Programming in C & Living in Unix - PowerPoint Presentation

Slide1

Programming in C & Living in Unix

15-213: Introduction to Computer Systems

Recitation 6: Monday, Sept. 30, 2013

Marjorie Carlson

Section A

Slide2

Weekly Update

Buffer Lab is due Tuesday (tomorrow), 11:59PM

This is another lab you don’t want to waste your late days on.

Cache Lab is out Tuesday (tomorrow), 11:59 PM

Due

Thursday October

10

th

Cache Lab has two parts.

Write a 200- to 300-line C program that simulates the behavior of cache. (Let the coding in C begin!)

Optimize a small matrix transpose function in order to minimize the number of cache misses.

Next recitation will address part 2. For part 1, pay close attention in class this week.



Slide3

Agenda

Living in

Unix

Beginner

The Command Line

Basic Commands

Pipes & Redirects

Intermediate

More Commands

The Environment

Shell Scripting

Programming

in

C

Refresher

Compiling

Hunting Memory

Bugs

GDB

Valgrind

Slide4

“In the Beginning was the Command Line…”

Command Line Interface

“Provides a means of communication between a user and a computer that is based solely on textual input and output.”

Shell

A shell is a program that

reads

and

executes the commands entered on the command line. It provides “an interface between the user and the internal parts of the operating system (at the very core of which is the kernel).”The original UNIX shell is sh (the Bourne shell).Many other versions exist: bash, csh, etc.

The Linux Information Project: Shell

The Linux Information Project: Command Line

Slide5

Unix – Beginner: Basic Commands

Moving

Around

Manipulating

Files

ls

List directory contents

mvMove (rename) filescd

Change working directory

cp

Copy files

(and

directories

with “-

r

”)

pwd

Display

present

working directory

rm

Remove files

(or directories with “-

r

”)

ln

Make links between files/directories

cat

Concatenate and print files

mkdir

Make

directories

chmod

Change file

permission

bits

Working

Remotely

Looking

Up Commands

ssh

Secure remote login program

man

Interface

to online reference manuals

sftp

Secure remote file transfer

program

which

Shows the full path of shell commands

scp

Secure remote

file copy program

locate

Find files by name

Managing

Processes

Other

Important Commands

ps

Report

current processes status

echo

Display a line of text

kill

Terminate a process

exit

Cause the shell

to exit

jobs

Report

current shell’s job status

history

Display the command history list

fg

(

bg

)

Run

jobs in foreground (background)

who

Show who is logged on the system

Slide6

Quick Aside: Using the

rm

Command

rm

./filename

– deletes file “filename” in current dir.

rm

./*ame – deletes all files in the current directory that end in “ame.”rm –r

./*

– deletes all files and directories inside the current directory.

rm

-

r

./directory

– deletes all files inside the given directory and the directory itself.

sudo

rm

–

rvf

/*

– deletes the entire hard drive.

DO NOT DO THIS!!!

sudo

runs the command as root, allowing you to delete anything.

-

v

(verbose) flag will list all the files being deleted.

-

f

(force) flag prevents it from asking for confirmation before deleting important files.

Slide7

Quick Aside: Man Page Sections

man

foo

prints the manual page for the

foo

, where

foo is any user command, system call, C library function, etc.However, some programs, utilities, and functions have the same name, so you have to specify which section you want (e.g., man 3 printf gets you the man page for the C library function printf, not the Unix

command

printf

).

How do you know which section you want

?

whatis

foo

man

–

f

foo

man –

k

foo

lists all man pages mentioning “

foo

.”

Slide8

Unix – Beginner: Pipes & Redirects

A pipe (

|

) between

two commands sends the first command’s output to the second command as its input.

A redirect (

<

or >) does basically the same thing, but with a file rather than a process as the input or output.Remember this slide? 

Option 1: Pipes

$ cat

exploitfile

| ./hex2raw | ./

bufbomb

-

t

andrewID

Option 2: Redirects

$ ./hex2raw <

exploitfile

>

exploit-

rawfile

$ ./

bufbomb

-t

andrewID

<

exploit-

rawfile

Slide9

Agenda

Living in

Unix

Beginner

The Command Line

Basic Commands

Pipes & Redirects

IntermediateMore CommandsThe EnvironmentShell ScriptingProgramming in C

Refresher

Compiling

Hunting Memory

Bugs

GDB

Valgrind

Slide10

Unix – Intermediate: More Commands

Transforming Text

Useful

with Other Commands

cut

Remove sections from each line of files (or redirected

text)

screenScreen manager with terminal emulationsed

Stream editor

for filtering and transforming text

sudo

Execute a command as another user (typically root)

tr

Translate or delete characters

sleep

Delay for a specified amount

of time

Archiving

Looking

Up Commands

zipPackage and compress filesaliasDefine or display aliasestarTar file creation, extraction and manipulationexport (setenv)*Exposes variables to the shell environment and its following commandsManipulating File AttributesSearching Files and File ContenttouchChange file timestamps (creates empty file, if nonexistent)findSearch for files in a directory hierarchyumaskSet file mode creation maskgrepPrint lines matching a pattern

* – Bash uses

export

.

Csh

uses

setenv

.

Slide11

Unix – Intermediate:

Environment

Variables

Your shell’s environment is a set of

variables,

including information such as your username, your home directory, even your language

.

env lists all your environment variables.echo $VARIABLENAME prints a specific one.$PATH is a colon-delimited list of directories to search for executables.Variables can be set in config files like

.

login

or

.

bashrc

, or

by scripts, in which case the script must use

export

(bash) or

setenv

(csh) to export changes to the scope of the shell.

Slide12

Unix – Intermediate: Shell Scripting

Shell scripting is a powerful tool that lets you integrate command line tools quickly and easily to solve complex problems

.

Shell scripts are written in a very simple, interpreted language. The simplest shells scripts are simply a list of commands to execute

.

Shell scripting syntax

is not at all user-friendly, but shell scripting remains popular for its real power

.Teaching shell scripting is depth is beyond the scope of this course – for more information, check out Kesden’s old 15-123 lectures 3, 4 and 5.

Slide13

Unix – Intermediate: Shell Scripting

To write a shell script:

Open your preferred editor.

Type

#!/bin/

sh

. W

hen you run the script, this will tell the OS that what follows is a shell script.Write the relevant commands.Save the file. (You may want its extension to be .sh.)Right now it’s just a text file – not an executable. You need to chmod +x

foo.sh

it to make it executable

.

Now run it as

./

foo.sh

!

Slide14

Unix – Intermediate: Shell Scripting

hello.sh

hello.sh

with variables

#!/bin/

sh

# Prints “Hello, world.” to STDOUT

echo

“Hello, world.”

#!/bin/

sh

str

=“Hello,

world.”

echo $

str

# Also prints “Hello, world.”

Anything after a ‘#’ is a comment.

Variables

Setting

a variable takes the form ‘varName=VALUE’.There CANNOT be any spaces to the left and right of the “=“.Evaluating a variable takes the form “$varName”.Shell scripting is syntactically subtle. (Unquoted strings, “quoted strings”, ‘single-quoted strings’ and `back quotes` all mean

differe

nt

things!)

Slide15

Agenda

Living in

Unix

Beginner

The Command Line

Basic Commands

Pipes & Redirects

IntermediateMore CommandsThe EnvironmentShell ScriptingProgramming in C

Refresher

Compiling

Hunting Memory

Bugs

GDB

Valgrind

Slide16

C – Refresher: Things to Remember

If you allocate it,

free

it.

int

*array= malloc(5 *

sizeof(int)); … free(array);If you use Standard C Library functions that involve pointers, make sure you know if you need to free it. (This will be relevant in proxylab.)

Slide17

C – Refresher: Things to Remember

There is no String type. Strings are just NULL-terminated char arrays.

Setting pointers to NULL after freeing them is a good habit, so is checking if they are equal to NULL.

Global variables are evil, but if you must use make sure you use

extern

where appropriate.

Define functions with prototypes, preferably in a header file, for simplicity and clarity.

Slide18

C – Refresher: Command Line Arguments

If

you want to pass arguments on the command line to your C functions, your main function’s parameters must be

int

main(int

argc, char **argv)argc is the number of arguments. (Always ≥ 1)

argv

is an array of char*

s

(

more or less an array of

strings) corresponding to the command line input.

argv

[0]

is always the name of

the program.

The

rest of the array are the arguments, parsed on space.

Slide19

C – Refresher: Command Line Arguments

$

ls

–

l

private

argc

= 3argv = {“ls”, “-l”, “etc”}

Slide20

C – Refresher: Headers & Libraries

Header files make your functions available

to other source files.

Implementation

in

.

c

, and declarations in .h.forward declarationsstruct prototypes#definesIt should be wrapped in #ifndef

LINKEDLIST_H

#define LINKEDLIST_H

...

#

endif

bits.h

int

bitNor(int

,

int

);int test_bitNor(int, int);int isNotEqual(int, int);int test_isNotEqual(int, int);int anyOddBit();int test_anyOddBit();int rotateLeft(int, int);int test_rotateLeft(int, int);int bitParity(int);int

test_bitParity(int

);

int

tmin

();

int

test_tmin

();

int

fitsBits(int

,

int

);

int

test_fitsBits(int

,

int

);

int

rempwr2(int,

int

);

int

test_rempwr2(int,

int

);

int

addOK(int

,

int

);

int

test_addOK(int

,

int

);

int

isNonZero(int

);

int

test_isNonZero(int

);

int

ilog2(int);

int

test_ilog2(int);

unsigned

float_half(unsigned

);

unsigned

test_float_half(unsigned

);

unsigned

float_twice(unsigned

);

unsigned

test_float_twice(

unsignned

);

Slide21

C – Refresher: Headers & Libraries

#

include <*.

h

>

- Used for including header files found in the C include path: standard C libraries.

#

include “*.h” – Used for including header files in the same directory.There are command-line compiler flags to control where header files are searched for, but you shouldn’t need to worry about that.

Slide22

C – Refresher:

“Reverse”

Demo

reverse

takes

arguments from the command

line

and prints each of them backwards.It calls reverse_strdup.To compile: gcc –Wall –reverse.c –

reverse_strdup.c

–

o

reverse

Slide23

Agenda

Living in

Unix

Beginner

The Command Line

Basic Commands

Pipes & Redirects

IntermediateMore CommandsThe EnvironmentShell ScriptingProgramming in C

Refresher

Compiling

Hunting Memory

Bugs

GDB

Valgrind

Slide24

C – Compiling: Command Line

gcc

GNU

project C and C++ compiler

When compiling C code, all dependencies must be specified

.

Remember how we just compiled?

gcc

–Wall –

reverse.c

–

reverse_strdup.c

–

o

reverse

This will not

compile:

gcc

–Wall –

reverse.c –o reverse

Slide25

C – Compiling: Command Line

gcc

GNU

project C and C++ compiler

gcc

does not requires these flags, but they encourage people to write better C code.

Useful

Flags

-Wall

Enables

all construction warnings

-

Wextra

Enables even more warnings not enabled by Wall

-

Werror

Treat

all warnings as Errors

-pedantic

Issue all mandatory diagnostics listed in C

standard-ansiCompiles code according to 1989 C standards-gProduces debug information (GDB uses this information)-O1Optimize-O2Optimize even more-o filenameNames output binary file “filename”

Slide26

C – Compiling: Makefiles

Make

GNU make utility to maintain groups of programs

Projects can get very complicated very

fast,

and it can take a long time to have GCC recompile the whole project for a small change.

Makefiles

are designed to solve this problem;

make

recompiles only the parts of the project that have changed and links them to those that haven’t

changed

.

Makefiles

commonly separate compilation (

.

c



.

o) and linking (.o’s  executable).

Slide27

C – Compiling: Makefiles

Make

GNU make utility to maintain groups of programs

Make

files consist of one or more rules in the following form.

Makefile

Rule Format

Makefile

for “

gcc

foo.c

bar.c

baz.c

–

o

myapp

”target : source(s)[TAB]command[TAB]commandmyapp: foo.o bar.o baz.o gcc foo.o bar.o baz.o –o myappfoo.o: foo.c foo.h gcc –c foo.cbar.o: bar.c bar.h gcc –c bar.cbaz.o: baz.c baz.h gcc –c baz.c

Slide28

C – Compiling: Makefiles

Comments are any line beginning with ‘#’

The first line of each command must be a TAB

.

If you want help identifying dependencies, you can try:

gcc

–MM

foo.c outputs foo’s dependencies to the console.makedepend adds dependencies to the

Makefile

for

you, if you already have one.

E.g.,

foo.c

bar.c

baz.c

.

Make

GNU make utility to maintain groups of programs

Slide29

C – Compiling: Makefiles

Use macros

– similar to shell

variables – to avoid retyping the same stuff

for every .

o

file.

For more information on Makefiles, check out Kesden’s old 15-123 lecture 16.

Make

GNU make utility to maintain groups of programs

Makefile

Rule Format

CC =

gcc

CCOPT = -

g

–DDEBUG –DPRINT

#CCOPT = -02

foo.o

:

foo.c foo.h $(CC) $(CCOPT) –c foo.c

Slide30

Agenda

Living in

Unix

Beginner

The Command Line

Basic Commands

Pipes & Redirects

IntermediateMore CommandsThe EnvironmentShell ScriptingProgramming in C

Refresher

Compiling

Hunting Memory

Bugs

GDB

Valgrind

Slide31

C – Hunting Memory Bugs: GDB

Useful for debugging the occasional easy

segfault

(among other things!).

You’re used to using

disas

and

stepi/nexti to look at and step through Assembly.If you compile with –g, you can use list and step/next to look at and step through the C, too.This even works after you’ve segfaulted!Other useful commands:where

(print function stack and lines)

up/down

(traverse the function stack)

display/print

variables (like you do now with registers).

Slide32

C – Hunting Memory Bugs: Valgrind

Great

for

finding memory problems in C programs. Has a ton of tools:

memcheck

,

leakfind

, cachegrind.Valgrind’s memcheck tool can:Track memory leaks & (definitely/possibly) lost blocksTrack origin for uninitialized valuesTell you what line of code seems to have been the problemSyntax (including the verbose flag):

valgrind

--tool=

memcheck

-

v

./

myprogram

args

valgrind

–-tool=leak-check=full -

v ./myprogram argsvalgrindA suite of tools for debugging and profiling programs

Slide33

Sources and Useful Links

The Linux Information Project: Command Line Definition

Introduction to Linux: A Hands-On Guide (Garrels)

You should be comfortable with chapters 2, 3, 4 and 5.

The On-line Manual Database

Kesden’s

15-213

: Effective Programming in C and UnixLectures 3, 4 and 5 cover the basics of Shell Scripting.Lecture 16 covers Makefiles and lecture 15 covers Valgrind.