Advanced Operating Systems CE5301
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Questions and Answers

What is a File Descriptor in the context of operating systems?

a struct with all the info about the files

What is a Device Driver responsible for?

  • Interacting directly with the device hardware
  • Registers/Unregisters itself with the kernel
  • Handler functions for file operations
  • All of the above (correct)

    • Can files be viewed as communication channels in operating systems?

    True

    What is the analog of 'open' in the context of connected queues over the Internet?

    <p>Socket</p> Signup and view all the answers

    Which operating system first introduced the concept of a Socket?

    <p>4.2 BSD UNIX</p> Signup and view all the answers

    Low-level I/O in operating systems is directly supported at the system call level.

    <p>True</p> Signup and view all the answers

    What is the return value from the 'fork' system call?

    <p>integer</p> Signup and view all the answers

    Which of the following statements is correct regarding the return value from 'fork'?

    <p>When = 0: Running in new Child process</p> Signup and view all the answers

    The 'wait()' system call waits for the next child to exit and returns the PID of the terminating child. The argument passed is a pointer to an integer variable to hold the ________.

    <p>exit status</p> Signup and view all the answers

    What does the 'execl' function do in the context of the code snippet provided?

    <p>start new program</p> Signup and view all the answers

    Study Notes

    Operating System Fundamentals

    • The Unix Operating System Structure consists of:
      • Applications (User Mode)
      • Standard Libraries (User Mode)
      • Kernel Mode (Hardware Abstraction)
      • Hardware (Device Drivers, Platform Support)

    Process Management

    • The fork() system call:
      • Returns an integer value
      • Value > 0: return value is the PID of the new child process (running in parent process)
      • Value = 0: running in new child process
      • Value < 0: error
    • The wait() system call:
      • Waits for the next child to exit
      • Return value is the PID of the terminating child
      • Argument is a pointer to an integer variable to hold the exit status
    • The exec() family of calls:
      • Replaces the process with a new executable

    I/O Design Concepts

    • Uniformity:
      • File operations, device I/O, and interprocess communication through open, read/write, close
      • Allows simple composition of programs
    • Open before use:
      • Provides opportunity for access control and arbitration
      • Sets up the underlying machinery (data structures)
    • Byte-oriented:
      • Even if blocks are transferred, addressing is in bytes
    • Kernel buffered reads:
      • Streaming and block devices look the same
      • Read blocks process, yielding processor to other tasks
    • Kernel buffered writes:
      • Completion of outgoing transfer decoupled from the application, allowing it to continue

    File System Abstraction

    • High-level idea:
      • Files live in a hierarchical namespace of filenames
    • File:
      • Named collection of data in a file system
      • File data: text, binary, linearized objects
      • File metadata: information about the file (size, modification time, owner, security info)
    • Directory:
      • "Folder" containing files and directories
      • Hierarchical (graphical) naming
      • Path through the directory graph uniquely identifies a file or directory

    C High-Level File API

    • Operates on "streams" - a sequence of bytes, whether text or data, with a position
    • FILE *fopen(const char *filename, const char *mode)
    • int fclose(FILE *fp)

    C Low-Level File API

    • Operations on file descriptors - an OS object representing the state of a file
    • int open(const char *filename, int flags [, mode_t mode])
    • int creat(const char *filename, mode_t mode)
    • int close(int filedes)

    File System Implementation

    • Internal OS file descriptor:
      • A struct with all the information about the file
      • Where it resides
      • Its status
      • How to access it
    • File system driver:
      • vfs_read() function:
        • Checks if the file has a read method
        • Calls the read method if available
        • Otherwise, calls do_sync_read()### File System: from syscall to driver
    • vfs_read function in fs/read_write.c is responsible for handling read system calls
      • Checks if the file is open for reading and if the read operation is valid
      • Verifies the area of memory where the data will be written
      • Calls the read function of the file operations structure
      • Updates the number of bytes read and notifies the file system of the access

    Lower Level Driver

    • Associated with a particular hardware device
    • Registers/unregisters itself with the kernel
    • Handler functions for each of the file operations

    Device Drivers

    • Device-specific code in the kernel that interacts directly with the device hardware
    • Supports a standard, internal interface
    • Same kernel I/O system can interact easily with different device drivers
    • Typically divided into two pieces: top half and bottom half
      • Top half: accessed in call path from system calls, implements standard interface
      • Bottom half: runs as interrupt routine

    Life Cycle of an I/O Request

    • User program initiates I/O request
    • Kernel I/O subsystem handles the request
    • Device driver is called to perform the I/O operation
    • Device driver interacts with the hardware
    • Data is transferred between the device and the kernel
    • Kernel notifies the user program of the I/O completion

    Communication between processes

    • Files can be viewed as communication channels
    • Producer and consumer of a file may be distinct processes
    • May be separated in time
    • Data written once and consumed once can be implemented using a queue

    Request Response Protocol

    • Client issues requests to a server
    • Server performs operations and responds to the client
    • Requests and responses are sent over a communication channel

    Client-Server Models

    • Multiple clients accessing a common server
    • File servers, web, FTP, databases, etc.

    Sockets

    • An abstraction of a network I/O queue
    • Mechanism for inter-process communication
    • Embodies one side of a communication channel
    • Same interface regardless of location of other end
    • Can be local (UNIX socket) or remote (network socket)

    Silly Echo Server and Client

    • Client and server communicate using sockets
    • Client sends request, server responds
    • Data is transferred over the network

    Server Protocol

    • Server creates a socket and binds it to an address
    • Server listens for connections
    • Client connects to server
    • Server accepts connection and forks off a new process to handle it
    • Child process closes listen socket and parent process waits for child to finish
    • Server closes connection socket and exits

    Server Protocol with Protection and Parallelism

    • Server creates a socket and binds it to an address
    • Server listens for connections
    • Client connects to server
    • Server accepts connection and forks off a new process to handle it
    • Child process closes listen socket and parent process waits for child to finish
    • Server closes connection socket and exits
    • Multiple clients can connect to the server concurrently

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    Description

    Lecture on Introduction to I/O, Sockets, and Networking, covering fork, wait, and exec functions in process management.

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