Introduction to UNIX System Calls

Outline

• System Calls: fork, exec, exit, wait, open, read, write, close
• Case Study: Unix/xv6 shell (simplified)

System Calls

• "Kernel functions" that perform privileged operations on behalf of the process. As an OS designer, one of the goals is to minimize the system call interface.

Case study: Unix/xv6 shell (simplified)

• provides an interactive command execution and programming language
• typically handles login session, runs other processes
• look at some simple examples of shell operations, how they use different OS abstractions, and how those abstractions fit together. See Unix paper if you are unfamiliar with the shell.
• Basic structure:

  	while (1) {
  	    write (1, "$ ", 2);			// 1 = STDOUT_FILENO
  	    readcommand (0, command, args);	// parse user input, 0 = STDIN_FILENO
  	    if ((pid = fork ()) == 0) {		// child?
  		exec (command, args, 0);
  	    } else if (pid > 0) {		// parent?
  		wait (0);			// wait for child to terminate
  	    } else {
  		perror ("Failed to fork\n");
  	    }
  	}
  

• system calls: read, write, fork, exec, wait. conventions: -1 return value signals error, error code stored in errno, perror prints out a descriptive error message based on errno.
• What's the shell doing? fork, exec, wait: process diagram (PID, address space -- memory of the process, parent links). fork returns twice, in some sense!
• why call "wait"? to wait for the child to terminate and collect its exit status. (if child finishes, child becomes a zombie until parent calls wait.)
• Example:

  	$ ls
  

• how does ls know which directory to look at? (cwd variable is copied during fork)
• how does it know what to do with its output? (fd[1] is initialized by the shell)
• I/O: process has file descriptors, numbered starting from 0.
• system calls: open, read, write, close
• numbering conventions:
  • file descriptor 0 for input (e.g., keyboard). read_command:

         read (0, buf, bufsize)
    

  • file descriptor 1 for output (e.g., terminal)

         write (1, "hello\n", strlen("hello\n"))
    

  • file descriptor 2 for error (e.g., terminal)
• on fork, child inherits open file descriptors from parent (show in process diagram).
• on exec, process retains file descriptors, except those specifically marked as close-on-exec: fcntl(fd, F_SETFD, FD_CLOEXEC)
• How does the shell implement:

       $ ls > tmp1
  

  just before exec insert:

  	close(1);
  	creat("tmp1", 0666);   // fd will be 1
  

  The kernel always uses the first free file descriptor, 1 in this case. Could use dup2() to clone a file descriptor to a new number.

• Good illustration for why fork + exec vs. CreateProcess on Windows. (CreateProcess takes 10 arguments.)