Week 1 / Lecture 2 8 March 2017 NWEN 241 C Fundamentals. Alvin Valera. School of Engineering and Computer Science Victoria University of Wellington

Transcription

1 Week 1 / Lecture 2 8 March 2017 NWEN 241 C Fundamentals Alvin Valera School of Engineering and Computer Science Victoria University of Wellington

2 Admin stuff People Course Coordinator Lecturer Alvin Valera AM 401 Tutors Alex Mitchell Brady Hanna Celine Young Daniel Ko Inti Mateus Resende Albuquerque Jakob Pfender Sean Stevenson Yueying Stella Chang Class Representatives Hannah Pachoud Christian Lee Stefan Andelic NWEN 241: Systems Programming 2

3 Admin stuff Helpdesk sessions Mon Tue Wed Thu Fri [09:00-11:00] Helpdesk CO 246 [14:00-16:00] Helpdesk CO 246 [14:00-16:00] Helpdesk CO 246 [14:00-16:00] Helpdesk CO 246 Tutors will be on duty from Weeks 2-11 during Helpdesk sessions Helpdesk sessions are optional; no signup is necessary NWEN 241: Systems Programming 3

4 Admin stuff Tutorial setup & preparation First tutorial will be a walk-through on how to write and debug C programs using Plain text editor and command line interface Integrated development environment If you want to follow the walk-through session using your laptop*: (1) Install Oracle VM VirtualBox (2) Download NWEN 241 VM to your laptop hard disk *Detailed instructions are available in the course web page: NWEN 241: Systems Programming 4

5 Recap A C program consists of one or more functions, and one of those functions must be main Program execution begins with the main function /* A simple program */ #include <stdio.h> int main(void) { printf("hello world\n"); } return 0; NWEN 241: Systems Programming 5

6 This lecture Program structure (continued) Compilation process Data types NWEN 241: Systems Programming 6

7 Program structure Indicates start of compound statement /* A simple program */ #include <stdio.h> String start and end int main(void) { printf("hello world\n"); Indicates end of compound statement } return 0; Indicates end of statement NWEN 241: Systems Programming 7

8 Whitespaces A note on whitespaces in C source code: Compiler ignores whitespaces (space, tab, newline), except in string literals and for separating tokens int main(void) { printf("hello world\n"); } return 0; int main(void){printf("hello world\n");return 0;} NWEN 241: Systems Programming 8

9 Header files #include <stdio.h> A header file usually contains function prototypes, constant definitions, type definitions, etc. Which header file to include? Include header files that contain the function prototype, constant definition, type definition, etc., used in your program NWEN 241: Systems Programming 9

10 Header files C provides a standard library* which consists of the following headers: <assert.h> <ctype.h> <errno.h> <float.h> <limits.h> <locale.h> *C99 and C11 added more header files. <math.h> <setjmp.h> <signal.h> <stdarg.h> <stddef.h> <stdio.h> <stdlib.h> <string.h> <time.h> To know more about the C standard library, visit NWEN 241: Systems Programming 10

11 Identifiers /* Program to calculate the area of a circle */ #include <stdio.h> #define PI 3.14 float sq(float); int main(void) { float radius, area; } /* Ask user to input */ printf("radius = "); scanf("%f", &radius); area = PI * sq(radius); printf("area = %f\n", area); return 0; To uniquely identify constants, variables, functions and labels, you need to name them with an identifier float sq(float r) { return (r * r); } NWEN 241: Systems Programming 11

12 Identifiers in C An identifier is a sequence of letters and digits The first character must be a letter The underscore character _ counts as a letter Upper and lower case letters are diferent Identifiers may have any length Usually, only the first 31 characters are significant For macro names, only the first 63 characters are significant C reserved keywords cannot be used as identifiers! Use meaningful names when naming, i.e., a name that describes the purpose of the entity NWEN 241: Systems Programming 12

13 Examples counter Valid: consists of letters _Temp_variable_2 Valid: consists of letters and digits 1myVariable steps{2} continue Invalid: first character is not a letter Invalid: uses non-letter and nondigit characters Invalid: reserved word NWEN 241: Systems Programming 13

14 Writing a program & compilation Using a text editor and command line interface % vi hello.c /* A simple program */ #include <stdio.h> int main(void) This opens hello.c (if it exists) or creates a new one { printf("hello world\n"); return 0; } NWEN 241: Systems Programming 14

15 Writing a program & compilation Afer saving your changes to the file, you can then compile it % gcc hello.c -o hello This invokes gcc to compile hello.c and generate executable output file called hello %./hello Hello world This executes hello NWEN 241: Systems Programming 15

16 Compilation process hello.c Source program (text) Preprocessor (cpp) hello.i Compiler (cc1) Modified source program (text) Preprocessing phase. hello.s Assembly program (text) Assembler (as) printf.o hello.o Relocatable object programs (binary) Linker (ld) The preprocessor (cpp) modifies the original C program according to directives that begin with the '#' character, e.g., #include <stdio.h> command in line 3 of hello.c tells the preprocessor to read the contents of the system header file stdio.h and insert it directly into the program text. The result is another C program, typically with the.i sufix. hello Executable object program (binary) NWEN 241: Systems Programming 16

17 Compilation process printf.o hello.c Source program (text) Preprocessor (cpp) hello.i Compiler (cc1) Modified source program (text) hello.s Assembly program (text) Assembler (as) hello.o Relocatable object programs (binary) Linker (ld) hello Executable object program (binary) Compilation phase. The compiler (cc1) translates the text file hello.i into the text file hello.s, which contains an assemblylanguage program. NWEN 241: Systems Programming 17

18 Compilation process hello.c Source program (text) Preprocessor (cpp) hello.i Compiler (cc1) Modified source program (text) hello.s Assembly program (text) Assembler (as) printf.o hello.o Relocatable object programs (binary) Linker (ld) hello Executable object program (binary) Assembly phase. The assembler (as) translates hello.s into machine-language instructions, packages them in a form known as a relocatable object program, and stores the result in the object file hello.o. This file is a binary file containing 17 bytes to encode the instructions for function main. If you try to open hello.o with a text editor, it would appear to be gibberish. NWEN 241: Systems Programming 18

19 Compilation process hello.c Source program (text) Preprocessor (cpp) hello.i Compiler (cc1) Modified source program (text) hello.s Assembly program (text) Assembler (as) printf.o hello.o Relocatable object programs (binary) Linker (ld) Linking phase. printf function, which is part of the standard C library provided by every C compiler. printf function resides in a separate pre-compiled object file called printf.o, which must be merged with our hello.o program. The linker (ld) performs this merging, creating an executable object file (or simply executable) that is ready to be loaded into memory and executed by the system. hello Executable object program (binary) NWEN 241: Systems Programming 19

20 Data types What do you see? NWEN 241: Systems Programming 20

21 Data types What do you see? NWEN 241: Systems Programming 21

22 Data types Programming is about describing data and algorithms How data is represented in memory? Four basic data types: int (integer quantity) char (single character) float (floating-point number) double (double-precision floating-point number) Note: There are also qualifiers associated with some of the types: short / long, and signed / unsigned. NWEN 241: Systems Programming 22

23 Data types Two groups of types Integral types: int and char Can be used to hold integer values Floating types: float and double Can be used to hold real values NWEN 241: Systems Programming 23

24 char char is meant to hold 1 ASCII character see 0 NUL 1 SOH 2 STX 3 ETX 4 EOT 5 ENQ 6 ACK 7 BEL 8 BS 9 HT 10 NL 11 VT 12 NP 13 CR 14 SO 15 SI 16 DLE 17 DC1 18 DC2 19 DC3 20 DC4 21 NAK 22 SYN 23 ETB 24 CAN 25 EM 26 SUB 27 ESC 28 FS 29 GS 30 RS 31 US 32 SP 33! 34 " 35 # 36 $ 37 % 38 & 39 ' 40 ( 41 ) 42 * , / : 59 ; 60 < 61 = 62 > 63? 65 A 66 B 67 C 68 D 69 E 70 F 71 G 72 H 73 I 74 J 75 K 76 L 77 M 78 N 79 O 80 P 81 Q 82 R 83 S 84 T 85 U 86 V 87 W 88 X 89 Y 90 Z 91 [ 92 \ 93 ] 94 ^ 95 _ 96 ` 97 a 98 b 99 c 100 d 101 e 102 f 103 g 104 h 105 i 106 j 107 k 108 l 109 m 110 n 111 o 112 p 113 q 114 r 115 s 116 t 117 u 118 v 119 w 120 x 121 y 122 z 123 { } 126 ~ 127 DEL NWEN 241: Systems Programming 24

25 Data types What do you see? Interpreted as an integer: 65 Interpreted as an ASCII character: A NWEN 241: Systems Programming 25

26 Floating types Most computers use the IEEE 754 standard for representing floating types IEEE 754 defines two representations Single precision (32-bit) Positive range: Double precision (64-bit) Positive range: NWEN 241: Systems Programming 26

27 Floating types single: 8 bits double: 11 bits single: 23 bits double: 52 bits S E M x = ( 1) S (1+M ) 2 ( E Bias) S: sign bit (0 non-negative, 1 negative) E: exponent in excess representation Bias = 127 in single Bias = 1023 in double M: Mantissa or fraction part NWEN 241: Systems Programming 27

28 Data type size Sizes of diferent types Use sizeof() to find out The sizes may vary from machine to machine The following rules are always guaranteed: sizeof(char) = 1 sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) sizeof(signed) = sizeof(unsigned) = sizeof(int) sizeof(float) <= sizeof(double) <= sizeof(long double) Does Java have varied sizes between systems? NWEN 241: Systems Programming 28

29 Declaring variables A variable must be declared before it can be used A declaration specifies a type, and contains a list of one or more variables of that type int lower, upper, step; char c, line[1000]; Variables can be distributed among declarations in any fashion int lower; int upper, step; char c; char line[1000]; NWEN 241: Systems Programming 29

30 Declaring variables A variable may be initialized in its declaration If variable name is followed by an equals sign and an expression, the latter serves as an initializer int i = 0; char c = 'A'; float f = 1.25; Possible initializers Constant Expression (depends on whether variable is automatic or not) To be discussed in detail in the future NWEN 241: Systems Programming 30

31 Type casting Type casting is a way to convert a variable from one data type to another data type C performs automatic type casting int i = 2; double d = 2.5; i = (int)d; /* explicit type casting */ i = d; /* d is converted to an int * and then assigned to i. */ NWEN 241: Systems Programming 31

32 Constants Constants integer constants floating-point constants character constants string constants enumeration constants Naming constants Use the const qualifier const float PI = 3.14; Use the preprocessor #define PI 3.14 NWEN 241: Systems Programming 32

33 To be continued More on data types in the next lecture NWEN 241: Systems Programming 33