CS Exam 2

exit(status)

Terminate process execution and return status to OS, status \= \> 0 ERROR, status \= 0 no error

file

structure used to access a stream; every stream has its own FILE associated w/stream

File *f \= fopen ("chipmunk", "r")

chipmunk is name of file, r is mode; if fopen fails it returns null-\> file not found/binary file (diff function and don't have permission to open)

fopen modes

r \= read, if you pass a file that doesnt exist it fails; w \= write, will create file if one doesn't exit or if it does exist will delete and replace with new data; a \= append, will create file if it doesnt exist or will add to it if file does exist

scanf()

reads user input from the command prompt; reads from standard input

returns \# of input values converted or EOF

fscanf()
int fscanf(FILE *stream, char *format, ... );

reads input from the stream given as an argument

scanf if is a special case of fscanf.

scanf("%d", &number); // is the same as
fscanf(stdin, "%d", &number);

returns \# of input values converted or EOF

sscanf()

A function that accepts input from a string. It is a variation of the scanf() function. returns \# of input values converted or EOF

structures in c can have \____ to structures of the \____ \____

pointers; same type

if you need to use a field within a typedef for ex:
typedef struct node {
int data;
struct node *next;
} Node;

Node *head; (also pointer to a node)

struct node *next is a pointer to a node; needs incomplete type for the compiler, cant have field of the actual node w/o declaration of Node; this is why you make a ptr to structure bc compiler knows how much mem to allocate for ptr but not for structure

Node *head

in order to change head have to pass in ptr to ptr so if you dereference it you get actual pointer to modify it

IN C EVERYTHING IS PASSED BY \__________

VALUE

Graphs basics

need type Vertex (Edge * edge list, char * name, *next vert) , Edge (destination, next edge, weight), and String_graph(num vertices, num edges, Vertex *vertex list)

each graph has a head and each vertex has ptr to next vertex

each vert has a head pointer to first edge

each edge has a ptr to next edge

double linked list

A linked list where each node has a pointer to the previous and next nodes

4 case: insert to empty, beginning of list(change head), end of list (replace tail), middle of list

What/why is there runstack/heap?

runtime stack allows for recursion because for a recursive call to work every call needs it own local vars & param

heap -\> dynamically allocated memory, avoid compile time limits, heap is anonymous, if it has a variable name NOT heap

where do linked structures belong and why?

HEAP; allows you to creat memory that exists beyond the scope it was created; each node must be dynamically allocated bc otherwise you list will fall out of scope.

Binary search tree set up

typedef struct node {
int date;
struct node *left;
struct node *right;
} Node;

left is less than, right is greater than

what is the precedence of this?
int *q[10];

array subscripting has precedence;

array of ptrs to int

what is the precedence of this?
int (*f) (void);

ptr to a function; can point to any function that has those param

char * (*h[5]) (int);

array of 5 ptrs to functions that take in an int and return a char

typedef void (*Ifp) (int);
Ifp p;

name of a function w/o any parentheses -\> ptr to the memory address of the function

you can call a function through a ptr

argc

\# of cmd line arguments that appeared on cmd line including program

argv

array of pointers to strings of arguments that appeared on the cmd line

argv[argc] \=?

last element is always NULL

3 types of main

main(void);
main(int argc, char*argv[]);
main (int argc, char*argv[], char *envp[]);

shell

executes the command on the command line

shell variables

Variables that control the shell; you can set the shell variables to male the shell diff

Ex:
1. HOME
2. PATH

environmental variables

affect the current shell but are also seen by every program thats run by the current shell

HOME

Identifies a users home directory

PATH

Specifies the directories the shell should look when it tries to execute any command.

what does commans setenv

how you set an environmental variable

char * envp[]

array of NULL terminated strings, key/value of terms

Preprocessor steps

1. deletes comments
2. inserts code for \#include
3. substitutes the \#defines
4. handles conditional compilation

Compilation of C program Steps:

1. preprocessing
2. compilation
3. compilers produce assembly code and run the assembler on it
4. linker -\> links object files together

Compilation steps

1. scanner -\> takes the prgm and breaks it up into the low level syntax entities
2. parser -\> figures out which language constructs they belong to
3. type checker -\> makes sure things are being applied to the right type of operands
4. code generator -\> generates code \= relatively inefficient code
5. optimizer -\> makes code more efficient

Macros
\#define NAME(parameters) replacement
ex: \#define sum(a,b) a+b
x \= sum (2,5)
what does x \=?

when program is compiled, SUM(2,5) replaced 2+5,
x \= 7
compiler sees the expression after the preprocessor has replaced it w/ "replacement"

what is a problem w/macros?

numeric expressions can yield unexpected results, side effects, macros can invoke another

what does this equal?
s \= SQUARE(a+1)
s \= a + 1 * a + 1
s \= 5 + 1 * 5 + 1

s \= 11 instead of 36 b cof precedence INCLUDE parentheses to avoid unexpected behavior

do macros have a type?

no typeless bc the preprocessor replaces the code

kernal

The part of the operating system that controls memory and file access

runs directly on the hardware

what are priveledged instructions?

handles through kernal mode by making a system call

how does a system call relate to a user?

ordinary users don't have permission to make changes/access memory files so need to make system call

process

instance of a program loaded into memory and running

whats the diff bw program and process

program isnt alive, just code on a disk

process is alive and actually running

multitasking

more than one process can be alive/active in memory at a time; more efficient

1 CPU 1 core -\> how many processes can run at once

1

how does cpu give illusion of multiple processes running at once

cycle between processes

program counter

register in the CPU that stored the mem address of the next instruction to be executed

process table

to keep track of all the processes, the scheduler maintains a list of all active processes at a point in main memory

page table

stores processes' address spaces (memory)

file table

lists which files are opened by each process

process state transitions: ready

ready but waiting for CPU

process state transitions: waiting

waiting for smth to finish before it can go back to ready state

process state transitions: interrupt

os sends process back to ready, why? to make sure time running is shared between processes

what are background processes and what are they used for?

processes running but not using the terminal, because you can run commands from the terminal while the processes run

what are signals?

how processes communicate; the kernal notifies processes using signals

can users send signals?

yes - you can define diff actions for when a process gets a signal, also default actions exist

which signals can't be ignored?

SIGKILL and SIGSTOP, but processes can also block/ignore signals

SIGSEGV

signal for seg fault

SIGFPE

Floating point exception

SIGILL

illegal instruction

how do processes communicate with each other?

send signals via kernal

how does a shell communicate with processes?

A shell can send signals to other processes using the "kill" command, or program can have kill()

how is a process created?

splits itself into two processes (forking)

how many times does pid_t fork(void) return?

once in each process, parent and child

what does parent return is child isn't created properly?

-1

what does child and parent return when forking is successful?

child \= 0
parent \> 0 -\> also child's ID

what is unique to the child process?

process ID, own copies of parent file descriptors and streams, process times, resource utilizations initially set to 0

what is inherited from parent to child process?

copy of parents memory (program code, runtime stack, heap), current working directory, the environment and all environment variables, controlling terminal (child and parent read/write input from same place)

pid_t getpid(void) vs
pid_t getppid(void)

returns current process ID vs return process id of parent

Fork Bomb

Works by quickly creating a large number of processes to saturate the available processing space in the computer's operating system. can only hold a finite \# of processes

what are the ways for process termination?

process terminates itself using exit() system call, OS kernal kills a process, a process can kill another but only if they belong to the same user or one has priviledged process (kernal/root)

what is root?

the OS process

deceased children?

process exits, still tracked by the kernal, needs to be released from the table

how are deceased children released from table?

reaped by parent using wait() of waitpid(), system calls

what are zombie children?

no cpu cycles, no memory, just an entry on the process table

why is a zombie process still tracked?

parent might still want to get info from it

orphaned process

if the parent terminated first, child is adopted by closest ancestor that can adopt all the way up to systemd

What is systemd?

first process when system boats, automatically reaps its children when they terminate

pid_t wait(int *status)

reaps the most recently terminated child, blocking wait, pass a ptr to an int will be filled w info (exit status of child) about reaped child, returns process ID of reaped child if successful

what happens if you call wait w/o child

returns -1 and doesn't block

what happens if you call wait w some terminated children but not reaped

will wait and reap on of them

what happens if you call wait while all children are still running?

blocks parent

pid_t waitpid(pid_t pid, int *status, int options)

wait for a specific child, blocking wait but you can turn it off, with options (WNOHANG)

you can only wait for a \_____ child process

direct

WEXITSTATUS(status)

returns exit status of child, only valid if WIFEXITED is true

how to invoke a new program in a process?

use execv function

what does exec return if sucesfull and if failed?

sucesfull-\> NEVER returns
failed-\> return -1 and sets errno to indicate cause of error

types of exec

v: call uses an argument vector (an array) that has the same form as ARGV

l: call uses an argument list where last argument must be null

p: means the call searches PATH to find the program to exec

e: specify environment for the exec'd program

what maintains a file descriptor table

kernel

file descriptor table

unique to each process and holds information about all files for each process

open()

process uses this to open a file, kernal returns a file descriptor, returns -1 if error

close()

returns 0 if sucessful, -1 on error
closed by kernal, open files use system resources so they need to be closed once theyre not needed

what is a function pointer?

pointer that stores mem adress of function

how do fgets() and fscanf() compare?

fgets() will read an entire line from the input and stops only for newline
fscanf reads a string using %s and stops at the first whitespace character

strcpy(str1, str2)

copies str2 to str1
str1 does not have to be null terminated but str2 does

what does sizeof(str) return

number of characters + null char

conditional compilation: can compilation fail if the compiler sees two def of a structure?

no, not if its because main includes a header and that header includes another with a struc

errno? how to print erno?

external global int variable and indicates an error occurred during any function call
perror(error)

why would you use text stream or binary stream?

text stream is readable but binary is faster

how to determine exit status of a child process

call wait() - returns the process ID of the terminated child and the exit status of the child process

0 \= successfull, nonzero \= error