Computer Science, Computer Architecture
Content
Compsci 250 / ECE 250:
“C Programming”
Alvin R. Lebeck
Some slides based on those from Daniel Sorin,
Andrew Hilton, Amir Roth, Gershon Kedem
Administrivia
• Homework #1
• No class Monday
• Today
! Finish Data Representations
! C programming & Memory
• Updated Academic Honesty Policy
! Do not provide solutions to any archive (digital, paper, etc.)
! This is a simple extension of existing policy of not providing
solutions to another student.
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
2
Review: Answer
What floating-point number is
0xC1580000?
1100 0001 0101 1000 0000 0000 0000 0000
31 30
23 22
0
1 1000 0010 101 1000 0000 0000 0000 0000
S
•
•
•
•
N
M
Sign = 1 means this is a negative number
Exponent = (128+2)-127 = 3
Mantissa = 1.1011
-1.1011x23 = -1101.1 = -13.5
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
3
Special values in IEEE Floating Point
• How do you represent 0.0 in IEEE Floating Point?
! 0.0 = 000000000
! But need 1.XXXXX representation?
• Exponent = 0 is denormalized
! Implicit 0. instead of 1. in mantissa
! Allows 0000….0000 to be 0
! Helps with very small numbers near 0
• Results in +/- 0 in FP (but they are equal )
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
4
Other Weird FP numbers
• Exponent = 1111 1111 also not standard
! If all 0 mantissa: +/- ∞
1/0 = +∞
-1/0 = -∞
! If non zero mantissa: Not a Number (NaN)
sqrt(-42) = NaN
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
5
FP Addition/Substraction
• Addition/Subtraction
! Align the exponents
! Perform operation
! Renormalize the result (put into 1.M x 2E-127 form)
• What can possibly go wrong?
• Well a lot
• Search for disasters caused by numerical errors
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
6
Software Implication! Rounding Errors
• We only have 32-bits to represent floats
! Must approximate some values
•
•
•
•
Limited bits for mantissa
Does (x+y)*z = (x*z+y*z)?
Mathematically yes, but assumes infinite precision
Assume base 10, four digits available (two to left, two to
right of decimal point)
!
!
!
!
x = 99.96 x 103
x = x + 0.07
x = 100.03 x 103
x = 10.00 x 104
• Numerical Analysis (CS 220) studies some of these
issues
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
7
Floating Point Representation
• Double Precision Floating point:
64-bit representation:
! 1-bit sign
! 11-bit (biased) exponent
! 52-bit fraction (with implicit 1).
• double in Java, C, C++, …
S
Exp
1
11-bit
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Mantissa
52 - bit
Compsci / ECE 250
8
Oct. Char
000
010
020
030
040
050
060
070
100
110
120
130
140
150
160
170
nul
bs
dle
can
sp
(
0
8
@
H
P
X
`
h
p
x
ASCII Character Representation
001
011
021
031
041
051
061
071
101
111
121
131
141
151
161
171
soh
ht
dc1
em
!
)
1
9
A
I
Q
Y
a
i
q
y
002
012
022
032
042
052
062
072
102
112
122
132
142
250
162
172
stx
nl
dc2
sub
"
*
2
:
B
J
R
Z
b
j
r
z
003
013
023
033
043
053
063
073
103
113
123
133
143
153
163
173
etx
vt
dc3
esc
#
+
3
;
C
K
S
[
c
k
s
{
004
014
024
034
044
054
064
074
104
114
124
134
144
154
164
174
eot
np
dc4
fs
$
,
4
<
D
L
T
\
d
l
t
|
005
015
025
035
045
055
065
075
105
115
125
135
145
155
165
175
enq
cr
nak
gs
%
5
=
E
M
U
]
e
m
u
}
006
016
026
036
046
056
066
076
106
116
126
136
146
156
166
176
ack
so
syn
rs
&
.
6
>
F
N
V
^
f
n
v
~
007
017
027
037
047
057
067
077
107
117
127
137
147
157
167
177
bel
si
etb
us
'
/
7
?
G
O
W
_
g
o
w
del
• Each character represented by 7-bit ASCII code (packed into 8-bits)
• Convert upper to lower case ‘A’ + 3210 = ‘a’
• Other tables with decimal and HEX values.
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
9
Unicode
• Many types
• UTF-8: variable length encoding backward compatible
with ASCII
! Linux
• UTF-16: variable length
! Windows, Java
• UTF-32: fixed length
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
10
Data Representation Summary
•
•
•
•
Need efficient representations
2’s complement for signed integers
IEEE Floating Point for Real numbers
Bits are bits are bits, same bits can be interpreted as
different types (unsigned, 2’s complement, float, etc.)
• Next up: C programming & memory
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
11
What you know today
• JAVA
...
System.out.println("Please Enter In Your First Name: ");
String firstName = bufRead.readLine();
System.out.println("Please Enter In The Year You Were Born: ");
String bornYear = bufRead.readLine();
System.out.println("Please Enter In The Current Year: ");
String thisYear = bufRead.readLine();
int bYear = Integer.parseInt(bornYear);
int tYear = Integer.parseInt(thisYear);
int age = tYear – bYear ;
System.out.println("Hello " + firstName + ". You are " + age + " years
old");
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
12
How does a Java program execute?
• Compile Java Source to Java Byte codes
• Java Virtual Machine (JVM) interprets/translates Byte
codes
• JVM is a program executing on the hardware…
• Java has lots of things that make it easier to program
without making mistakes
• JVM handles memory for you
! What do you do when you remove an entry from a hash table,
binary tree, etc.?
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
13
The C Programming Language
• No virtual machine
! No dynamic type checking, array bounds, garbage collection, etc.
! Compile source file directly to machine (this is a little simplified)
• Closer to hardware
! Easier to make mistakes
! Can often result in faster code
• Generally used for ‘systems programming’
! operating systems, embedded systems, database implementation
! There is object oriented C++ (C is a strict subset of C++)
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
14
The C Programming Language (Continued)
• No objects
• Procedural, not object oriented
! No objects with methods
• Structures, unions like objects
! Member variables (no methods)
•
•
•
•
Pointers – memory, arrays
External standard library – I/O, other facilities
Macro preprocessor (#<directive>)
Additional Resources
! Kernighan & Richie book The C Programming Language
! MIT open course Practical Programming in C link in ‘docs’ of web
site
! Drew Hilton Video Snippets
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
15
Compiling and Running a C Program
•
•
•
•
•
Use the gcc program to create an executable file
gcc –o <outputname> <source file name>
gcc –o hello hello.c (must be in same directory as hello.c)
If no –o option, then default output name is a.out (e.g., gcc hello.c)
Type the program name on the command line
! ./ before “hello” means look in current directory for hello program
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
16
Debugging
• Print debugging…
! Just output information at different points in the program
! Not the most efficient, but often works.
• gdb <executable filename>
• Good for stopping at set points in program and inspecting
variable values.
! If you get good at using a debugger it is easier/better than printf
debugging…
! See GDB Essentials video off https://www.cs.duke.edu/courses/
spring15/compsci250/docs.html
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
17
Variables, operators, expressions
• C variables are similar to Java
! Data types: int, float, double, char, void
! Signed and unsigned int
! char, short, int, long, long long can all be integer types
» These specify how many bits to represent an integer
• Constants
! Use #define C preprocessor
! E.g.,: #define MAX_SCORE 100
• Operators:
! Mathematical +, -, *, /, %,
! Logical !, &&, ||, ==, !=, <, >, <=, >=
! Bitwise &, |, ~, ^ , <<, >> (we’ll get to these next)
• Expressions: var1 = var2 + var3;
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
18
Bitwise AND / OR / XOR
•
•
•
•
& operator performs bitwise AND
| operator performs bitwise OR
^ operator performs bitwise Exclusive OR (XOR)
~ operator performs bitwise NOT
• Per bit
0&0=0
0|0=0
0^0=0
0&1=0
0|1=1
0^1=1
1&0=0
1|0=1
1^0=1
1&1=1
1|1=1
1^1=0
• For multiple bits, apply operation to individual bits in same position
AND
011010
101110
001010
© Alvin R. Lebeck
From Sorin, Hilton, Roth
OR
011010
101110
111110
XOR
011010
101110
110100
Compsci / ECE 250
19
The SHIFT operator
• >> is shift right, << is shift left, operands are unsigned int
and number of positions to shift
• (1 << 3) is …000001 -> …0001000 (it’s 23)
• 0xff00 is 0xff << 8, and 0xff is 0xff00 >> 8
! Unsigned ints, be careful shifting signed integers (more later this
semester)
• 0xAB & 0x0F = 0x0B
• 0xAB & 0xF0 = 0xA0
• 0x0F and 0xF0 are called a bit mask
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
20
Sakai In Class Quiz 2.1
button
y
x
val = 01 1010 0011 0100 1100
• 32-bit word with x,y and button fields
!
!
!
!
bits 0-7 contain x position
bits 8-15 contain y position
bits 16-17 contain button (0 = left, 1 = middle, 2 = right)
Bits 18-31 don’t matter
• Which C Statements correctly set xpos, ypos & button?
A.
xpos = val && 0x000ff;
ypos = (val && 0x0ff00) >> 8;
button = (val && 0x30000) >> 16;
C.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) << 8;
button = (val & 0x30000) << 16;
© Alvin R. Lebeck
From Sorin, Hilton, Roth
B.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) >> 8;
button = (val & 0x30000) >> 16;
D.
xpos = val & 0x000ff;
ypos = val & 0x0ff00 >> 1;
button = val & 0x30000 >> 2;
Compsci / ECE 250
21
Sakai In Class Quiz 2.1: Answer
button
y
x
val = 01 1010 0011 0100 1100
• 32-bit word with x,y and button fields
!
!
!
!
bits 0-7 contain x position
bits 8-15 contain y position
bits 16-17 contain button (0 = left, 1 = middle, 2 = right)
Bits 18-31 don’t matter
• Which C Statements correctly set xpos, ypos & button?
A.
xpos = val && 0x000ff;
ypos = (val && 0x0ff00) >> 8;
button = (val && 0x30000) >> 16;
Logical AND
C.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) << 8;
button = (val & 0x30000) << 16;
Shift wrong direction; value not
in least significant bits
© Alvin R. Lebeck
From Sorin, Hilton, Roth
B.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) >> 8;
button = (val & 0x30000) >> 16;
D.
xpos = val & 0x000ff;
ypos = val & 0x0ff00 >> 1;
button = val & 0x30000 >> 2;
Precedence…shifts mask first; shift
amount wrong
Compsci / ECE 250
22
Functions
• Encapsulate computation
! Reuse or clarity of code
! Cannot define functions within functions
• Must be declared before use!
int div2(int x,int y); /* declaration here */
main() {
int a;
a = div2(10,2);
}
int div2(int x, int y) { /* implementation here */
return (x/y);
}
• Or put functions at top of file (doesn’t always work)
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
23
Global Variables
• Global variables: Accessible from any function
#include <stdio.h>
float f = 0;
void bar () {
f = 0.5;
}
int main()
{
f =0.31234;
bar();
printf(“The value is %f \n”, f);
}
Output is:
The value is 0.5
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
24
Memory Partitions
2n-1
• Text for instructions
Stack
! add res, src1, src2
! mem[res] = mem[src1] +
mem[src2]
Typical
Address
Space
• Data
! static (constants, global variables)
! dynamic (heap, malloc / new
allocated)
! grows up
Heap
• Stack
Data
! local variables
! grows down
Text
• Variables are names for
memory locations
0
Reserved
! int x;
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
25
A Simple Program’s Memory Layout
...
int result; // global var
int main()
{
int x;
...
result = x + result;
...
}
For now, think of this as performing:
mem[0x208] = mem[0x400] + mem[0x208]
2n-1
x 0x400
Compsci / ECE 250
5
Heap
result 0x208
Data
3
Text
0
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Stack
Reserved
26
Sakai In Class Quiz 2.2
#include <stdio.h>
int a = 0;
float f = 0.5;
void setvals() {
float f = 0.5;
a = 78;
}
int main()
{
int a = 23;
f =0.31234;
setvals();
printf(“The values are %d, %f \n”,a,f);
}
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
• What is the result?
a)
b)
c)
d)
a = 78, f = 0.5
a = 78, f = 0.31234
a = 23, f = 0.5
a = 23, f = 0.31234
27
Sakai In Class Quiz 2.2
#include <stdio.h>
int a = 0;
float f = 0.5;
void setvals() {
float f = 0.5;
a = 78;
}
int main()
{
int a = 23;
f =0.31234;
setvals();
printf(“The values are %d, %f \n”,a,f);
}
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
2n-1
a 0xF0020
f 0x F0000
Stack
Heap
f 0x20004
a 0x 20000
Data
Text
0
Reserved
28
Reference vs. Pointer
Java
! “The value of a reference type variable, in contrast to that of a
primitive type, is a reference to (an address of) the value or set of
values represented by the variable” http://java.sun.com/docs/books/tutorial/java/
nutsandbolts/datatypes.html
! Cannot manipulate the value of the reference
C
! A pointer is a variable that contains the location of another
variable
! A pointer is a memory location that contains the address of
another memory location
! Can manipulate the value of pointer (double edge sword)
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
29
Pointers
• Declaration of pointer variables
! int *x_ptr; char *c_ptr; void *ptr;
• How do we get the location (address) of a variable?
Use one of the following:
1. Use the & ‘address of’ operator
!
x_ptr = &intvar;
2. From another pointer (yes we can do arithmetic on them)
!
x_ptr = y_ptr + 18;
3. Return from memory allocator
!
!
x_ptr = (int *) malloc(sizeof(int));
Similar to java new, but must explicitly free memory (i.e., free(x_ptr));
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
30
Pointers
• address of operator &
! don t confuse with bitwise AND operator
Given
int x; int* p; // p points to an int
p = &x;
Then
*p = 2; and x = 2; produce the same result
Note: p is a pointer, *p is an int
• What happens for p = 2?;
On 32-bit machine, p is 32-bits
x 0x26cf0
...
p 0x26d00 0x26cf0
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
31
Pointers
• De-reference using *ptr to get what is pointed at
statement
x
x_ptr
int x;
??
??
int *x_ptr;
??
??
x=2
2
??
x_ptr = &x;
2
&x
*x_ptr = 68;
68
&x
x_ptr = 200;
68
200
*x_ptr = 42
68
200
• Be careful with assignment to a pointer variable
! You can make it point anywhere…can be very bad
! You will, this semester, likely experience a “segmentation fault”
! What is 200?
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
32
Sakai In Class Quiz 2.3
Which Version Correctly Swaps Values?
void swap (int x, int y){
int temp = x;
x = y;
y = temp;
}
main() {
int a = 3;
int b = 4;
swap(a, b);
printf(“a = %d, b= %d\n”,
a, b);
}
void swap (int *x, int *y){
int temp = *x;
*x = *y;
*y = temp;
}
main() {
int a = 3;
int b = 4;
swap(&a, &b);
printf(“a = %d, b= %d\n”,
a, b);
}
A.
© Alvin R. Lebeck
From Sorin, Hilton, Roth
B.
Compsci / ECE 250
33
Strings as Arrays
s t
0 1
•
•
•
•
r i
15 16
g \0
42 43
A string is an array of characters with \0 at the end
Each element is one byte, ASCII code
\0 is null (ASCII code 0)
char str1[256] is similar to str2 = (char *) malloc(256);
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
34
Structures
• Loosely like objects
! Have member variables
! Do not have methods!
• Structure definition with struct keyword
struct student_record {
int id;
float grade;
} rec1, rec2;
• Declare a variable of the structure type with struct keyword
struct student_record onerec, *tworec;
• Access the structure member fields with
! ‘.’ structvar.member (e.g., onerec.id = 12;)
! ‘->’ structprt->member (e.g., tworec->id;)
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
35
Summary
• C is different from Java
! Procedural
! Structures not objects…
! Pointers!
• Next time
! Explicit memory deallocation
! Pointer manipulation
! Linked Structures
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
36
“C Programming”
Alvin R. Lebeck
Some slides based on those from Daniel Sorin,
Andrew Hilton, Amir Roth, Gershon Kedem
Administrivia
• Homework #1
• No class Monday
• Today
! Finish Data Representations
! C programming & Memory
• Updated Academic Honesty Policy
! Do not provide solutions to any archive (digital, paper, etc.)
! This is a simple extension of existing policy of not providing
solutions to another student.
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
2
Review: Answer
What floating-point number is
0xC1580000?
1100 0001 0101 1000 0000 0000 0000 0000
31 30
23 22
0
1 1000 0010 101 1000 0000 0000 0000 0000
S
•
•
•
•
N
M
Sign = 1 means this is a negative number
Exponent = (128+2)-127 = 3
Mantissa = 1.1011
-1.1011x23 = -1101.1 = -13.5
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
3
Special values in IEEE Floating Point
• How do you represent 0.0 in IEEE Floating Point?
! 0.0 = 000000000
! But need 1.XXXXX representation?
• Exponent = 0 is denormalized
! Implicit 0. instead of 1. in mantissa
! Allows 0000….0000 to be 0
! Helps with very small numbers near 0
• Results in +/- 0 in FP (but they are equal )
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
4
Other Weird FP numbers
• Exponent = 1111 1111 also not standard
! If all 0 mantissa: +/- ∞
1/0 = +∞
-1/0 = -∞
! If non zero mantissa: Not a Number (NaN)
sqrt(-42) = NaN
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
5
FP Addition/Substraction
• Addition/Subtraction
! Align the exponents
! Perform operation
! Renormalize the result (put into 1.M x 2E-127 form)
• What can possibly go wrong?
• Well a lot
• Search for disasters caused by numerical errors
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
6
Software Implication! Rounding Errors
• We only have 32-bits to represent floats
! Must approximate some values
•
•
•
•
Limited bits for mantissa
Does (x+y)*z = (x*z+y*z)?
Mathematically yes, but assumes infinite precision
Assume base 10, four digits available (two to left, two to
right of decimal point)
!
!
!
!
x = 99.96 x 103
x = x + 0.07
x = 100.03 x 103
x = 10.00 x 104
• Numerical Analysis (CS 220) studies some of these
issues
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
7
Floating Point Representation
• Double Precision Floating point:
64-bit representation:
! 1-bit sign
! 11-bit (biased) exponent
! 52-bit fraction (with implicit 1).
• double in Java, C, C++, …
S
Exp
1
11-bit
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Mantissa
52 - bit
Compsci / ECE 250
8
Oct. Char
000
010
020
030
040
050
060
070
100
110
120
130
140
150
160
170
nul
bs
dle
can
sp
(
0
8
@
H
P
X
`
h
p
x
ASCII Character Representation
001
011
021
031
041
051
061
071
101
111
121
131
141
151
161
171
soh
ht
dc1
em
!
)
1
9
A
I
Q
Y
a
i
q
y
002
012
022
032
042
052
062
072
102
112
122
132
142
250
162
172
stx
nl
dc2
sub
"
*
2
:
B
J
R
Z
b
j
r
z
003
013
023
033
043
053
063
073
103
113
123
133
143
153
163
173
etx
vt
dc3
esc
#
+
3
;
C
K
S
[
c
k
s
{
004
014
024
034
044
054
064
074
104
114
124
134
144
154
164
174
eot
np
dc4
fs
$
,
4
<
D
L
T
\
d
l
t
|
005
015
025
035
045
055
065
075
105
115
125
135
145
155
165
175
enq
cr
nak
gs
%
5
=
E
M
U
]
e
m
u
}
006
016
026
036
046
056
066
076
106
116
126
136
146
156
166
176
ack
so
syn
rs
&
.
6
>
F
N
V
^
f
n
v
~
007
017
027
037
047
057
067
077
107
117
127
137
147
157
167
177
bel
si
etb
us
'
/
7
?
G
O
W
_
g
o
w
del
• Each character represented by 7-bit ASCII code (packed into 8-bits)
• Convert upper to lower case ‘A’ + 3210 = ‘a’
• Other tables with decimal and HEX values.
© Alvin R. Lebeck
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9
Unicode
• Many types
• UTF-8: variable length encoding backward compatible
with ASCII
! Linux
• UTF-16: variable length
! Windows, Java
• UTF-32: fixed length
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10
Data Representation Summary
•
•
•
•
Need efficient representations
2’s complement for signed integers
IEEE Floating Point for Real numbers
Bits are bits are bits, same bits can be interpreted as
different types (unsigned, 2’s complement, float, etc.)
• Next up: C programming & memory
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11
What you know today
• JAVA
...
System.out.println("Please Enter In Your First Name: ");
String firstName = bufRead.readLine();
System.out.println("Please Enter In The Year You Were Born: ");
String bornYear = bufRead.readLine();
System.out.println("Please Enter In The Current Year: ");
String thisYear = bufRead.readLine();
int bYear = Integer.parseInt(bornYear);
int tYear = Integer.parseInt(thisYear);
int age = tYear – bYear ;
System.out.println("Hello " + firstName + ". You are " + age + " years
old");
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How does a Java program execute?
• Compile Java Source to Java Byte codes
• Java Virtual Machine (JVM) interprets/translates Byte
codes
• JVM is a program executing on the hardware…
• Java has lots of things that make it easier to program
without making mistakes
• JVM handles memory for you
! What do you do when you remove an entry from a hash table,
binary tree, etc.?
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13
The C Programming Language
• No virtual machine
! No dynamic type checking, array bounds, garbage collection, etc.
! Compile source file directly to machine (this is a little simplified)
• Closer to hardware
! Easier to make mistakes
! Can often result in faster code
• Generally used for ‘systems programming’
! operating systems, embedded systems, database implementation
! There is object oriented C++ (C is a strict subset of C++)
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14
The C Programming Language (Continued)
• No objects
• Procedural, not object oriented
! No objects with methods
• Structures, unions like objects
! Member variables (no methods)
•
•
•
•
Pointers – memory, arrays
External standard library – I/O, other facilities
Macro preprocessor (#<directive>)
Additional Resources
! Kernighan & Richie book The C Programming Language
! MIT open course Practical Programming in C link in ‘docs’ of web
site
! Drew Hilton Video Snippets
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15
Compiling and Running a C Program
•
•
•
•
•
Use the gcc program to create an executable file
gcc –o <outputname> <source file name>
gcc –o hello hello.c (must be in same directory as hello.c)
If no –o option, then default output name is a.out (e.g., gcc hello.c)
Type the program name on the command line
! ./ before “hello” means look in current directory for hello program
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16
Debugging
• Print debugging…
! Just output information at different points in the program
! Not the most efficient, but often works.
• gdb <executable filename>
• Good for stopping at set points in program and inspecting
variable values.
! If you get good at using a debugger it is easier/better than printf
debugging…
! See GDB Essentials video off https://www.cs.duke.edu/courses/
spring15/compsci250/docs.html
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17
Variables, operators, expressions
• C variables are similar to Java
! Data types: int, float, double, char, void
! Signed and unsigned int
! char, short, int, long, long long can all be integer types
» These specify how many bits to represent an integer
• Constants
! Use #define C preprocessor
! E.g.,: #define MAX_SCORE 100
• Operators:
! Mathematical +, -, *, /, %,
! Logical !, &&, ||, ==, !=, <, >, <=, >=
! Bitwise &, |, ~, ^ , <<, >> (we’ll get to these next)
• Expressions: var1 = var2 + var3;
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18
Bitwise AND / OR / XOR
•
•
•
•
& operator performs bitwise AND
| operator performs bitwise OR
^ operator performs bitwise Exclusive OR (XOR)
~ operator performs bitwise NOT
• Per bit
0&0=0
0|0=0
0^0=0
0&1=0
0|1=1
0^1=1
1&0=0
1|0=1
1^0=1
1&1=1
1|1=1
1^1=0
• For multiple bits, apply operation to individual bits in same position
AND
011010
101110
001010
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OR
011010
101110
111110
XOR
011010
101110
110100
Compsci / ECE 250
19
The SHIFT operator
• >> is shift right, << is shift left, operands are unsigned int
and number of positions to shift
• (1 << 3) is …000001 -> …0001000 (it’s 23)
• 0xff00 is 0xff << 8, and 0xff is 0xff00 >> 8
! Unsigned ints, be careful shifting signed integers (more later this
semester)
• 0xAB & 0x0F = 0x0B
• 0xAB & 0xF0 = 0xA0
• 0x0F and 0xF0 are called a bit mask
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20
Sakai In Class Quiz 2.1
button
y
x
val = 01 1010 0011 0100 1100
• 32-bit word with x,y and button fields
!
!
!
!
bits 0-7 contain x position
bits 8-15 contain y position
bits 16-17 contain button (0 = left, 1 = middle, 2 = right)
Bits 18-31 don’t matter
• Which C Statements correctly set xpos, ypos & button?
A.
xpos = val && 0x000ff;
ypos = (val && 0x0ff00) >> 8;
button = (val && 0x30000) >> 16;
C.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) << 8;
button = (val & 0x30000) << 16;
© Alvin R. Lebeck
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B.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) >> 8;
button = (val & 0x30000) >> 16;
D.
xpos = val & 0x000ff;
ypos = val & 0x0ff00 >> 1;
button = val & 0x30000 >> 2;
Compsci / ECE 250
21
Sakai In Class Quiz 2.1: Answer
button
y
x
val = 01 1010 0011 0100 1100
• 32-bit word with x,y and button fields
!
!
!
!
bits 0-7 contain x position
bits 8-15 contain y position
bits 16-17 contain button (0 = left, 1 = middle, 2 = right)
Bits 18-31 don’t matter
• Which C Statements correctly set xpos, ypos & button?
A.
xpos = val && 0x000ff;
ypos = (val && 0x0ff00) >> 8;
button = (val && 0x30000) >> 16;
Logical AND
C.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) << 8;
button = (val & 0x30000) << 16;
Shift wrong direction; value not
in least significant bits
© Alvin R. Lebeck
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B.
xpos = val & 0x000ff;
ypos = (val & 0x0ff00) >> 8;
button = (val & 0x30000) >> 16;
D.
xpos = val & 0x000ff;
ypos = val & 0x0ff00 >> 1;
button = val & 0x30000 >> 2;
Precedence…shifts mask first; shift
amount wrong
Compsci / ECE 250
22
Functions
• Encapsulate computation
! Reuse or clarity of code
! Cannot define functions within functions
• Must be declared before use!
int div2(int x,int y); /* declaration here */
main() {
int a;
a = div2(10,2);
}
int div2(int x, int y) { /* implementation here */
return (x/y);
}
• Or put functions at top of file (doesn’t always work)
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23
Global Variables
• Global variables: Accessible from any function
#include <stdio.h>
float f = 0;
void bar () {
f = 0.5;
}
int main()
{
f =0.31234;
bar();
printf(“The value is %f \n”, f);
}
Output is:
The value is 0.5
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24
Memory Partitions
2n-1
• Text for instructions
Stack
! add res, src1, src2
! mem[res] = mem[src1] +
mem[src2]
Typical
Address
Space
• Data
! static (constants, global variables)
! dynamic (heap, malloc / new
allocated)
! grows up
Heap
• Stack
Data
! local variables
! grows down
Text
• Variables are names for
memory locations
0
Reserved
! int x;
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25
A Simple Program’s Memory Layout
...
int result; // global var
int main()
{
int x;
...
result = x + result;
...
}
For now, think of this as performing:
mem[0x208] = mem[0x400] + mem[0x208]
2n-1
x 0x400
Compsci / ECE 250
5
Heap
result 0x208
Data
3
Text
0
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Stack
Reserved
26
Sakai In Class Quiz 2.2
#include <stdio.h>
int a = 0;
float f = 0.5;
void setvals() {
float f = 0.5;
a = 78;
}
int main()
{
int a = 23;
f =0.31234;
setvals();
printf(“The values are %d, %f \n”,a,f);
}
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
• What is the result?
a)
b)
c)
d)
a = 78, f = 0.5
a = 78, f = 0.31234
a = 23, f = 0.5
a = 23, f = 0.31234
27
Sakai In Class Quiz 2.2
#include <stdio.h>
int a = 0;
float f = 0.5;
void setvals() {
float f = 0.5;
a = 78;
}
int main()
{
int a = 23;
f =0.31234;
setvals();
printf(“The values are %d, %f \n”,a,f);
}
© Alvin R. Lebeck
From Sorin, Hilton, Roth
Compsci / ECE 250
2n-1
a 0xF0020
f 0x F0000
Stack
Heap
f 0x20004
a 0x 20000
Data
Text
0
Reserved
28
Reference vs. Pointer
Java
! “The value of a reference type variable, in contrast to that of a
primitive type, is a reference to (an address of) the value or set of
values represented by the variable” http://java.sun.com/docs/books/tutorial/java/
nutsandbolts/datatypes.html
! Cannot manipulate the value of the reference
C
! A pointer is a variable that contains the location of another
variable
! A pointer is a memory location that contains the address of
another memory location
! Can manipulate the value of pointer (double edge sword)
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Compsci / ECE 250
29
Pointers
• Declaration of pointer variables
! int *x_ptr; char *c_ptr; void *ptr;
• How do we get the location (address) of a variable?
Use one of the following:
1. Use the & ‘address of’ operator
!
x_ptr = &intvar;
2. From another pointer (yes we can do arithmetic on them)
!
x_ptr = y_ptr + 18;
3. Return from memory allocator
!
!
x_ptr = (int *) malloc(sizeof(int));
Similar to java new, but must explicitly free memory (i.e., free(x_ptr));
© Alvin R. Lebeck
From Sorin, Hilton, Roth
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30
Pointers
• address of operator &
! don t confuse with bitwise AND operator
Given
int x; int* p; // p points to an int
p = &x;
Then
*p = 2; and x = 2; produce the same result
Note: p is a pointer, *p is an int
• What happens for p = 2?;
On 32-bit machine, p is 32-bits
x 0x26cf0
...
p 0x26d00 0x26cf0
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31
Pointers
• De-reference using *ptr to get what is pointed at
statement
x
x_ptr
int x;
??
??
int *x_ptr;
??
??
x=2
2
??
x_ptr = &x;
2
&x
*x_ptr = 68;
68
&x
x_ptr = 200;
68
200
*x_ptr = 42
68
200
• Be careful with assignment to a pointer variable
! You can make it point anywhere…can be very bad
! You will, this semester, likely experience a “segmentation fault”
! What is 200?
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32
Sakai In Class Quiz 2.3
Which Version Correctly Swaps Values?
void swap (int x, int y){
int temp = x;
x = y;
y = temp;
}
main() {
int a = 3;
int b = 4;
swap(a, b);
printf(“a = %d, b= %d\n”,
a, b);
}
void swap (int *x, int *y){
int temp = *x;
*x = *y;
*y = temp;
}
main() {
int a = 3;
int b = 4;
swap(&a, &b);
printf(“a = %d, b= %d\n”,
a, b);
}
A.
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B.
Compsci / ECE 250
33
Strings as Arrays
s t
0 1
•
•
•
•
r i
15 16
g \0
42 43
A string is an array of characters with \0 at the end
Each element is one byte, ASCII code
\0 is null (ASCII code 0)
char str1[256] is similar to str2 = (char *) malloc(256);
© Alvin R. Lebeck
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Compsci / ECE 250
34
Structures
• Loosely like objects
! Have member variables
! Do not have methods!
• Structure definition with struct keyword
struct student_record {
int id;
float grade;
} rec1, rec2;
• Declare a variable of the structure type with struct keyword
struct student_record onerec, *tworec;
• Access the structure member fields with
! ‘.’ structvar.member (e.g., onerec.id = 12;)
! ‘->’ structprt->member (e.g., tworec->id;)
© Alvin R. Lebeck
From Sorin, Hilton, Roth
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35
Summary
• C is different from Java
! Procedural
! Structures not objects…
! Pointers!
• Next time
! Explicit memory deallocation
! Pointer manipulation
! Linked Structures
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From Sorin, Hilton, Roth
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36
