Instructions: Answer all questions.

What are system calls? Explain about the various types of systems calls used in OS.

Marks: 15

System calls are the fundamental interface between a user program and the operating system kernel. They are special functions that a program can invoke to request services from the operating system. These services can range from simple operations like reading a file or creating a new process, to more complex tasks such as managing memory or communicating with other processes.

When a program needs to perform an operation that requires privileged access to hardware or system resources, it cannot do so directly. Instead, it makes a system call. The operating system kernel then executes the requested service on behalf of the program, ensuring that system resources are managed securely and efficiently. This mechanism provides a crucial layer of abstraction and protection, preventing user programs from interfering with each other or corrupting the operating system itself.

System calls are typically implemented as a set of library functions (e.g., in C's standard library) that wrap the actual kernel trap instructions. When a library function is called, it sets up the necessary parameters, executes a special instruction (like `INT` on x86 or `SYSCALL` on ARM) to transfer control to the kernel, and then returns the result to the calling program.

Types of System Calls:

Operating systems categorize system calls into several functional groups. The most common types are:

• Process Control: These system calls manage the creation, termination, and execution of processes.
  • Create Process: Creates a new process.

    fork()
    exec()
    spawn()
    

  • Terminate Process: Ends the execution of a process.

    exit()
    abort()
    

  • Load/Execute Program: Loads a program into memory and executes it.

    exec()
    

  • Get Process Attributes: Retrieves information about a process (e.g., PID).

    getpid()
    getppid()
    

  • Wait for Process: Suspends the calling process until a child process terminates.

    wait()
    waitpid()
    

  • Change Process Attributes: Modifies attributes of a process.

    nice()
    

• File Management: These calls allow processes to interact with the file system.
  • Create File: Creates a new file.

    open()
    creat()
    

  • Delete File: Deletes an existing file.

    unlink()
    remove()
    

  • Open File: Opens an existing file for reading, writing, or both.

    open()
    

  • Close File: Closes an open file.

    close()
    

  • Read File: Reads data from an open file.

    read()
    

  • Write File: Writes data to an open file.

    write()
    

  • Seek: Changes the current file position.

    lseek()
    

  • Get File Attributes: Retrieves information about a file (e.g., size, permissions).

    stat()
    fstat()
    

  • Set File Attributes: Modifies file attributes.

    chmod()
    chown()
    

• Device Management: These calls handle interactions with hardware devices.
  • Request Device: Requests access to a device.

    open()
    

  • Release Device: Releases access to a device.

    close()
    

  • Read Device: Reads data from a device.

    read()
    

  • Write Device: Writes data to a device.

    write()
    

  • Get Device Attributes: Retrieves device status and capabilities.

    ioctl()
    

  • Attach/Detach Device: Manages device connections.

    mount()
    umount()
    

• Information Maintenance: These calls provide system information or allow time/date manipulation.
  • Get Time or Date: Retrieves the current system time and date.

    time()
    gettimeofday()
    

  • Set Time or Date: Sets the system time and date.

    settimeofday()
    

  • Get System Information: Retrieves various system parameters (e.g., memory usage, number of processes).

    uname()
    getrusage()
    

  • Get Process File Usage: Information about resource usage by a process.

    getrusage()
    

• Communication: These calls facilitate inter-process communication (IPC) and network communication.
  • Create/Delete Communication Connection: Establishes or removes communication channels.

    pipe()
    socket()
    

  • Send/Receive Message: Transmits or receives data between processes.

    send()
    recv()
    write()
    read()
    

  • Transfer Status Information: Shares status information.

    ioctl()
    

  • Attach/Detach Remote Device: Manages remote connections.

    mount()
    

• Protection: These calls control access to system resources.
  • Change Protection: Modifies access permissions for files or resources.

    chmod()
    

  • Get Protection Information: Retrieves access control information.

    stat()
    

  • Grant/Revoke Privileges: Manages user or process privileges.

    setuid()
    seteuid()
    

Each type of system call serves a distinct purpose, contributing to the overall functionality and security of the operating system. Understanding these categories helps in comprehending how user programs interact with the kernel to perform essential tasks.