C For Java Programmers

1. C For Java Programmers Tom Roeder CS415 2005sp

2. Why C? • The language of low-level systems programming • Commonly used (legacy code) • Trades off safety for speed • Clear mapping from C statements to operations • Simple to understand, imperative language • Well-understood common optimizations

3. Why not C? • Explicit memory management • Memory leaks • Invalid pointers • No (built-in) exception handling • Not type safe • Poor separation of concerns • No good language support for modularization • Too close to assembly

4. Common Constructs in C and Java • Loops • if() {} • for(;;) {} • while() {} • do {} while() • Functions • int call_me(float a) {return (int)a;} • int temp = call_me(3.14);

5. C Types: Primitives • Often architecture-dependent • int, short, long • Can count on short <= int <= long • float, double • Can usually count on float <= double • char • One byte per character • Unicode WCHAR in Windows: two bytes

6. C Types: struct • Often need to define object-like storage units • C only encapsulates data, not methods • struct is the unit of encapsulation struct pos { float x; float y; } p; p.x = 0.7; p.y = 0.1;

7. C Types: enum • When you need to write code like Color x = RED; • enum { BLUE, RED, GREEN }; • We’ll get to how to define the Color symbol • enums are actually underlying ints. • Can have any value at all • enum { BLUE = 7, RED = 137, GREEN }; • GREEN will have value 138.

8. C Types: pointers • A pointer contains the starting address of the memory for a given value int y = 0; int* x = &y; *x = 10; /* y is now 10 */ • Explicit dynamic memory management int* x = malloc(sizeof(int)*z); free(x);

9. C Types: arrays and strings • An array is just a pointer (0-based) int x[5] = {10, 20, 30, 40, 50}; x[3] == *(x + 3); • A string is just an array of characters int main(int argc, char** argv) { printf(“arg 1: %s\n”, argv[0]); printf(“char 1: %c\n”, argv[0][0]); } • Remember to terminate your string with ‘\0’ • See string.h for functions: strcmp, strcpy, …

10. C Types: typedef • When you want to define a new type: typedef int bool; typedef enum { FALSE = 0, TRUE } bool; typedef struct queue_t { void* elt; queue_t* next; } queue; • Can be ill-used: (as in Microsoft) typedef int* INTP

11. C Types: void* • void* is a pointer to any type • Extremely useful in generic data structures typedef struct queue_t { void* elt; queue_t* next; } queue; queue* q; … *(int*)(q->elt) = 137;

12. C Functions: function pointers • Unlike in Java, we don’t have any reflection • We can nonetheless pass functions around int call_me(float a) { return (int)a;} … int (*fp)(float) = &call_me; printf(“fp gives %d\n”, (*fp)(3.0)); • Simply passing the address of the function

13. C Functions: parameter passing • Two methods in C • By value • By reference int swap(int a, int b); int swap(int* a, int* b); int swap(int& a, int& b); • In Java, all reference types are passed by reference

14. C Functions: prototypes • A function in C must be declared before used • Thus you often give the signature twice: int call_me(float a); int main(int argc, char** argv) { return call_me(3.0); } int call_me(float a) {return 1;}

15. C Traps: memory management • Don’t forgot to free memory you’ve alloc’ed • Arrays are not bounds checked in C • Set all pointers to NULL • when they are initialized • when they are freed • Requires strict discipline, and you will forget • Use Purify (if on *NIX, use valgrind) • Always check to see if a pointer is NULL before using it.

16. C Traps: local variables • Simple example: char* get_name() { char temp[NAME_LENGTH]; /* get something into temp */ return temp; } • temp is allocated on the stack; you will have an invalid pointer.

17. C Preprocessor • Used for constants and simple replacements #define PI 3.14 #define DEBUG 1 • Also used for macros with arguments #define max(x,y) ((x)>=(y) ? (x):(y)) • Conditional compilation #if DEBUG #endif • includes: #include <stdio.h>

18. Most Common C Libraries • <stdio.h> : standard I/O • printf (format, …); • printf(“Hello there, %s\n”, “Tom”); • %s – strings • %c – characters • %d – integers • %f – float • %lf – double • <stdlib.h>: useful functions: • exit, malloc, free

19. Multi-file project layout • Divide the code into functional units • Each unit has a .h and a .c file • Put the prototypes in the .h and the functions in .c • To make sure that .h files aren’t included more than once, for myfile.h, we do: #ifndef __MYFILE_H_ #define __MYFILE_H_ … /* file contents */ #endif /* __MYFILE_H_ */

20. Multi-file project layout • Normally have a file main.c • Include the other .h files • Has a function int main(int argc, char** argv) {} • To compile, use make/nmake • create object files (*.o) and then link with libraries • We won’t go into make here.

21. Where to go from here? • This is not an exhaustive discussion of C • Read the older notes online • Write some sample programs Get going on the assignment so that if you have simple C problems, we can help you solve them quickly. • Look in • K&R: The C Programming Language • Visual Studio’s help facility