COMP26912: Algorithms and Imperative Programming

Transcription

1 COMP26912 Lecture C4 1/28 COMP26912: Algorithms and Imperative Programming Lecture C4: C - Coping without Classes Pete Jinks School of Computer Science, University of Manchester Spring 2012

2 OMP26912 Lecture C4 2/28 What have you learnt about C? Most surprising? Most annoying? Most useful or impressive? Hardest to understand? Would like to know more about?

3 COMP26912 Lecture C4 Java classes let us control... 4/28 Lecture Outline Java Classes let us control partitioning, implementation hiding etc.... creating new (user-defined) types... name visibility... variable lifetime So what mechanisms does C give us?

4 COMP26912 Lecture C4 Java classes let us control... 6/28 Compiling and running Java: compile interpret compile: javac Fred.java ( Fred.class) interpret: java Fred C: compile link run compile+link: gcc fred.c -o fred run:./fred (-o filename: name of output) compile: gcc -c fred.c -o fred.o link: gcc fred.o -o fred (-c: compile but don t link) (more compile flags: gcc -g -std=c99 -Wall... ) (more link flags: gcc -lm... )

5 COMP26912 Lecture C4 Java classes let us control... 7/28 Partitioning javac One.java Two.java (or just: javac One.java) java One gcc -c one.c -o one.o gcc -c two.c -o two.o gcc one.o two.o -o both./both.class.o package library (-l flag) both dealt with during linking

6 COMP26912 Lecture C4 Java classes let us control... 8/28 Checking partitions for consistency filename.h = interface, filename.c = implementation import One; #include "one.h" (or #include <stdio.h>) (include own.h as a check) private int i static int i public int j int j + extern int j in.h file One.x x

7 OMP26912 Lecture C4 Java classes let us control... 9/28 e.g. one.h: (all the public things in one.c) extern int x; two.h: (all the public things in two.c) extern int y; one.c: #include "one.h" #include "two.h" int x = 0; (the public variable) static int z = 0; (a private variable) static void f(){ (a private function) x= y + z; (uses y from two.c) } two.c:...

8 OMP26912 Lecture C4 Java classes let us control... 10/28 Q: Rewrite this Java as C One.java: class One { public static int x; private static int y; private static void a (int b) {y= y - b; x= x + b; Two.z= b; Two.c (b);} } Two.java: class Two { public static int z; private static int w; public static void c (int d) {z= w - d; One.x= d;} }

9 Answer COMP26912 Lecture C4 Java classes let us control... 12/28

10 COMP26912 Lecture C4 Java classes let us control... 13/28 Using make to support partitioning makefile rules: result file(s): input file(s) command(s) to make result(s) from input(s) Built-in Rules: file.o is made automatically from file.c by e.g. gcc -c file.c -o file.o file is made automatically from file.o by e.g. gcc file.o -o file

11 OMP26912 Lecture C4 Java classes let us control... 15/28 Q: make a partitioned program Given files: one.h, one.c, two.h, two.c, where one.c and two.c both #include one.h and two.h, write a makefile to create a single binary program both from one.c and two.c so that the minimum recompilation etc. is done if any of these files is edited

12 COMP26912 Lecture C4 Java classes let us control... 18/28 Types in C scalar: int, float, char etc. array, string: char [], char *, int [][], etc. pointer: *, & struct: list of contents ( fields ) union: list of alternatives enum: list of names

13 COMP26912 Lecture C4 Java classes let us control... 19/28 User-defined types in C e.g. struct my struct {...}; union my union {...}; enum my enum {...}; e.g. typedef... my type... ; e.g. #define my type 2...

14 COMP26912 Lecture C4 Java classes let us control... 20/28 Why use typedef? char *x[10]; Is x a pointer to an array, or an array of pointers? typedef char* ptr2char; ptr2char a[10]; typedef char arrayof10char[10]; arrayof10char *b;

15 COMP26912 Lecture C4 Java classes let us control... 22/28 Identifier visibility ( Scope ) Java C private (i.e. class) static (i.e. file) public (i.e. program) extern inside { } (and nesting etc.) same in C class/object. name just name in C C only: struct. field (and union. field and pointer >field) C only: #define { int b; // not a float #define int float int c; // a float } int d; // also a float convention: #define INT...

16 COMP26912 Lecture C4 Java classes let us control... 23/28 C has 5 Namespaces ( overloading classes ) e.g. struct foo {int foo;} foo; variables, functions, parameters, typedefs, enum values tags (i.e. names of struct, union, enum) field-names (inside a struct or union) preprocessor macros (#define) labels Don t do this!

17 COMP26912 Lecture C4 Java classes let us control... 25/28 Variable lifetime ( Extent ) Java C { to } same in C ( stack ) (recursion: many copies at the same time) instance (from new to release of owning object) heap (from malloc to free) class (for whole runtime of program) static!

18 COMP26912 Lecture C4 Java classes let us control... 27/28 e.g. static void insert (int values[], int nextvalue) { static int nextinsert= 0; values[nextinsert]= nextvalue; nextinsert++; } static... insert... visibility = file {static... nextinsert...} visibility = brackets (function) exists for whole of runtime, only copy

19 COMP26912 Lecture C4 Java classes let us control... 28/28 Lecture Review Java classes let us control partitioning... user-defined types... visibility... lifetime Next week: Milan Trees & Hashing