CS61C Machine Structures Lecture 3 Introduction to the C Programming Language 1 23 2006 John Wawrzynek www cs berkeley edu johnw www inst eecs berkeley edu cs61c CS 61C L03 Introduction to C 1 Wawrzynek Spring 2006 UCB Administrivia Near term Get cardkeys from CS main office Soda Hall 3rd floor Reading for this week K R Ch 1 4 today Ch 5 6 W F HW HW1 due Wednesday 11 59pm HW2 will be posted Wednesday Project 1 C Programming Goes online tomorrow AM Due Monday 2 6 2 weeks from today CS 61C L03 Introduction to C 2 Wawrzynek Spring 2006 UCB Introduction to C Why learn C CS 61C L03 Introduction to C 3 Wawrzynek Spring 2006 UCB Disclaimer Important You will not learn how to fully code in C in these lectures You ll still need your C reference for this course K R is a great reference But check online for more sources JAVA in a Nutshell O Reilly Chapter 2 How Java Differs from C Brian Harvey s course notes On class website CS 61C L03 Introduction to C 4 Wawrzynek Spring 2006 UCB Compilation Overview C compilers take C and convert it into an architecture specific machine code string of 1s and 0s Unlike Java which converts to architecture independent bytecodes Unlike most Scheme environments which interpret the code These differ mainly in when your program is converted to machine instructions For C generally a 2 part process of compiling c files to o files then linking the o files into executables CS 61C L03 Introduction to C 5 Wawrzynek Spring 2006 UCB Compilation characteristics Great run time performance generally much faster than Scheme or Java for comparable code because it optimizes for a given architecture OK compilation time enhancements in compilation procedure Makefiles allow only modified files to be recompiled CS 61C L03 Introduction to C 6 Wawrzynek Spring 2006 UCB Compilation Disadvantages All compiled files including the executable are architecture specific depending on both the CPU type and the operating system Executable must be rebuilt on each new system Called porting your code to a new architecture The change compile run repeat iteration cycle is slow CS 61C L03 Introduction to C 7 Wawrzynek Spring 2006 UCB C vs Java Overview 1 2 Java C Object oriented OOP No built in object abstraction Data separate from methods Methods Class libraries of data structures Functions Automatic memory management Manual memory management C libraries are lower level Pointers CS 61C L03 Introduction to C 8 Wawrzynek Spring 2006 UCB C vs Java Overview 2 2 Java C High memory overhead from class libraries Low memory overhead Relatively Slow Arrays initialize to zero Relatively Fast Arrays initialize to garbage Syntax Syntax comment comment System out print comment printf Newer C compilers allow Java style comments as well CS 61C L03 Introduction to C 9 Wawrzynek Spring 2006 UCB C Syntax Variable Declarations Very similar to Java but with a few minor but important differences All variable declarations must go before they are used at the beginning of the block A variable may be initialized in its declaration Examples of declarations correct int a 0 b 10 incorrect for int i 0 i 10 i C compiler now allow this in the case of for loops CS 61C L03 Introduction to C 10 Wawrzynek Spring 2006 UCB C Syntax True or False What evaluates to FALSE in C 0 integer NULL pointer more on this later no such thing as a Boolean What evaluates to TRUE in C everything else same idea as in scheme only f is false everything else is true CS 61C L03 Introduction to C 11 Wawrzynek Spring 2006 UCB C syntax flow control Within a function remarkably close to Java constructs in methods shows its legacy in terms of flow control if else switch while and for do while CS 61C L03 Introduction to C 12 Wawrzynek Spring 2006 UCB C Syntax main To get the main function to accept arguments use this int main int argc char argv What does this mean argc will contain the number of strings on the command line the executable counts as one plus one for each argument Example unix sort myFile argv is a pointer to an array containing the arguments as strings more on pointers later CS 61C L03 Introduction to C 13 Wawrzynek Spring 2006 UCB Address vs Value Consider memory to be a single huge array 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 Negative address Don t confuse the address referring to a memory location with the value stored in that location 101 102 103 104 105 CS 61C L03 Introduction to C 14 23 42 Wawrzynek Spring 2006 UCB Pointers An address refers to a particular memory location In other words it points to a memory location Pointer A variable that contains the address of another variable Location address 101 102 103 104 105 23 42 104 x y p name CS 61C L03 Introduction to C 15 Wawrzynek Spring 2006 UCB Pointers How to create a pointer operator get address of a variable int p x x 3 p x p x p x 3 x 3 p Note the gets used 2 different ways in this example In the declaration to indicate that p is going to be a pointer and in the printf to get the value pointed to by p How get a value pointed to dereference operator get value pointed to printf p points to d n p CS 61C L03 Introduction to C 16 Wawrzynek Spring 2006 UCB Pointers How to change a variable pointed to Use dereference operator on left of p 5 CS 61C L03 Introduction to C 17 p x 3 p x 5 Wawrzynek Spring 2006 UCB Pointers and Parameter Passing Java and C pass a parameter by value procedure function gets a copy of the parameter so changing the copy cannot change the original void addOne int x x x 1 int y 3 addOne y y is still 3 CS 61C L03 Introduction to C 18 Wawrzynek Spring 2006 UCB Pointers and Parameter Passing How to get a function to change a value void addOne int p p p 1 int y 3 addOne y y is now 4 CS 61C L03 Introduction to C 19 Wawrzynek Spring 2006 UCB Pointers Of course pointers are used to point to any data type int char a struct etc Normally a particular pointer variable can only point to one type void is a type that can point to anything generic pointer Use sparingly to help avoid program bugs and security issues and a lot of other bad things CS 61C L03 Introduction to C 20 Wawrzynek Spring 2006 UCB Find the Errors void main int p x 5 y init y p x 10 int z flip sign p printf x d y d p d n x y p flip sign int n n n How many errors CS 61C L03 Introduction to C 21 Wawrzynek Spring 2006 UCB …
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