System Calls and Functions

• This section is amazing in implication.
• How do you think the kernel works?
  • Probably a program that is running somewhere.
• The exact opposite is true.
• Step one Hardware
  • Most hardware has at least two modes: normal an privileged.
  • You can imagine what runs in each.
• When your program needs something from the operating system it makes a system call.
• A system call is a controlled entry point into the kernel allowing a process to request that the kernel perform some action on the process's behalf.
• These are the calls we will be discussing.
• The interesting part is what happens when a system call is made.
  • He outlines this on page 45.
  • Then discusses in more detail what the x86 does immediately following this.
  • And points out that this is further detailed in syscalls (2) section of the man page.
    • Which also provides a list of these calls!
• The program makes a call to the api, either as a system call or a library function which eventually evokes the system call.
  1. The function copies relative information to the correct place.
  2. Causes the chip to switch to privileged mode
  3. Causes the system call function to execute.

  • // user space                // kernel space
    ....
    system call() ----------->  
                                 kernel does os type things (maintenance)
                                 kernel does system call requested things
                                 kernel returns
                  <-----------
    return from system call
    user code continues
            

  • What this does not include is the fact that it could be.

  • // user space                // kernel space
    Program 1 for user 1
    system call() ----------->  
                                 kernel does os type things (maintenance)
                                 kernel does system call requested things
                                 kernel returns
                  <-----------
    Program 2 for user 2
    return from system call
    user code continues
    system call() ----------->  
                                 kernel does os type things (maintenance)
                                 kernel does system call requested things
                                 kernel returns
                  <-----------
    Program 1 for user 1
    return from system call
    user code continues