Operating Systems 2019W Lecture 4

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Video from the lecture given on January 16, 2019 is now available.


Things discussed in lecture

  • conducting experiments to understand system behavior vs. reading documentation
  • Basic UNIX commands: cat, grep, wc, top
  • static and dynamic linking, system calls (hello.c from Tutorial 1)
  • /proc/self/exe
  • many system calls from strace of hello
  • C library (libc)
  • static linking includes more than necessary
  • dpkg -S to find package containing a file
  • ldd to see which dynamic libraries a file depends on
  • stand alone shell, sash (called ash in lecture)
  • BusyBox
    • statically linked
    • little version of common parts of UNIX userland
    • BusyBox plus Linux kernel = almost complete small Linux system
  • disassembly of hello using objdump -d
    • functions invoked using call, system calls invoked using syscall
  • X Window system versus Wayland
    • X Window system: standard way of displaying graphical applications on UNIX-like systems
    • Wayland: modern replacement of X used with lots of backwards compatibility
  • investigation of what happens when a window is closed (socket connection is terminated, but thought signal was sent)
  • signals
    • short messages sent to processes
    • can be sent by the kill system call (and kill command)
    • kill <process ID> sends TERM signal by default
    • KILL causes process to die without cleanup (TERM allows for handler to do cleanup)
    • have default handlers, most can be changed except for STOP and KILL
    • Could catch bad memory access by catching SEGV signal
    • can stop and restart processes using signals (STOP and CONT), CONT doesn't always work though