NOTES.md

2016-01-12

Basic language features in c

operators

• arithmetic + - * / % ++ --
• relational == != < > <= >=
• bitwise ~ & | ^ >> <<
• logical && || !
• conditional ?:
• assignment = += -+ *= /= %=

operator characteristics

• arity (unary,binary, ternary)
• precedence
• associativity: order in which orperators of the same predecence execute

expressions, statements, blocks

• expression: sequence of operators that resolves to a single value
• statement: expression terminated by a semi-colon
• block: sequence of statements between a pair of matches braces { }
• scope: part of the program where a variable can be used:
  • local: inside a block
  • global: outside blocks

control structures, data structures:

• conditional - if/else
• selective - switch statement
• iterative - for, while, do-while (segfaults often)
• jump - break, continue

2016-01-19

commenting

• you need to document sideeffects of a function: unintended/unexpected
• do not document every line of code or i will CHRISTINE you

section 2: data representation

• 2.1. primitives DTs
• 2.2. compound DTs
• 2.3. pointers

s. 2.1.1 Bit models

• bit: (0|1)

• byte: 8 consecutive bits

• nibble: 4 consecutive bits; maps to one hexadecimal digit

• base conversion:

  • binary base 2
  • octal base 8
  • hexadecimal base 16

magnitude-only bit model

- unsigned non-negative whole numbers
- each bit represents non-negative integer power of 2
- 8 bits → 2^8 ⇒ 256 max val
- least significant bits → lowest powers of 2 ("lower order bits")
- most significant bits → highest powers of 2 ("higher order bits")
1. unsigned char is 1 byte
2. unsigned int is 4 bytes (on 32bit arch)
3. unsigned short int is 2 bytess

• in c {int, char} is always signed int

two's complement

- positive integers and zero - use magnitude-only bit model
- negative integers: write bits for positive number; invert; add 1

• fixed point: NOT USED ANYMORE

  • number of digits in fraction are fixed
  • inefficient use of avialable bits
  • fine for fraction powers of 2 but imprecise for powers of 10

• floating point:

  • format: +/- 1.f x 2^e

2016-01-21

• two's complement: only applies when you put in a negative number
  • to reverse, run the procedure again

floating point

float

• 4 bytes

double

• 8 bytes

bitwise operations

• why? store data that is only one bit long; modify
• smallest c data type is 8 bits, so if we have single-bit data, we waste 7 bits
• bitwise ops allow using those 7 bits
• can use bitwise ops on literals and vars
• bit-shift operators are not 'circular'
  • << 3 left shift by 3 (adding zeros to end)
  • >> 2 right shift by 2
    • (if twos complement, adding sign bit to beginning)
    • (if otherwise, adding zeros to begninng )

2016-02-04

THESE ARE THE ONLY MEMORY ALLOCATION FUNCTIONS YOU MAY USE FOR THIS COURSE. OR ELSE I WILL CRY. IAMPROF.

malloc

• void \*malloc(size_t size) where size is a number of bytes (duh)
• returns the start memory address reserved
• if you don't store this pointer, how in the hell ya gonna free the memory, honey?
• in stdlib.h

calloc

• clears memory first, allocs a number of elements
• mallocs
• in stdlib.h

void

• both malloc and calloc return the pointer to void
• void means: no datatype || any data type

typecasting

• outside scope of course
• usually used on pointers
• can be used on non-pointer vars

2016-03-15

Header files

• never contains function implementations
• never includes .c files

gcc

• preprocessor

• compile

• link

• gcc -c : we will use -c from now on

• gcc *.o -o bin

why separate compiling and linking?

• different programming languages
• compilation may be slow

For the confused: What the hello?

source → compiler → object files → linking → binary executable 

how to use a library?

1. include the header file
2. link in the object file
3. remember: libc.a is always linked in by default

static linking v. dynamic linking

• dynamic - default - runtime as needed - smaller exe but slower execution time
• static - library code is copied into exe - larger exe but faster exe time

2016-03-17 Concurrent

concurrency :: doing > 1 thing at a time

concurrent computing ::

• a program with more than one control flow
• a large system can be:
  • multithreaded
  • multiprocess
  • distributed

thread :: one control flow

• !== processes
• share resources with process tree

distributed system ::

• execution over multiple physical host
• each host has different resources (fs, processor...)
• hosts msust be networked togeter in order to commnicate
  • either by intranet or internet
• why?
  • users are in different physical locations
  • server hosts are in different physical locatoins
  • single host has insufficient resources/processor power

process :: running exe

multiprocess ::

• can be DIFFERENT exe OR MULTIPLE instance of same exe
• why?
  • system has different tasks to perform
  • tasks are independent
  • tasks use different resources

multithreaded ::

• process with multiple control flows
• share the process (virtual memeory, address space, resources)
• different threads may need to synchronize
• why?
  • different things to do within a process
  • tasks ~dependent upon each other
  • example: one thread blocks and waits for user input

process management ::

• launching, killing, signals

process

• process id (PID)
• parent process id - PID of process that started this process
• usually managed by OS; can be: shell, program
• ps -ef

forking

• clone of itself
• new process is the child
• child gets an instantaneous copy of the parent's address space

exec

• morphing itself
• char *args = {processname,arg1,arg2};
• execvp(buffer, args);

fork bomb

• OS keeps process table
• all tables have finite capacity

wait()

• wait until any of the children terminator
• or wait until a specific process terminates

system()

• fork-exec the stupid way

signals

• just an integer
• USR1, USR2
• typically used for errors
• procesess

2016-03-31 Code Organization

types of storage classes

• auto(matic): stored on stack, incl. params, local vars
• static: retains value until program termination, only visible in block
• extern: global and declared in another file
• register: stored in CPU register rather than in memory; may be ignored
  • good compiler will perform this optimization anyway
• mutable: allows parent's const status to change (i.e. struct/c++ class)
• see http://www.csegeek.com/csegeek/view/tutorials/cpp_lang/cpp_storage_class.php

block scope

• automatic variable disappear at the closing brace

file scope === "global scope"

• != global memory

function parameter scope

• a paramter name specificed within a function prototype is useable only in that prototype
• the name is not binding wihtin the implementation of hte function

function scope

• this is really → labels!
• do not use these unless you're forced to and you want Christine to Christine you!

2016-03-31 Libraries

• (these notes are not comprehensive for this course.)
• libraries are in /usr/include - duh. honestly this is first-year/highschool material. what is this curriculum?
• Hot tip from the prof. Don't try to grep through *.o in /usr/lib it's object code. yawn. why am i here?

how do i use a library? (most first year material)

• include the header files
  • specify additonal location with gcc -I
• linking -lpthread -lm
  • specify additional location with gcc -L

how to make a library

• create object files, package them
• supply header *.h files and archive *.a files to
• where is my libc.a? → this is cool gcc --print-file-name=libc.a
• include a library → #include <yomammagotsalib.h>

2016-03-31 Shell Scripts

• ( really? can i just get a 90% on the exam for writing every answer as a portable sh script without bashisms? )
• ( cause he's got a big ego. he talks like dis cuz he can back it up. )
• ( who needs twitter when you can read my course notes. )
• ( My dream is that we would cover POSIX-defined constructs instead of backticks. )
• not on the exam -_-

2016-04-21 Supplementary: Memory Areas of a Program

1. Data Segment / Global Segment
   • static vars, global vars
2. Text Segment / Code Segment:
   • program instructions
3. Function Call Stack:
   • automatic vars, parameters
4. Heap
   • dynamically allocated with malloc, calloc, c++ new