSomeWork( ) 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)

• but both output values are always the same! Why?

Job started Wed Dec 14 2005 131439 PM Job
finished Wed Dec 14 2005 131439 PM
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!

Static buffer
char start ctime(time1)
Tue Dec 13 2005 100 AM
char start
Wed Dec 14 2005 1300 PM
char stop ctime(time2)
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 buf80
• ...
• return buf

stack
Wed Dec 14 2005 130000 PM\n
char time ctime(time1) ... ... x
fun(10,20,30) ... printf("s\n", time)
000000000000010100000000000001010000000000000011
110
13
Weird functions why static memory?

• why not use malloc (heap dynamic) variable?
• ctime( time_t time )
• char buf (char )malloc(80)
• ...
• return buf

heap
Wed Dec 14 2005 1100 AM Wed Dec 14 2005 1200
AM Wed Dec 14 2005 1300 PM
char time time ctime(time1) ... time
ctime(time2) ... time ctime(time3)
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

Which of these assignments are allowed? Is the
test condition true?
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?
17
Constants

• Manifest constant (literal)
• const int BUFSIZE 1024
• char bufBUFSIZE
• Compile time (everything is known)
• const int BUFSIZE 8 128
• Load time (determined when program is loaded)
• static final String user System.getenv("user
  name") / java /
• Run-time
• int fun(const int n) const size nn ...
• 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 ltmath.hgt / hypot is an error in C but
not in C ! / float hypot(float x, float y)
return sqrt(xx yy)
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