inst eecs berkeley edu cs61c Lecture 3 Introduction to the C Programming Language pt 1 2010 01 25 Lecturer SOE Dan Garcia www cs berkeley edu ddgarcia These 5 integer encodings have different benefits 1s complement and sign mag have most problems 00000 MIPS Supercomputer China s next supercomputer the Dawning 6000 will be built using the Loongson MIPS processor and run Linux Currently the top 500 supercomputers are mostly x86 chips You ll learn MIPS in CS61C www technologyreview com computing 24374 Garcia Spring 2010 UCB Introduction to C 00001 01111 10000 11111 2 s complement C99 s intN t universal learn 00000 00001 01111 10000 11110 11111 Overflow numbers computers finite errors CS61C L03 Introduction to C pt 1 2 META Ain t no free lunch Garcia Spring 2010 UCB Has there been an update to ANSI C Officially some C What to do if you re experience is required in that 10 before CS61C Start early ask 2010Sp an experiment We represent things in computers as particular bit patterns N bits 2N things unsigned C99 s uintN t CS61C L03 Introduction to C pt 1 1 META We often make design decisions to make HW simple And in review CS61C Machine Structures questions as soon as they come up Yes It s called the C99 or C9x std You need gcc std c99 to compile References http en wikipedia org wiki C99 http home tiscalinet ch t wolf tw c c9x changes html Highlights Declarations in for loops like Java 15 Java like comments to end of line 10 Variable length non global arrays 33 inttypes h explicit integer types 38 stdbool h for boolean logic def s 35 CS61C L03 Introduction to C pt 1 3 Garcia Spring 2010 UCB Disclaimer CS61C L03 Introduction to C pt 1 4 Garcia Spring 2010 UCB Compilation Overview 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 must have reference Check online for more sources JAVA in a Nutshell O Reilly Chapter 2 How Java Differs from C http oreilly com catalog javanut excerpt Brian Harvey s course notes 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 bytecode 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 Assembling is also done but is hidden i e done automatically by default On CS61C class website CS61C L03 Introduction to C pt 1 5 Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 6 Garcia Spring 2010 UCB Compilation Advantages Compilation Disadvantages Great run time performance generally much faster than Scheme or Java for comparable code because it optimizes for a given architecture All compiled files including the executable are architecture specific depending on both the CPU type and the operating system OK compilation time enhancements in compilation procedure Makefiles allow only modified files to be recompiled 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 Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 7 Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 8 C Syntax main C Syntax Variable Declarations To get the main function to accept arguments use this Very similar to Java but with a few minor but important differences 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 Here argc is 2 All variable declarations must go before they are used at the beginning of the block A variable may be initialized in its declaration if not it holds garbage Examples of declarations correct int a 0 b 10 unix sort myFile argv is a pointer to an array containing the arguments as strings more on pointers later Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 9 Incorrect for int i 0 i 10 i C99 overcomes these limitations Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 10 Address vs Value Pointers Consider memory to be a single huge array An address refers to a particular memory location In other words it points to a memory location 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 Pointer A variable that contains the address of a variable Location address Don t confuse the address referring to a memory location with the value stored in that location 101 102 103 104 105 23 CS61C L03 Introduction to C pt 1 11 42 101 102 103 104 105 23 42 104 x y p name Garcia Spring 2010 UCB CS61C L03 Introduction to C pt 1 12 Garcia Spring 2010 UCB Pointers How to create a pointer Pointers How to change a variable pointed to operator get address of a variable int p x p x 3 p x p p x x 3 x 3 Use dereference operator on left of 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 p 5 p x 3 p x 5 How get a value pointed to dereference operator get value pointed to printf p points to d n p CS61C L03 Introduction to C pt 1 13 Garcia Spring 2010 UCB Pointers and Parameter Passing Java and C pass parameters by value procedure function method gets a copy of the parameter so changing the copy cannot change the original void addOne int x x x 1 CS61C L03 Introduction to C pt 1 14 Garcia Spring 2010 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 int y 3 addOne y y is now 4 y is still 3 CS61C L03 Introduction to C pt 1 15 Garcia Spring 2010 UCB Garcia Spring 2010 UCB Peer Instruction Question Pointers Pointers are used to point to any data type int char a struct etc Normally a pointer can only point 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 a lot of other bad things CS61C L03 Introduction to C pt 1 17 CS61C L03 Introduction to C pt 1 16 Garcia Spring 2010 UCB 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 …
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