COMS 3157 Advanced Programming

11 - Introduction to UNIX

Intro to OS

What is OS?

• software that sits between hardware and other software
• core part is called the “kernel”
• ex) Windows, Linux, Mac OS

What does OS do?

• a dictator and a servant at the same time
• controls hardware resources and logical resources
• provides a (virtual) environment in which programs run
  • linear address space
  • exclusive use of CPU
  • hardware devices that responds to nice, easy commands

How does OS do that?

• privileged operations (aided by CPU)
• periodic timer interrupts
• predefined entry points into the kernel: system calls

Software organization

    applications: emacs, gcc, firefox, bash, mdb-lookup-cs3157
    -----------------------------------------------------------------
    library functions: printf(), strcpy(), malloc(), fopen(), fread()
    -----------------------------------------------------------------
    system calls: open(), read(), fork(), signal()
    -----------------------------------------------------------------
    OS kernel
    -----------------------------------------------------------------
    hardware: processor, memory, disk, video card, keyboard, printer

History of OS

1945-1970:

• vacuum tubes
• mainframes with punch cards
• IBM 360
• MULTICS

1970: Ken Thompson & Dennis Ritchie invent UNIX and C

Since then, many UNIX variants come and go including:

• AT&T System V Release 4 (SVR4)
• 4.4BSD (Berkeley Software Distribution)
• and others:
  • Microsoft Xenix
  • IBM AIX
  • HP-UX
  • IRIX

Currently, four main competitors remain:

• Linux: created by Linus Torvalds in 1991
• Solaris: SVR4-based commercial offering by Sun
• FreeBSD: based on 4.4BSD
• macOS: combo of Mach kernel and FreeBSD

OS for personal computers

• 1977: CP/M by Kildall - dominant OS for 8-bit PCs
• 1977: Apple II by Steve Jobs and Steve Wozniak
• Early 80s: MS-DOS for IBM PC by Microsoft
• 1984: Apple Macintosh
• 1985-1996: NeXT by Steve Jobs
  • precursor to Mac OS X
• Late 80s & early 90s: MS Windows up to 3.11
  • shell on top of MS-DOS
• Mid 90s to the present: Windows NT, 2000, XP, 7, 8
  • true 32/64-bit OS comparable to UNIX
• 2001-present: Mac OS X

UNIX Overview

User name, User ID, Group, Permission

• every user is equal, except “root” (uid 0)

• example:

  jae@tbilisi:~/cs3157-pub/bin$ ls -al      
  total 84
  drwxr-xr-x 2 jae phd  4096 2011-10-26 00:06 .
  drwxr-xr-x 7 jae phd  4096 2011-10-25 23:35 ..
  -rwxr-xr-x 1 jae phd 16740 2011-10-25 23:47 mdb-add
  -rwsr-xr-x 1 jae phd 16755 2011-10-25 23:58 mdb-add-cs3157
  -rw-r--r-- 1 jae phd  5480 2011-10-29 16:58 mdb-cs3157
  -rwxr-xr-x 1 jae phd 20905 2011-10-25 23:47 mdb-lookup
  -rwxr-xr-x 1 jae phd    83 2011-10-26 00:06 mdb-lookup-cs3157
  jae@tbilisi:~/cs3157-pub/bin$
  

File system

• single root directory: “/”
• relative path v. absolute path
• everything is a file: even a directory, even a hardware device!

UNIX I/O using file descriptors

• file descriptors are small integers representing open files
• when a program starts, kernal opens 3 files without being asked
  • stdin, stdout, stderr on descriptors 0, 1, 2
  • keyboard, screen, screen unless redirected
  • subsequent open files get 3, 4, 5, 6,…
• unbuffered

• file descriptors are used for sockets too

• example:

  int fd = open("myfile", O_RDONLY, 0); 
  if (fd == -1) {
      // open error
  }
  
  int n; 
  char buf[BUF_SIZE];  
  while ((n = read(fd, buf, BUF_SIZE)) > 0)
      if (write(1, buf, n) != n) { 
          // write error
      }
  if (n < 0) {
      // read error
  

Processes

• program v. process

• process ID: getpid()

• a process is created by fork & exec by an existing process

• example:

  // NOTE: this is pseudo-code
  
  ......
  
  if ((pid = fork()) < 0) { // "called once, returns twice"
          die("fork err");
      
  } else if (pid == 0) { 
          // comes here in child process
          exec("ls"); 
          die("exec err");
      
  } else { 
          // comes here in parent process
          ......
  

• kernel starts “init” process, which in turn starts various login managers: getty, xdm, sshd, etc.
• ps command: try ps auxfww

Signals (optional topic)

• OS’s way of telling a process something happened. For example:
  • user pressed Ctrl-C
  • you did something wrong: divide by zero, illegal memory access, etc.
  • one of your child process has quit
  • etc.
• it can come anytime; a process can either:
  • let the default action take place
  • explicitly ignore the signal (not always possible)
  • catch the signal and do your own thing (not always possible)

• you can generate a signal: kill command or kill() function

• example:

  #include <stdio.h>
  #include <stdlib.h>
  #include <signal.h>
  #include <unistd.h>
  
  static void sig_int(int signo)
  {
      printf("stop pressing ctrl-c!\n");
  }
  
  int main()
  {
      if (signal(SIGINT, &sig_int) == SIG_ERR) {
          perror("signal() failed");
          exit(1);
      }
  
      int i = 0;
      for (;;) {
          printf("%d\n", i++);
          sleep(1);
      }
  }