6.087-1 Introduction to C- Language

6.087 Lecture 1 – January 11, 2010Introduction to C Writing C Programs Our First C Program 1 1 WhatisC?DennisRitchie–AT&TBellLaboratories–197216-bitDECPDP-11computer(right)Widelyusedtodayextendstonewersystemarchitecturesefficiency/performancelow-levelaccessFeatures of CC features: • Few keywords • Structures, unions – compound data types • Pointers – memory, arrays • External standard library – I/O, other facilities • Compiles to native code • Macro preprocessor 2 Versions of CEvolved over the years: 1972 – C invented • • 1978 – The C Programming Language published; first specification of language • 1989 – C89 standard (known as ANSI C or Standard C) • 1990 – ANSI C adopted by ISO, known as C901999 – C99 standard• • mostly backward-compatible • not completely implemented in many compilers 2007 – work on new C standard C1X announced • In this course: ANSI/ISO C (C89/C90) 3 What is C used for?Systems programming: • OSes, like Linux • microcontrollers: automobiles and airplanes • embedded processors: phones, portable electronics, etc. • DSP processors: digital audio and TV systems...• 4 C vs. related languages• More recent derivatives: C++, Objective C, C# • Influenced: Java, Perl, Python (quite different) C lacks: • • exceptions • range-checking • garbage collection• object-oriented programming• polymorphism...• Low-level language faster code (usually) • ⇒ 5 Warning: low-level language!Inherently unsafe: • No range checking • Limited type safety at compile time • No type checking at runtime Handle with care. • Always run in a debugger like gdb (more later. . . ) Never run as root• Never test code on the Athena servers • Athena is MIT's UNIX-based computing environment. OCW does not provide access to it. 6 1 16.087 Lecture 1 – January 11, 2010Introduction to C Writing C Programs Our First C Program 7 Editing C code.c extension • • Editable directly More later. . . • 7 Compiling a program• gcc (included with most Linux distributions): compiler .o extension • • omitted for common programs like gcc 8 More about gcc• Run gcc: athena% gcc -Wall infilename.c -o outfilename.o • -Wall enables most compiler warnings • More complicated forms exist • multiple source files • auxiliary directories• optimization, linking• Embed debugging info and disable optimization: athena% gcc -g -O0 -Wall infilename.c -o outfilename.o Athena is MIT's UNIX-based computing environment. OCW does not provide access to it. 9 1 1DebuggingFigure: gdb: command-line debugger 10 Using gdbSome useful commands: • break linenumber – create breakpoint at specified line • break file:linenumber – create breakpoint at line in file • run – run programc – continue execution• next – execute next line • • step – execute next line or step into function • quit – quit gdb • print expression – print current value of the specified expression • help command – in-program help 11 Memory debuggingFigure: valgrind: diagnose memory-related problems 12 The IDE – all-in-one solution• Popular IDEs: Eclipse (CDT), Microsoft Visual C++ (Express Edition), KDevelop, Xcode, . . . • Integrated editor with compiler, debugger • Very convenient for larger programs Courtesy of The Eclipse Foundation. Used with permission. 13 Using Eclipse• Need Eclipse CDT for C programs (see http://www.eclipse.org/cdt/) • Use New > C Project • choose “Hello World ANSI C Project” for simple project • “Linux GCC toolchain” sets up gcc and gdb (must be installed separately) • Recommended for final project 14 6.087 Lecture 1 – January 11, 2010Introduction to C Writing C Programs Our First C Program 15 Hello, 6.087 students• In style of “Hello, world!” .c file structure • • Syntax: comments, macros, basic declarations • The main() function and function structure • Expressions, order-of-operations • Basic console I/O (puts(), etc.) 15 Structure of a .c file/* Begin with comments about file contents */Insert #include statements and preprocessor definitions Function prototypes and variable declarations Define main() function { Function body } Define other function { Function body } . . . 16 Comments• Comments: /∗ this is a simple comment ∗/• Can span multiple lines /∗ This commentspansmultiple lines ∗ /• Completely ignored by compiler • Can appear almost anywhere /∗ hello .c −− our first C program Created by Daniel Weller , 01/11/2010 ∗ /17 The #include macro• Header files: constants, functions, other declarations #include – read the contents of the header file • stdio.h • stdio.h: standard I/O functions for console, files /∗ hello .c −− our first C program Created by Daniel Weller , 01/11/2010 ∗ /#include /∗ basic I/O facilities ∗ /18 More about header files• stdio.h – part of the C Standard Library • other important header files: ctype.h, math.h, stdlib.h, string.h, time.h • For the ugly details: visit http: //www.unix.org/single_unix_specification/(registration required) • Included files must be on include path • -Idirectory with gcc: specify additional include directories • standard include directories assumed by default • #include "stdio.h" – searches ./for stdio.h first 19 Declaring variablesMust declare variables before use • Variable declaration: • int n;float phi;• int -integer data type • float -floating-point data type • Many other types (more next lecture. . . ) 20 Initializing variables• Uninitialized, variable assumes a default value • Variables initialized via assignment operator: n = 3;Can also initialize at declaration:• float phi = 1.6180339887; • Can declare/initialize multiple variables at once: int a, b, c =0, d=4; 21 Arithmetic expressionsSuppose x and y are variables • x+y, x-y, x*y, x/y, x%y: binary arithmetic • A simple statement:y = x+3∗x/(y−4);• Numeric literals like 3 or 4 valid in expressions • Semicolon ends statement (not newline) • x += y, x -= y, x *= y, x /= y, x %= y: arithmetic and assignment 22 Order of operations• Order of operations: Operator Evaluation direction +,-(sign) right-to-left *,/,% left-to-right +,-left-to-right =,+=,-=,*=,/=,%= right-to-left • Use parentheses to override order of evaluation 23 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 24 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 2. Evaluate multiplies and divides, from left-to-right float z = x+3∗x/2.0; float z = x+6.0/2.0; float z = x+3.0; →→ 24 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 2. Evaluate multiplies and divides, from left-to-right float z = x+3∗x/2.0; float z = x+6.0/2.0; float z = x+3.0; →→ 3. Evaluate additionfloat z = x+3.0; float z = 5.0;→ 24 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 2. Evaluate multiplies and divides, from left-to-right float z = x+3∗x/2.0; float z = x+6.0/2.0; float z = x+3.0; →→ 3. Evaluate additionfloat z = x+3.0; float z = 5.0;→ 4. Perform initialization with assignmentNow, z =5.0.24 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 2. Evaluate multiplies and divides, from left-to-right float z = x+3∗x/2.0; float z = x+6.0/2.0; float z = x+3.0; →→ 3. Evaluate additionfloat z = x+3.0; float z = 5.0;→ 4. Perform initialization with assignmentNow, z =5.0.How do I insert parentheses to get z =4.0? 24 Order of operationsAssume x =2.0 and y =6.0. Evaluate the statement float z = x+3∗x/(y−4); 1. Evaluate expression in parenthesesfloat z = x+3∗x/(y−4); float z = x+3∗x/2.0;→ 2. Evaluate multiplies and divides, from left-to-right float z = x+3∗x/2.0; float z = x+6.0/2.0; float z = x+3.0; →→ 3. Evaluate additionfloat z = x+3.0; float z = 5.0;→ 4. Perform initialization with assignmentNow, z =5.0.How do I insert parentheses to get z =4.0? float z = (x+3∗x)/(y−4); 24 Function prototypesFunctions also must be declared before use • • Declaration called function prototype • Function prototypes: int factorial ( int ); or int factorial (int n); • Prototypes for many common functions in header files for C Standard Library 25 Function prototypesGeneral form: • return_type function_name(arg1,arg2,...); • Arguments: local variables, values passed from caller • Return value: single value returned to caller when function exits • void – signifies no return value/arguments int rand(void); 26 The main() function• main(): entry point for C program • Simplest version: no inputs, outputs 0 when successful, and nonzero to signal some error int main(void); • Two-argument form of main(): access command-line arguments int main(int argc, char ∗∗argv); • More on the char **argv notation later this week. . . 27 Function definitionsFunction declaration { declare variables; program statements; } • Must match prototype (if there is one)variable names don’t have to match• no semicolon at end • • Curly braces define a block – region of code • Variables declared in a block exist only in that block • Variable declarations before any other statements 28 Our main() function/∗ The main() function ∗ /int main ( void )/∗ entry point ∗ /{ /∗ write message to console ∗ /puts ( "hello, 6.087 students" ); return 0; /∗ exit (0 => success) ∗ /} • puts(): output text to console window (stdout) and end the line • String literal: written surrounded by double quotes • return 0;exits the function, returning value 0 to caller29 Alternative main() function• Alternatively, store the string in a variable first: int main ( void )/∗ entry point ∗ /{const char msg[] = "hello, 6.087 students" ;/∗ write message to console ∗ /puts (msg); • const keyword: qualifies variable as constant • char: data type representing a single character; written in quotes: ’a’, ’3’, ’n’ • const char msg[]: a constant array of characters 30 More about strings• Strings stored as character array • Null-terminated (last character in array is ’\0’ null) • Not written explicitly in string literals • Special characters specified using \ (escape character): • \\ – backslash, \’ – apostrophe, \” – quotation mark • \b, \t, \r, \n – backspace, tab, carriage return, linefeed • \ooo, \xhh – octal and hexadecimal ASCII character codes, e.g. \x41 – ’A’, \060 – ’0’ 31 Console I/O• stdout, stdin: console output and input streams • puts(string): print string to stdout • putchar(char): print character to stdout • char = getchar(): return character from stdin • string = gets(string): read line from stdin into string • Many others -later this week 32 Preprocessor macros• Preprocessor macros begin with # character #include • #define msg "hello, 6.087 students" defines msg as “hello, 6.087 students” throughout source file • many constants specified this way 33 Defining expression macros• #define can take arguments and be treated like a function #define add3(x,y,z) ((x)+(y)+(z)) • parentheses ensure order of operations • compiler performs inline replacement; not suitable for recursion 34 Conditional preprocessor macros• #if, #ifdef, #ifndef, #else, #elif , #endif conditional preprocessor macros, can control which lines are compiled • evaluated before code itself is compiled, so conditions must be preprocessor defines or literals • the gcc option -Dname=value sets a preprocessor define that can be used • Used in header files to ensure declarations happen only once 35 Conditional preprocessor macros• #pragma preprocessor directive • #error, #warning trigger a custom compiler error/warning • #undef msg remove the definition of msg at compile time 36 Compiling our codeAfter we save our code, we run gcc: athena% gcc -g -O0 -Wall hello.c -o hello.o Assuming that we have made no errors, our compiling is complete. Athena is MIT's UNIX-based computing environment. OCW does not provide access to it. 37 1 1Running our codeOr, in gdb, athena% gdb hello.o.. . Reading symbols from hello.o...done. (gdb) run Starting program: hello.o hello, 6.087 students Program exited normally.(gdb) quitathena%Athena is MIT's UNIX-based computing environment. OCW does not provide access to it. 38 1 1SummaryTopics covered: • How to edit, compile, and debug C programs • C programming fundamentals:comments• • preprocessor macros, including #include • the main() function • declaring and initializing variables, scope • using puts() – calling a function and passing an argument • returning from a function 39 MIT OpenCourseWare http://ocw.mit.edu 6.087 Practical Programming in C IAP 2010 For information about citing these materials or our Terms of Use,visit: http://ocw.mit.edu/terms.