03LecSp11CIntroIx6 - 1/23/11 Agenda •  •  • ...

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Unformatted text preview: 1/23/11 Agenda •  •  •  •  •  •  CS 61C: Great Ideas in Computer Architecture (Machine Structures) Introduc)on to C (Part I) Instructors: Randy H. Katz David A. PaGerson hGp://inst.eecs.Berkeley.edu/~cs61c/sp11 Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers Summary 1/23/11 Levels of RepresentaOon/ InterpretaOon temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; High Level Language Program (e.g., C) Compiler Assembly Language Program (e.g., MIPS) lw lw sw sw Assembler Machine Language Program (MIPS) $t0, 0($2) $t1, 4($2) $t1, 0($2) $t0, 4($2) 0000 1010 1100 0101 1001 1111 0110 1000 1100 0101 1010 0000 Anything can be represented as a number, i.e., data or instrucOons 1010 0000 0101 1100 2 Agenda We are here! 0110 1000 1111 1001 Spring 2011  ­ ­ Lecture #3 1111 1001 1000 0110 0101 1100 0000 1010 1000 0110 1001 1111 ! Machine Interpreta4on Hardware Architecture DescripCon (e.g., block diagrams) •  •  •  •  •  •  Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers Summary Architecture Implementa4on Logic Circuit DescripCon 1/23/11 (Circuit SchemaCc Diagrams) Spring 2011  ­ ­ Lecture #3 3 1/23/11 IntroducOon to C “the Universal Assembly Language” •  “Some” C experience is required before CS61C C++ or Java OK 83% already know JAVA 54% already know C++ 34% already know C 7% already know C# About 10% have not taken 61B or equivalent •  If you have no experience in these languages, then start early and ask a lot of quesOons in discussion! 1/23/11 Spring 2011  ­ ­ Lecture #3 4 Disclaimer •  Based on pre ­semester survey: –  –  –  –  –  Spring 2011  ­ ­ Lecture #3 5 •  You will not learn how to fully code in C in these lectures! You’ll sOll need your C reference for this course –  K&R is a must ­have •  Check online for more sources –  “JAVA in a Nutshell,” O’Reilly •  Chapter 2, “How Java Differs from C ” •  hGp://oreilly.com/catalog/javanut/excerpt/ –  Brian Harvey’s helpful transiOon notes •  On CS61C class website •  hGp://inst.eecs.berkeley.edu/~cs61c/resources/ HarveyNotesC1 ­3.pdf •  Key C concepts: Pointers, Arrays, ImplicaOons for Memory management 1/23/11 Spring 2011  ­ ­ Lecture #3 6 1 1/23/11 C vs. Java C Basic C Concepts Java Type of Language Function Oriented Object Oriented Compiler Programming Unit Function Class = Abstract Data Type Typed variables Compilation gcc hello.c creates machine language code javac Hello.java creates Java virtual machine language bytecode Typed func)ons Execution a.out loads and executes program java Hello interprets bytecode hello, world #include<stdio.h>
 int main(void) {
 printf("Hello\n");
 return 0;
 } public class HelloWorld {
 public static void main (String args) { 
 System.out.printl("Hello");
 }
 } Storage Manual (malloc, free) Automatic (garbage collection) 1/23/11 Header files (.h) Structs Lists of predefined values Pointers Aliases to other variables These concepts disOnguish C from other languages you may know Spring 2011  ­ ­ Lecture #3 7 From hGp://www.cs.princeton.edu/introcs/faq/c2java.html 1/23/11 Spring 2011  ­ ­ Lecture #3 •  Excellent run ­Ome performance: generally much faster than Scheme or Java for comparable code (because it opOmizes for a given architecture) •  Fair compilaOon Ome: enhancements in compilaOon procedure (Makefiles) allow only modified files to be recompiled •  Why C?: we can write programs that allow us to exploit underlying features of the architecture – memory management, special instruc)ons, parallelism –  Unlike Java, which converts to architecture independent bytecode –  Unlike most Scheme environments, which interpret the code –  These differ mainly in exactly when your program is converted to low ­level machine instrucOons (“levels of interpretaOon”) –  For C, generally a two part process of compiling .c files to .o files, then linking the .o files into executables; Assembling is also done (but is hidden, i.e., done automaOcally, by default) 9 1/23/11 CompilaOon: Disadvantages int variable1 float variable2 char variable3 10 = 2; = 1.618; = 'A'; •  You have to declare the type of data a variable will hold –  Types can't change –  I.e., “porOng your code” to a new architecture •  “Change → Compile → Run [repeat]” iteraOon cycle can be slow, during the development cycle Spring 2011  ­ ­ Lecture #3 Spring 2011  ­ ­ Lecture #3 Typed Variables in C •  Compiled files, including the executable, are architecture ­specific, depending on CPU type and the operaOng system •  Executable must be rebuilt on each new system 1/23/11 8 CompilaOon: Advantages •  C compilers map C programs into architecture ­specific machine code (string of 1s and 0s) Spring 2011  ­ ­ Lecture #3 Enums CompilaOon: Overview 1/23/11 Creates useable programs from C source Kind of data that a variable contains The kind of data returned from a funcOon Declare funcOons and variables in a separate file Groups of related values 11 Type int unsigned int float char double long 1/23/11 DescripCon integer numbers, including negaOves integer numbers (no negaOves) floaOng point decimal numbers single text character or symbol greater precision FP number larger signed integer Spring 2011  ­ ­ Lecture #3 Examples 0, 78,  ­1400 0, 46, 900 0.0, 1.618,  ­1.4 'a', 'D', '?’ 12 2 1/23/11 Typed FuncOons in C int numberOfPeople () { return 3; } float dollarsAndCents () { return 10.33; } char firstLetter () { return 'A'; } 1/23/11 Structs in C •  You have to declare the type of data you plan to return from a funcOon •  Return type can be any C variable type, and is placed to the ley of the funcOon name •  You can also specify the return type as void •  Structs are structured groups of variables, e.g., •  Also necessary to define types for values passed into a funcOon •  Variables and funcOons MUST be defined before they are used song1.lengthInSeconds = 213; song1.yearRecorded = 1994; –  Just think of this as saying that no value will be returned typedef struct { int lengthInSeconds; int yearRecorded; } Song; Song song1; Song song2; song2.lengthInSeconds = 248; song2.yearRecorded = 1988; Spring 2011  ­ ­ Lecture #3 13 1/23/11 Consts and Enums in C •  •  •  •  •  const float goldenRatio = 1.618; const int daysInWeek = 7; •  You can have a constant version of any of the standard C variable types •  Enums: a group of related constants used to parameterize libraries Spring 2011  ­ ­ Lecture #3 14 Agenda •  Constant is assigned a value once in the declaraOon; value can't change unOl the program is restarted 1/23/11 Dot notaOon: x.y = value Spring 2011  ­ ­ Lecture #3 15 Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers 1/23/11 Administrivia Spring 2011  ­ ­ Lecture #3 16 Agenda •  We promised you that CS61c would be relentless! –  This week: Lab #2, HW #2 –  Lab #2, Amazon EC2, will soon be posted –  HW #2 will soon be posted •  TAs have their office hours late in the week – don’t wait for last minute to get quesOons answered on HWs and Labs –  Take a quick look when posted and come to discussion with quesOons •  We are amazed that there students who sOll haven’t signed up for the class discussion/announcements group! •  •  •  •  •  Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers –  Wonderful to see the valuable discussion and help going on there! 1/23/11 Spring 2011  ­ ­ Lecture #3 17 1/23/11 Spring 2011  ­ ­ Lecture #3 18 3 1/23/11 A Second C Program: Compute Table of Sines A First C Program: Hello World Original C: #include <stdio.h> #include <math.h> ANSI Standard C: main() { printf("\nHello World\n"); } #include <stdio.h> int main(void) { printf("\nHello World\n"); return (0); } printf("angle void main() { int angle_degree; double angle_radian, pi, value; /* Print a header */ printf("\nCompute a table of the sine function\n\n"); Spring 2011  ­ ­ Lecture #3 Compute a table of the sine function Value of PI = 3.141593 angle 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 1/23/11 Sine 0.000000 0.173648 0.342020 0.500000 0.642788 0.766044 0.866025 0.939693 0.984808 1.000000 0.984808 0.939693 0.866025 0.766044 0.642788 0.500000 0.342020 0.173648 0.000000 19 1/23/11 C Syntax: main! –  int main (int argc, char *argv)! •  What does this mean? –  argc contains the number of strings on the command line (the executable counts as one, plus one for each argument). Here argc is 2: unix% sort myFile –  argv is a pointer to an array containing the arguments as strings (more on pointers later) 21 1/23/11 C Syntax: Variable DeclaraOons 22 •  Within a funcOon, remarkably close to Java constructs in methods (shows its legacy) in terms of flow control –  if-else •  if (expression) statement •  if (expression) statement1 else statement2 –  while –  Correct: {! !! ! ! int a = 0, b = 10;! !! ! ! !...! − Incorrect: for (int i = 0; i < 10; i++)! }! Spring 2011  ­ ­ Lecture #3 Spring 2011  ­ ­ Lecture #3 C Syntax : Flow Control (1/2) •  Similar to Java, but with a few minor but important differences •  All variable declaraOons must appear before they are used (e.g., at the beginning of the block) •  A variable may be iniOalized in its declaraOon; if not, it holds garbage! •  Examples of declaraOons: 1/23/11 20 •  To get arguments to the main funcOon, use: -0.173648 -0.342020 -0.500000 -0.642788 -0.766044 -0.866025 -0.939693 -0.984808 -1.000000 -0.984808 -0.939693 -0.866025 -0.766044 -0.642788 -0.500000 -0.342020 -0.173648 -0.000000 Spring 2011  ­ ­ Lecture #3 } } Spring 2011  ­ ­ Lecture #3 Second C Program Sample Output 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 /* loop until angle_degree > 360 */ { angle_radian = pi*angle_degree/180.0; value = sin(angle_radian); printf (" %3d %f \ n " , angle_degree, value); angle_degree = angle_degree + 10; /* increment the loop index */ /* obtain pi once for all */ /* or just use pi = M_PI, where */ /* M_PI is defined in math.h */ pi = 4.0*atan(1.0); printf("Value of PI = %f \n\n", pi); 1/23/11 Sine \n"); angle_degree = 0; /* initial angle value */ /* scan over angle */ while (angle_degree <= 360) •  while (expression) statement •  do statement while (expression); 23 1/23/11 Spring 2011  ­ ­ Lecture #3 24 4 1/23/11 C Syntax : Flow Control (2/2) C Syntax: True or False •  Within a funcOon, remarkably close to Java constructs in methods (shows its legacy) in terms of flow control •  What evaluates to FALSE in C? –  0 (integer) –  NULL (a special kind of pointer: more on this later) –  No explicit Boolean type –  for •  for (initialize; check; update) statement –  switch •  What evaluates to TRUE in C? •  switch (expression){ case const1: statements case const2: statements default: statements } •  break 1/23/11 Spring 2011  ­ ­ Lecture #3 –  Anything that isn’t false is true –  Same idea as in scheme: only #f is false, anything else is true! 25 1/23/11 Spring 2011  ­ ­ Lecture #3 Agenda •  •  •  •  •  Agenda Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers 1/23/11 Spring 2011  ­ ­ Lecture #3 26 •  •  •  •  •  27 Scheme vs. Java vs. C Administrivia Quick Start IntroducOon to C Technology Break Pointers 1/23/11 Spring 2011  ­ ­ Lecture #3 Pointers in C 28 Address vs. Value •  A pointer is just another kind of value •  Consider memory to be a single huge array –  A basic type in C –  Each cell of the array has an address associated with it –  Each cell also stores some value –  Do you think they use signed or unsigned numbers? NegaOve address?! int *ptr; Variable “ptr ” is a pointer to an “int ” •  Don’t confuse the address referring to a memory locaOon with the value stored there ... Some of these slides come from “Arrays and Pointers in C ” by Alan Cox and T.S. Ng, Rice University, www.clear.rice.edu/comp221/html/ppt/03 ­arrays ­pointers.ppt 1/23/11 Spring 2011  ­ ­ Lecture #3 29 1/23/11 101 102 103 104 105 ... 23 42 Spring 2011  ­ ­ Lecture #3 ... 30 5 1/23/11 Pointers Pointers in C •  An address refers to a parOcular memory locaOon; e.g., it points to a memory locaOon •  Pointer: A variable that contains the address of a variable •  If T is a type, T *p declares p a pointer to that type •  You can use p as a pointer to a T •  You can use *p as a T •  p++ increments p by the size of a T –  Important because of the way arrays are treated •  You can make a pointer to any variable –  If x is any variable, then &x is its address LocaOon (address) ... 104 101 102 103 104 105 ... 23 42 x name 1/23/11 p ... y Spring 2011  ­ ­ Lecture #3 31 Pointer OperaOons in C 1/23/11 32 Pointer Examples •  How to create a pointer: •  CreaOon & operator: get address of a variable & variable int *p, x; Returns variable’s memory address •  Dereference * pointer x = 3; Returns contents stored at address •  Indirect assignment p = &x; *pointer = val Stores value at address pointer = ptr Spring 2011  ­ ­ Lecture #3 33 1/23/11 int i1; int i2; int *ptr1; int *ptr2; –  Use the dereference operator * on ley of assignment operator = i1 = 1; i2 = 2; 3 ptr1 = &i1; ptr2 = ptr1; *p = 5; p x Spring 2011  ­ ­ Lecture #3 p ? x 3 x Note the “*” gets used 2 different ways in this example. In the declaraOon to indicate that p is going to be a pointer, and in the printf to get the value pointed to by p. ? 3 p Spring 2011  ­ ­ Lecture #3 34 Pointer Examples •  How to change a variable pointed to? x x !printf(“p points to %d\n”,*p); ! Pointer Examples p ? * “dereference operator”: get the value that the pointer points to Stores pointer in another variable 1/23/11 p • How get a value pointed to? •  Of course, sOll have … assignment 1/23/11 Spring 2011  ­ ­ Lecture #3 5 0x1014 … 0x1010 ptr2: 0x100C … 0x1008 ptr1: 0x1004 i2: 2 3 0x1000 i1: 3 1 0x1000 0x1000 *ptr1 = 3; i2 = *ptr2; 35 1/23/11 Spring 2011  ­ ­ Lecture #3 36 6 1/23/11 Pointer Examples int int int1 int2 = 1036; = 8; int *int_ptr1 = &int1; int *int_ptr2 = &int2; Pointer Examples /* some data to point to */ int int int1 int2 = 1036; = 8; /* get addresses of data */ int *int_ptr1 = &int1; int *int_ptr2 = &int2; *int_ptr1 = int_ptr2; */ /* get addresses of data */ int_ptr1 = *int_ptr2; *int_ptr1 = int2; /* some data to point to int_ptr1 = int_ptr2; What happens? What happens? Type check warning: int_ptr2 is not an int Type check warning: *int_ptr2 is not an int * int1 becomes 8 1/23/11 Changes int_ptr1 – doesn’t change int1 Spring 2011  ­ ­ Lecture #3 37 Pointers and Parameter Passing 38 •  How can we get a funcOon to change a value? –  Procedure/funcOon/method gets a copy of the parameter, so changing the copy cannot change the original void addOne (int *p) {
 !*p = *p + 1;
 }! int y = 3;! void addOne (int x) {
 ! x = x + 1;
 }! int y = 3;! addOne(y);! addOne(&y);! y is now equal to 4 y remains equal to 3 Spring 2011  ­ ­ Lecture #3 39 1/23/11 Pass By Reference •  In C, the default passing strategy is pass by copy •  To pass by reference, we use pass by copy – because in C, everything is pass by copy •  Value that we pass by copy is address of the actual argument: we achieve the through the address operator & •  In C, pass by reference is actually pass by copy – because you copy the address 1/23/11 Spring 2011  ­ ­ Lecture #3 Spring 2011  ­ ­ Lecture #3 Pointers and Parameter Passing •  Java and C pass parameters “by value” 1/23/11 1/23/11 41 Spring 2011  ­ ­ Lecture #3 40 Ge~ng Pass ­by ­Reference void set_x_and_y(int *x, int *y) { *x = 1001; *y = 1002; } a void f(void) { int a = 1; int b = 2; set_x_and_y(&a,&b); } 1/23/11 Spring 2011  ­ ­ Lecture #3 1001 1 b 1002 2 x y 42 7 1/23/11 Pointers Peer InstrucOon QuesOon !void main(); { int *p, x=5, y; // init y = *(p = &x) + 1; int z; flip-sign(p); printf("x=%d,y=%d,p=%d\n",x,y,p); } flip-sign(int *n){*n = -(*n)} •  Pointers are used to point to any kind of data (int, char, a struct, etc.) •  Normally a pointer only points to one type (int, char, a struct, etc.). –  void * is a type that can point to anything (generic pointer) –  Use sparingly to help avoid program bugs, and security issues, and other bad things! 1/23/11 Spring 2011  ­ ­ Lecture #3 How many syntax + logic errors in this C code? 43 1/23/11 Spring 2011  ­ ­ Lecture #3 More C Pointer Dangers Spring 2011  ­ ­ Lecture #3 46 •  Why use pointers? –  If we want to pass a large struct or array, it’s easier / faster / etc. to pass a pointer than the whole thing –  In general, pointers allow cleaner, more compact code •  So what are the drawbacks? –  Pointers are probably the single largest source of bugs in C, so be careful anyOme you deal with them •  Most problemaOc with dynamic memory management— which you will to know by the end of the semester, but not for the projects (there will be a lab later in the semester) •  Dangling references and memory leaks 1/23/11 Why Pointers in C? 47 •  “C99” or “C9x” standard –  gcc -std=c99 to compile –  Computers 25,000 Omes faster today, compilers beGer •  C designed to let programmer say what they want code to do without compiler ge~ng in way –  Even give compilers hints which registers to use! •  References –  http://en.wikipedia.org/wiki/C99 ! –  http://home.tiscalinet.ch/t_wolf/tw/c/ c9x_changes.html ! •  Highlights –  DeclaraOons in for loops, like Java –  Java ­like // comments (to end of line) –  Variable ­length non ­global arrays –  <inttypes.h>: explicit integer types –  <stdbool.h>: for boolean logic types and definiOons •  Today’s compilers produce much beGer code, so may not need to use pointers –  Compilers even ignore hints since they do it beGer! Spring 2011  ­ ­ Lecture #3 Spring 2011  ­ ­ Lecture #3 FYI—Update to ANSI C •  At Ome C was invented (early 1970s), compilers oyen didn’t produce efficient code 1/23/11 44 Pointers in C •  Declaring a pointer just allocates space to hold the pointer – it does not allocate the thing being pointed to! •  Local variables in C are not iniOalized, they may contain anything (aka “garbage”) •  What does the following code do? void f() { int *ptr; *ptr = 5; } 1/23/11 #Errors Red: 1 Orange: 2 Green: 3 Yellow: 4 Pink: 5 48 1/23/11 Spring 2011  ­ ­ Lecture #3 49 8 1/23/11 And In Conclusion, … •  All data is in memory –  Each memory locaOon has an address to use to refer to it and a value stored in it •  Pointer is a C version (abstracOon) of a data address –  * “follows” a pointer to its value –  & gets the address of a value –  Arrays and strings are implemented as variaOons on pointers •  C is an efficient language, but leaves safety to the programmer –  Array bounds not checked –  Variables not automaOcally iniOalized –  Use pointers with care: they are a common source of bugs in programs 1/23/11 Spring 2011  ­ ­ Lecture #3 50 9 ...
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