10/16/20091David Notkin Autumn 2009 CSE303 Lecture 8Lecture summary• History and characteristics of C• Major C language features– differences between C and Java• basic console input and output (printf and scanf)• Our learning objectives in C– procedural programming– deeper understanding of program compilation and execution– learn details of memory management– debugging skills– software development strategiesHistory• Created in 1972 by Dennis Ritchie of Bell Labsto accompany the Unix operating system– latest version standard: "C99" (1999)• Designed for creating system software(programs close to the OS that talk directly to hardware)– Also designed to be hardware-independent (portable)– C is also used to develop high-level applications• Currently one of the top two most widely used language worldwide• Based on ALGOL; has influenced the designs of many languages– C++, Java, C#, Perl, Eiffel, Objective-C, Modula, Pascal, ...Characteristics of C• fairly similar basic syntax and semantics to Java– if/else, for, while, int, double, {} [] () ; +- */% ++• Much smaller provided standard library than Java• More low-level (more work for programmer, less for compiler)• Procedural (not object-oriented)– C does not have objects as we know them– verb(noun); rather than noun.verb();• More unsafe (an incorrect program can cause more damage): C programs have more direct access to the system / hardwareFirst C program#include <stdio.h>int main(void) {printf("Hello, world!\n");return 0;}• Kernighan and Ritchie started the convention that the first program you show in a new language should be one that prints "Hello, world!"Dissecting Hello World#include <stdio.h>int main(void) {printf("Hello, world!\n");return 0;}like import in Java;links the program tothe standard I/O library(includes printf function)the main function header;you don't need to say public staticbecause these are the default in Cmain returns an int error code to the OS(0 on success, > 0 on failure)like println in Java (actually more like System.out.printf);prints output to console10/16/20092Second C program/* Computes greatest common divisor (GCD) with Euclid's algorithm. */#include <stdio.h>int main(int argc, char** argv) {int a, b, temp, r;printf("Please enter two positive integers: ");scanf("%d %d", &a, &b);if (b > a) {temp = a;a = b;b = temp;}while ((r = a % b) != 0) {a = b;b = r;}printf("The GCD is %d.\n", b);return 0;}Compiling/running• To compile a program– gcc -o target source.c– target is the name of the executable program to build• The compiler builds an actual executable file, not a .class like Java– example: gcc -o hi hello.c• To run your program, just execute that file– example: ./hicommand descriptiongccGNU C compilergcc options (partial)• Most common usage for this course:– gcc -g -Wall -o target source.c– the warnings from -Wall will protect usfrom unwise idioms-Waddress-Warray-bounds (only with „-O2‟)-Wc++0x-compat-Wchar-subscripts-Wimplicit-int-Wimplicit-function-declaration-Wcomment-Wformat-Wmain (only for C/ObjC and unless „-ffreestanding‟)-Wmissing-braces-Wnonnull-Wparentheses-Wpointer-sign-Wreorder-Wreturn-type-Wsequence-point-Wsign-compare (only in C++)-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-function-Wunused-label-Wunused-value-Wunused-variable-Wvolatile-register-varoption description-Wlevel of warnings to display(common usage: -Wall for all warnings)-ooutput executable file name(if omitted, compiles to file a.out )-ggenerates information for debugger toolsGuess: how many pages does the gcc manual have on gcc options?printf• printf("format string", parameters);• A format string contains placeholders to insert parameters into it:– %d or %i an integer– %lf a double ('long floating-point')– %s a string– %p a pointer (seen later)int x = 3; int y = 2;printf("(%d, %d)\n", x, y); // (3, 2)function descriptionprintf prints formatted output to stdoutprintf continued• A placeholder can specify the parameter's width or precision:– %8d an integer, 8 characters wide, right-aligned– %-8d an integer, 8 characters wide, left-aligned– %.4f a real number, 4 digits after decimal– %6.2f a real number, 6 total characters wide, 2 after decimal• Examples:int age = 45;double gpa = 1.2345678;printf("%8d %7.3f\n", age, gpa);printf("%8.2f %.1f %10.5f", gpa, gpa, gpa);Very much the same as Java• General syntax for statements, control structures, function calls• Types int, double, char, long– type-casting syntax• Expressions, operators, precedence+ - * / % ++ --= += -= *= /= %=< <= == != > >= && || !• Scope (within set of { } braces)• Comments: /* ... */, // ( // not officially legal until C99)10/16/20093Mostly the same as Java• Variables– can be used without being initialized (!)– must be declared at the start of a function or block (changed in C99)• for loops– variable cannot be declared in the loop header• if/else statements, while and do/while loops– there is no boolean type (changed in C99)– any type of value can be used as a test– 0 means false, every other number means true• Parameters / returns– C has certain features for values vs. references ("pointers")Very different from Java• Strings– very clunky to use in C; arrays of characters– are not objects; do not contain methods (external string functions)• I/O to/from console and files– no Scanner; must use input functions such as scanf– console I/O different than file I/O• Errors and exceptions– C has no try/catch and does not represent errors as objects– errors are usually returned as integer error codes from functions– crashes are mostly called "segmentation faults" and are not of much direct utility in figuring out what is wrongAlso very different• Arrays– are just bare contiguous blocks of memory– have no methods and do not know their own length (!)• Objects– C doesn't have them– closest similar feature: struct (a set of fields; no methods)• Memory management– most memory that you consume, you must explicitly free afterward• API and provided libraries– C doesn't have very many, compared to Java– you must write many things yourself (even data structures)scanf• scanf("format string", variables);• uses same syntax for formatted strings, placeholders as printf• Must precede each variable with an & (address-of operator)int
View Full Document
Unlocking...