L4: Input/Output

Question 1

Unix Directory Structure:

Important Directories(7)

Answer 1

/ - Root Directory

/bin - commonly used commands

/dev - Device Files

/etc - System Maintenance Files

/lib - System Libraries

/tmp - Temporary Files

/usr - User Files

Question 2

Unix Directory Structure:

User File Directory Subdirectories(4)

Answer 2

/usr

/usr/include - System Include Files

/usr/lib - More Library Files

/usr/bin - More Executable Files

/usr/man - Manual Pages

Question 3

Unix Innovations:

Shell

Answer 3

• Pipes
  
  • Simple model for connecting together programs
• Tools
  
  • Rich set of utilities for common text manipulation tasks

Question 4

Unix Innovations:

Programming

Answer 4

• C Language
  
  • As efficient and flexible as assembly
  • But more readable
• OS Interface
  
  • Convenient subroutine interface to all system services

Question 5

Unix Innovations:

Processes

Answer 5

• Simplicity
  
  • Unix has a simple model for spawning and coordinating processes
• Uniformity
  
  • Single model for:
    
    • jobs
    • concurrency
    • memory
    • etc

Question 6

Two Building Blocks

of

Unix Data

Answer 6

• Files
  
  • Actual storage areas on disk
  • Store data
• Processes
  
  • Manipulate data
  • Running programs in memory

Question 7

Unix Innovations:

File System

Answer 7

• Organization
  
  • Simple hierarchical structure
  • access control
• Content
  
  • Simple linear byte streams

Question 8

Unix Minimalist Philosophy

Answer 8

• Build it for programmers
• Keep capabilities few
• Make the capabilities powerful
• Compose complex capabilities FROM simple ones

Question 9

File Structure

Answer 9

• Regular Files are just arrays of bytes, starting at index 0
• Uses a byte offset to track the “position” while reading and writing to the file/data
• This increases flexibility:
  
  • Can reposition offset to overwrite or reread
  • Can move offset to end+1 to append without overwriting

Question 10

Unix File System Model:

3 Types of Files

Answer 10

• Regular Files
  
  • Array of bytes
  • Simple sequence with arbitrary file size
• Directory Files:
  
  • Single hierarchy of files
  • Very deep nesting
• Special Files
  
  • Things that LOOK like files, but are not actually on the disk
    
    • terminals,
    • network connections,
    • memory,
    • pipes,
    • etc

Question 11

Error Handling:

Using ERRNO

Answer 11

• Every system routine reports errors this way
• Returns an integer value
• Sets an extern int called errno, from errno.h
  
  • represents the specific error that occurred
• Use the function:
  
  • strerror(errno)
  • obtain a string with info about the error

Question 12

How the Shell launches a program

Answer 12

Example:

%ls -l -t foo.c bar.c < abc

1. Creates a new process
   
   • Finds the program file (ls)
   • Uses it for instructions and initial data
   • Creates an array of command line arguments
   • Sets file descriptors 0,1,2 (stdin, stdout,stderr)
2. Runs the Process
   
   • First instruction is C startup routine from library
   • Calls “main” with the command line arguments
3. Wait for process
   
   • Ends with the return
   • Shell waits( unless pipe or & )

Question 13

Library for Working with Files

Answer 13

fcntl.h

Question 14

File Operation Methods:

Opening a File

Answer 14

method:

int open( char* pathname, int flags, int mode)

• pathname
  
  • The pathname of the file
• flags
  
  • determine how the file is to be opened
  • predefined set of flags in fcntl.h
• mode
  
  • permissions(read/write)

Question 15

File Descriptors

Overview

Answer 15

File Descriptors

• Used to manipulate open files
• Integer numbers used as indices into a (secret) array managed by the kernel
• Each entry in the array has information about the state of an open file

Question 16

File Information

in the File Descriptor array

Answer 16

The File Descriptor Array manages information about the state of open files

• File location on disk
• Mode(read/write) the file is open in
• Current offset

Question 17

Changing the

Current Offset

of a File Descriptor

Answer 17

Use the “lseek()” function from the unistd.h library

lseek( fd, value, flag)

Returns a new offset, or -1 if the fd is not a disk file

The flag determines the kind of seek operation

• Set specific offset:
  
  • lseek(fd, i, SEEK_SET)
  • new offset = i
• Move by i:
  
  • lseek(fd, i, SEEK_CUR)
  • new offset = old_offset + i
• Move i bytes past end of file:
  
  • lseek(fd, i, SEEK_END)
  • new offset = size of file + i

Question 18

Reading from a File

Answer 18

Use the read() method from the unistd.h library

actual = read(fd, buffer, attempt);

• bytes from the file will be copied into the buffer
• the buffer must be allocated beforehand
• attempt defines the expected number of bytes to read
• returns the actual number of bytes read, or 0 if no more bytes to read
• Read begins at the current offset
• After reading, the offset is incremented by “actual”

Question 19

Opening a File:

Basic Flags

Answer 19

Used in the open(path, flag, mode_permissions)

Defined in the fcntl libary

• O_CREAT
  
  • Create File
• O_RDONLY
  
  • Open for reading
• O_TRUNC
  
  • Set size to 0
• O_WRONLY
  
  • Open for writing
• O_APPEND
  
  • Set offset to EOF before each write
• O_RDWR
  
  • Open for both reading and writing

Question 20

Writing to a File

Answer 20

Use the write() method from the unistd.h

actual = write(fd, buffer, actual)

• writing begins at the current offset
• bytes in buffer are written to the file
• if “fd” was opened with the O_APPEND flag, the offset is set to EOF + 1, so each write appends
• After the write, offset is incremented by “actual”, or the number of bytes written

Question 21

Layers of the Unix Architecture

Answer 21

• Kernel
  
  • Essential Basic Services
  • Only one kernel
  • General functionality
• Libraries
  
  • Useful additional services
  • Many libraries
  • Specific functionality
• Programs
  
  • Executable files
  • Lots of programs
  • Specific functionality
• Shell
  
  • Command Line environment
  • Few
  • Powerful
• Users
  
  • Humans using the computer

Question 22

Default File Descriptors

Answer 22

• 0 - stdin
• 1 - stdout
• 2 - stderr

Question 23

Debuggin/Error Handling:

Using Assert

Answer 23

• Include the “assert.h” library
• Verifies that a specified condition is true using an assert macro
• If condition is false, program will abort
• Specifying a condition in code:
  
  • assert( myVar < 2)
  • assert( myVar == 5), etc

Question 24

fprintf()

-Destination

Answer 24

Full function prototype

fprintf( destination, string, arg1, arg2, … )

• destination is a file descriptor
  
  • could be stdout, stderr, fd…
• arguments are substituted into the string following general C string substitution rules

Question 25

Common String Formatting

Substitution Symbols (5)

Answer 25

• %s - string value
• %c - char value
• %d - decimal integer
• %x - hex integer
• %f - float

Note: This is far from comprehensive

Question 26

Unix Innovations:

5 Areas

Answer 26

• File System
  
  • Organization
  • Content
• Processes
  
  • SImplicity
  • Uniformity
• Programming
  
  • C language
  • OS Interface
• Shell
  
  • Pipes
  • Tools
• Malleability
  
  • Portability
  • Open Source

Question 27

Bash command:

Pass a file to a program/process

Answer 27

%program < file_name

• This file supplies initial content for a new process

Question 28

Unix Innovations:

Malleability

Answer 28

• Portability
  
  • both OS and tools are written in C
  • both can be easily ported to other machines
• Open Source
  
  • source can be licensed and modified for experiments

Question 30

Specialized File Operations