CSIT 231: Function Pointers and Structs

• These notes cover topics from CSIT 231 Final Exam Study Guide

Lecture 11

• Pointers to functions often implement Polymorphism
• This changes behavior because it can change at run-time, which differs from static function calls
• Without parentheses around the pointer name in a function pointer, the compiler will make a mistake
• A struct is a user-defined data type used to group items of different types into a single type
• Items inside a struct are known as members
• Values in structs are stored contiguously in memory
• Members of a structure can be directly accessed using the "." operator
• Dynamic memory allocation is useful when the size or number of structures is unknown at compile time
• Dynamic memory allocation is useful for building data structures like linked lists, trees, and graphs
• When accessing members of a structure using pointers, the arrow operator (->) must be used
• Passing a pointer to a function is more efficient than passing the entire structure because only the memory address is passed, not a copy of the entire struct
• Explicit memory allocation is faster than implicit memory allocation
• Explicit memory allocation uses less memory but throws more errors

Lecture 12

• Enums are a user-defined data type to represent a group of global constants
• "The Days of the Week" is an example of an Enum
• Using an Enum instead of #define is better because it includes type checking and obeys scope rules
• #define can cause issues as it cannot group related variables
• The default value assigned to each member of the enum is 0
• Each value after the first increases by 1
• For the enum Weekdays { MONDAY = 20, TUESDAY = 30, WEDNESDAY = 40, THURSDAY = 50, FRIDAY = 60 };, Tuesday = 30, Thursday = 50
• A union is a user-defined data type consisting of a sequence of members whose storage overlaps
• When a union is defined, code can store a variety of data types, but since memory is all-shared memory space, only one member can hold a value at any given time
• Only one member can hold a value at a time
• The size of a union is based on the size of its largest member
• Unions are useful because they save space and memory and are useful for plugins and slicing network packets
• Members of a union are accessed using the "." operator

Lecture 13

• BASH, the Bourne Again Shell, is a command interpreter and a high-level programming language
• The special sequence of characters needed on the first line when scripting is the shebang (#!) / (#!/bin/bash)
• It tells the OS which shell should execute the file and follows the pathname
• When creating a script, permissions must be updated to read and execute the instructions
• Permission combinations:
  • 777 → Reads, writes, and executes for the owner, group, and others
  • 666 → Reads and writes for everyone
  • 444 → Read-only for everyone
• Permissions can be adjusted using numbers to set specific permissions for each category: owner, group, others
• Redirection encompasses the ways to alter where the standard input of a command comes from and where the standard output goes
  • < and > symbols are used to represent redirection
• A file descriptor is where a program sends its output and gets its input from
• There are three: Standard input, Standard output, standard error
• "/etc/passwd" is important for reading and identifying users
• The location where actual passwords are stored is important
• The first four fields of a /etc/passwd entry represent something important

Lecture 14

• The steps needed to add a node to the front of a list in a doubly-linked list are important
• The steps needed to add a node to the tail of a list in a doubly-linked list are important
• A doubly linked list is defined using a struct:
  • struct Node { int data; struct Node* next; struct Node* prev; };
  • struct Node* createNode (int data) { struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); newNode->data = data; newNode->next = NULL; newNode->prev = NULL; return newNode; }

Lecture 14_3

• Header files include #include <header.h>
• The C preprocessing directive #include requests the compiler to load the contents of a specific header file so it can be used in the program
• Double inclusion is when the including header file is in multiple files
• To solve double inclusion, include the same header file twice or use a Header Guard
• Header guards check if a header has been defined before and prevents being defined more than once
• Linking is the process of collecting and combining various pieces of code and data into a single file to be copied into memory and executed
• Linking can be done at compile time, load time, and run time
• Linking multiple files together leads to better optimization compared to creating one large source file

Lecture 15

• Symbol resolution associates each symbol reference with exactly one symbol definition
• A symbol table is an array of structs
• Each entry contains the name, size, and location of the symbol
• There are three object files:
  • Relocatable object (.o file)
  • Executable object file (a.out file)
  • Shared object file (.so file)
• There are global, external, and local symbols
• Rules for dealing with duplicate symbols:
  • Multiple strong symbols are not allowed, and each item can be defined only once
  • Using multiple strong symbols causes a Linker Error
  • Given a strong symbol and multiple weak symbols, choose the strong symbol
  • If there are multiple weak symbols, pick an arbitrary one

Lecture 16_2

• Putting all standard C functions into a single relocatable object module causes huge waste of space, and a change would require recompilation of the entire source code
• Static libraries are stored on disk in an archive file format
• Disadvantages of static libraries:
  • Duplication in stored executable files
  • Duplication in running executable files
  • Minor bug fixes of system libraries require each application to explicitly relink
• Shared Libraries are a modern solution

Lecture 17

• Dynamic linking occurs during runtime or load time, when a library is loaded at an arbitrary memory address and linked with a program in memory
• Shared libraries are "shared" either through one .so file or a single copy of the text section of a shared library in memory
• The simplest form of control flow is startup to shutdown, where a CPU reads and executes a sequence of instructions one at a time which is CPU Control Flow
• Events causing abrupt changes in the system: Instruction divides by zero, data from disk or network adapter, control, and hardware timer goes off
• These abrupt changes are referred to as Exceptional Control Flow (ECF)
• Exceptional control flow can occur at the hardware, OS, and application level
• An exception is the transfer of control to the OS kernel in response to some event, like a change in the processor state
• Events are triggered by any significant change

Cont.

• Each type of event has a unique exception number k
• k = index into exception table, also known as an interrupt vector
• Handler k is called each time exception k occurs
• The four classes of exceptions:
  • Interrupts
  • Traps
  • Faults
  • Aborts
• A process is an instance of a program executing
• While processes are running, using a logical control flow, they take turns using the processor
• While processes take turns, they are preempted, meaning they are temporarily suspended
• A logical flow whose execution overlaps in time with another flow is called a concurrent flow

Lecture 18

• Init. is the first process in the system
• The kernel assigns a unique ID number to every process
  • The unique ID is called the PID (Process ID)
  • Relationships between processes are named Parent-child
• The states a process may be in:
  • Ready/Runnable
  • Running
  • Zombie
  • Stopped
  • Terminated
  • Sleeping
• The process table contains all the info for processes
• a process control block (PCB) stores process info
• A context switch forcefully takes resources or the CPU from a process
• A context switch happens approximately every 1/100sec
• A thread is a path execution
• Every thread has its own:
  • Stack
  • PC - Program Counter
  • Set of registers
  • State
• Multithreading Advantages:
  • Fast creation, context switch, and communication across
• Multithreading Disadvantages:
  • Less robust or more sync problems
• Multiprocessing Advantages:
  • Reduced Execution Time, Parallel exec, Process Isolation
• Multiprocessing Disadvantages:
  • Memory and CPU Overhead *higher usage
• Fork: A running child process that is called once but returns twice

Lecture 19

• Code will be provided to read, understand multithreading and multiprocessing codes, along with a process graph
• A race condition is when threads can access shared data and try to change it simultaneously
• A mutex lock, also known as a mutual exclusion, is an operation used to prevent simultaneous possession of the shared resources
• The critical section is the portion of the code only one thread should execute at a time

Cont.

• Semaphores coordinate access to resources
  • The two types of semaphores are binary and counting
• The functions that lock/unlock a semaphore are Sem_post(), sem_wait()

Lecture 20

• A signal is a small message that notifies a process that an event of some type has occurred in the system
• A signal may be sent by sending or receiving
• When receiving a process, the process can do one of three things: Ignore, Catch, Terminate
• A pending signal has been sent but not yet received
• Blocked signals are signals a process can selectively block the receipt of certain signals
• There can be at most one pending signal of a particular type, the rest of the signals that may be of the same type are discarded
• Each signal type has a predefined default action. The four actions:
  • The process terminates
  • The Process terminates and dumps core
  • The process stops (suspends) until restarted by a SIGCONT signal
  • The process ignores the signal
• A transaction between a client and a server consists of four steps:
  • The client initiates a request to the server
  • The server verifies and approves the request
  • The server then sends a response to the client and waits
  • The client receives the response then processes it
• A network is a hierarchical system organized by geographical proximity
  • The three proximities: SAN, LAN, WAN
• IP addresses are stored in memory for a network and are referred to as "Dotted Decinal notation"
• Usually IPs are presented to humans in a dotted decimal format, which is a conversion from its original hex form
• A socket is an endpoint of a connection
  • A socket pair is (IP_adress:port)
• An Ephemeral port is assigned automatically by the client kernel when a client makes a connection request
  • Well-known port is associated with some service provided by a server
• Byte order is converted between the network and the client
• AF_INET is used for the internet

Lecture 21

• The main objective of multiprogramming is to have some process always running, maximizing CPU utilization
• CPU sitting idle is a waste, no work is being accomplished
• When one process waits, the OS gives the CPU to another process
• The CPU scheduler selects a process from the processes in memory that are ready to execute and allocates the CPU to that process when the CPU becomes idle
• The process control block shows all the process' stuffs
• A preemptive scheme is when the OS can interrupt a running process and switch to another process before the current one finishes
• A nonpreemptive scheme is when scheduling takes place under 1 and 4, The CPU holds, The OS does not forcibly take the CPU away
• First know is to create the gantt chart and calculate the average wait time using each process when the algorithms used:
  • First come first serve
  • Shortest job first (preemptive and nonpreemptive)
  • Round robin
• Then calculate exponential averaging using a formula