CS61C L03 Introduction to C (pt 1) (1)!Garcia, Spring 2010 © UCB!! !Lecturer SOE Dan Garcia!! !www.cs.berkeley.edu/~ddgarcia inst.eecs.berkeley.edu/~cs61c "CS61C : Machine Structures" Lecture 3 – Introduction to "the C Programming Language (pt 1) " 2010-01-25!MIPS Supercomputer ⇒"Chinaʼs 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. Youʼll learn MIPS in CS61C!!www.technologyreview.com/computing/24374/ CS61C L03 Introduction to C (pt 1) (2)!Garcia, Spring 2010 © UCB!And in review...!• We represent “things” in computers as particular bit patterns: N bits ⇒ 2N things !!• These 5 integer encodings have different benefits; 1s complement and sign/mag have most problems.!• unsigned (C99ʼs uintN_t) : "• 2ʼs complement (C99ʼs intN_t) universal, learn!!• "• Overflow: numbers ∞; computers finite,errors! !00000 00001 01111 ... 11111 11110 10000 ... META: We often make design decisions to make HW simple!META: Ainʼt no free lunch!00000 00001 01111 ... 10000 11111 ... CS61C L03 Introduction to C (pt 1) (3)!Garcia, Spring 2010 © UCB!Introduction to C!• What to do if youʼre in that 10%?!• Start early, ask questions as soon as they come up!!• Officially, “some” C experience is required before CS61C!• 2010Sp an experiment!!CS61C L03 Introduction to C (pt 1) (4)!Garcia, Spring 2010 © UCB!Has there been an update to ANSI C?!• Yes! Itʼs 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 defʼs (#35)!CS61C L03 Introduction to C (pt 1) (5)!Garcia, Spring 2010 © UCB!Disclaimer!• Important: You will not learn how to fully code in C in these lectures! Youʼll still need your C reference for this course.!• K&R is a must-have reference! Check online for more sources!• “JAVA in a Nutshell,” OʼReilly. ! Chapter 2, “How Java Differs from C”! http://oreilly.com/catalog/javanut/excerpt/!• Brian Harveyʼs course notes! On CS61C class website!CS61C L03 Introduction to C (pt 1) (6)!Garcia, Spring 2010 © 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 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)!CS61C L03 Introduction to C (pt 1) (7)!Garcia, Spring 2010 © UCB!Compilation : Advantages!• 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!CS61C L03 Introduction to C (pt 1) (8)!Garcia, Spring 2010 © 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!CS61C L03 Introduction to C (pt 1) (9)!Garcia, Spring 2010 © 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). Here argc is 2:!unix% sort myFile • argv is a pointer to an array containing the arguments as strings (more on pointers later).!CS61C L03 Introduction to C (pt 1) (10)!Garcia, Spring 2010 © 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; if not, it holds garbage!!• Examples of declarations:!• correct: { int a = 0, b = 10; ...!• Incorrect:* for (int i = 0; i < 10; i++) *C99 overcomes these limitations!CS61C L03 Introduction to C (pt 1) (11)!Garcia, Spring 2010 © 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?!!• Donʼt confuse the address referring to a memory location with the value stored in that location.!23" 42" ..." ..."101 102 103 104 105 ..."CS61C L03 Introduction to C (pt 1) (12)!Garcia, Spring 2010 © 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 a variable.!23" 42" ..." ..."101 102 103 104 105 ..."x"y"Location (address)"name"p"104"CS61C L03 Introduction to C (pt 1) (13)!Garcia, Spring 2010 © UCB!Pointers!• How to create a pointer:!& operator: get address of a variable!int *p, x; !p" ?" x" ?"x = 3; !p" ?" x" 3"p =&x; !p" x" 3"• How get a value pointed to?! * “dereference operator”: get value pointed to! printf(“p points to %d\n”,*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."CS61C L03 Introduction to C (pt 1) (14)!Garcia, Spring 2010 © UCB!Pointers!• How to change a variable pointed to?!• Use dereference * operator on left of = p" x" 5"*p = 5; p" x" 3"CS61C L03 Introduction to C (pt 1) (15)!Garcia, Spring 2010 © UCB!Pointers and Parameter Passing!• Java and C pass parameters “by value”!• procedure/function/method
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