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

18. Most Common C Libraries : standard I/O  printf (format, …);  printf(“Hello there, %s\n”, “Tom”); %s – strings %c – characters %d – integers %f – float %lf – double : 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