C For Java Programmers

Tom Roeder

CS415 2005sp

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

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

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);

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

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;

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.

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 managementint* x = malloc(sizeof(int)*z);

free(x);

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, …

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

C Types: void*

void* is a pointer to any type Extremely useful in generic data structurestypedef struct queue_t {

void* elt;

queue_t* next;

} queue;

queue* q;

…

*(int*)(q->elt) = 137;

C Functions: function pointers Unlike in Java, we don’t have any reflection We can nonetheless pass functions aroundint 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

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

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;}

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.

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.

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>

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

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_ */

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.

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