Kernel-Debugging/printf.html at master · kees-jan/Kernel-Debugging · GitHub

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<html>
  <head>
    <title>How does printf work?</title>
    <link rel="stylesheet" href="workshop.css" type="text/css" />
  </head>
  <body>
    <div id="column">
      <div id="main">
        <div id="head">
          <img src="logo-solo.png" id="logo" />
          <h1>How does printf work?</h1>
        </div>

        <div class="indexlink">
          (<a href="index.html">&lt;&lt; back to index</a>)
        </div>

        When you press a key in the Qemu window, it will appear on
        screen. This may appear as basic functionality, but a lot is going on
        under the covers:

        <ul>
          <li>A process is reading keypresses (from <tt>stdin</tt>)
          <li>Once it receives a key, it decides to put it on the screen
          (using libc's <tt>putc()</tt> to send it to <tt>stdout</tt>, for
          example)
          <li>libc requests the kernel to display the character by performing
          the <tt>write()</tt> system call.
          <li>This system call enters the kernel at the <tt>sys_write()</tt> function.
        </ul>

        <h2>Assignment</h2>

        Starting with a breakpoint in <tt>sys_write()</tt>, see if you can
        find out how the character ends up on screen.

        <p><b>Hint</b>: You should end up in a function
        called <tt>do_con_write</tt>, which calls <tt>scr_writew()</tt>

        <p><b>Question</b>: If you log in via ssh, will you use the same code?
        From where are things different?

        <p><b>Question</b>: Now that we've traced writing characters, reading
        them should be a piece of cake <tt>:-)</tt>
      </div>
      <address>
        <a href="mailto:kees-jan.dijkzeul@sioux.eu">Kees-Jan Dijkzeul</a>
      </address>
    </div>
  </body>
</html>