lecture7.1-unix filesystem

UNIX File System Overview

• Course: CompSci 337 - System Programming

Directory Hierarchy

• Overview of Linux File System Structure

  • All files are organized in a single directory hierarchy

  • Every file is a descendant of the root directory: /

  • Definitions:

    • Current Directory: .

    • Parent Directory: ..

Directory Fundamentals

• Structure of Directories

  • Every directory has at least two special entries:

    • .: mapping to the directory’s own inode number

    • ..: mapping to the parent directory’s inode number

  • Command to check entries: ls -a

  • Inode Number: Unique identifier for files and directories

Paths in the Directory Hierarchy

• Types of Paths

  • Absolute Path: Indicates location from the root node.

    • Example: /home/droh/hello.c

  • Relative Path: Indicates location relative to the current working directory.

    • Example: if droh is the current working directory, the relative path for hello.c is ./hello.c

Examples of Path Calculation

• Finding Absolute Paths

  • Absolute path for file01.txt located in /home/user1/ is /home/user1/file01.txt

  • Absolute path for file02.txt located in /home/user2/files2.txt is /home/user2/files2.txt

• Finding Relative Paths

  • If current working directory is user2, relative path for file02.txt is ../user2/file02.txt

  • If current working directory is user1, relative path for file02.txt is ../user2/file02.txt

Navigating Directories in C

• Standard I/O Functions do not permit directory navigation

• Using System Calls for Directory Navigation

User Mode vs Kernel Mode

• Access Levels in Operating Systems

  • User Mode:

    • Limited access to system resources

    • Applications like web browsers and text editors operate here.

  • Kernel Mode:

    • Full access to all system resources

    • Essential OS functions run in kernel mode

System Calls: Interaction with the OS

• Definition: Mechanism for user mode programs to request services from the kernel.

Example of System Call in Action: printf()

• Process:**

  1. Invoke printf() which formats the data.

  2. A system call is initiated, transitioning to kernel mode.

  3. The kernel manages data output to the console.

  4. Control returns to the program post-execution of printf().

System Calls in C

• High-Level I/O Functions:

  • scanf(), printf(), fopen(), fread(), fwrite(), fclose(), fseek()

• Low-Level System Calls:

  • Allow low-level operations like file access and directory management.

  • Functions include:

    • Directory I/O: opendir(), closedir(), readdir()

    • File I/O: open(), close(), read(), write(), lseek()

Accessing Directories Using System Calls

• opendir()

  • System call to open a directory given by path (absolute or relative).

  • Returns a pointer to a directory structure:

    const char *path = "./somedir";
    DIR *dir = opendir(path);
    if (dir == NULL) {
        perror(path);
        return;
    }
    closedir(dir);

• closedir():

  • Closes the directory pointed to by a DIR structure.

Reading Directory Contents

• readdir():

  • System call for retrieving directory entries

  • Utilizes a pointer of type DIR

  • Returns next entry or NULL when no more entries are found.

  • Entry structure contains:

    • d_ino: inode number

    • d_name: name of the file/directory

  • Example Usage:

    struct dirent *entry;
    while ((entry = readdir(dir)) != NULL) {
        printf("%s ", entry->d_name);
    }