C For Java Programmers

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C For Java Programmers

• Tom Roeder
• CS415 2005sp

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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

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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

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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)

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C Types Primitives

• Often architecture-dependent
• int, short, long
• Can count on short lt int lt long
• float, double
• Can usually count on float lt double
• char
• One byte per character
• Unicode WCHAR in Windows two bytes

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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

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C Types enum

• When you need to write code like
• Color x RED
• enum BLUE, RED, GREEN
• Well 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.

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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)

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C Types arrays and strings

• An array is just a pointer (0-based)
• int x5 10, 20, 30, 40, 50
• x3 (x 3)
• A string is just an array of characters
• int main(int argc, char argv)
• printf(arg 1 s\n, argv0)
• printf(char 1 c\n, argv00)
• Remember to terminate your string with \0
• See string.h for functions strcmp, strcpy,

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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

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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-gtelt) 137

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C Functions function pointers

• Unlike in Java, we dont 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

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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

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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

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C Traps memory management

• Dont forgot to free memory youve alloced
• 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.

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C Traps local variables

• Simple example
• char get_name()
• char tempNAME_LENGTH
• / get something into temp /
• return temp
• temp is allocated on the stack you will have an
  invalid pointer.

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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)gt(y) ? (x)(y))
• Conditional compilation
• if DEBUG
• endif
• includes include ltstdio.hgt

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Most Common C Libraries

• ltstdio.hgt standard I/O
• printf (format, )
• printf(Hello there, s\n, Tom)
• s strings
• c characters
• d integers
• f float
• lf double
• ltstdlib.hgt useful functions
• exit, malloc, free

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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 arent included more
  than once, for myfile.h, we do
• ifndef __MYFILE_H_
• define __MYFILE_H_
• / file contents /
• endif / __MYFILE_H_ /

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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 wont go into make here.

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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
• KR The C Programming Language
• Visual Studios help facility