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<ul>
<li><a href="#operating-systems-design-and-implementation-notes"
id="toc-operating-systems-design-and-implementation-notes">Operating
Systems Design and Implementation Notes</a></li>
<li><a href="#introduction-to-processes"
id="toc-introduction-to-processes">1. Introduction to Processes</a>
<ul>
<li><a href="#sequential-processes" id="toc-sequential-processes">1.
Sequential Processes</a>
<ul>
<li><a href="#process-vs-program" id="toc-process-vs-program">Process vs
Program:</a></li>
<li><a href="#key-idea" id="toc-key-idea">Key Idea:</a></li>
</ul></li>
<li><a href="#creation-and-termination"
id="toc-creation-and-termination">2. Creation and Termination</a>
<ul>
<li><a href="#creation---fork" id="toc-creation---fork">Creation ->
<code>fork()</code></a></li>
<li><a href="#termination" id="toc-termination">Termination</a></li>
</ul></li>
<li><a href="#process-hierarchies" id="toc-process-hierarchies">3.
Process Hierarchies</a></li>
<li><a href="#process-states" id="toc-process-states">4. Process
States</a>
<ul>
<li><a href="#process-scheduler" id="toc-process-scheduler">Process
scheduler</a></li>
</ul></li>
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</nav>
<h3 id="operating-systems-design-and-implementation-notes">Operating
Systems Design and Implementation Notes</h3>
<h1 id="introduction-to-processes">1. Introduction to Processes</h1>
<h5 id="by-jiawei-wang">By Jiawei Wang</h5>
<p>While, strictly speaking, at any instant of time, the CPU is running
only one program, in the course of 1 second, it may work on several
programs, thus giving the users the illusion of parallelism. called
<strong>pseudoparallelism</strong>.<br></p>
<h2 id="sequential-processes">1. Sequential Processes</h2>
<p><strong>This model (Sequential Processes) that makes parallelism
easier to deal with.</strong><br> <img src="Sources/Process.png"
alt="Process" /></p>
<h3 id="process-vs-program">Process vs Program:</h3>
<ul>
<li><strong>Process:</strong> A process is just an executing
<code>program</code> with values of
<code>program counter</code>,<code>registers</code> and
<code>variables</code>.</li>
<li><strong>Program:</strong> An algorithm expressed in some suitable
notation.</li>
</ul>
<h3 id="key-idea">Key Idea:</h3>
<p><strong>A process is an activity of some kind. It has a program,
input, output, and a state.<br>A single processor may be shared among
several processes, with some scheduling algorithm being used to
determine when to stop work on one process and service a different
one.</strong></p>
<h2 id="creation-and-termination">2. Creation and Termination</h2>
<p><strong>A new process is created by having an existing process
execute a process creation system call.</strong><br></p>
<h3 id="creation---fork">Creation -> <code>fork()</code></h3>
<ul>
<li><p><strong>After the <code>fork</code>, the two processes, the
parent and the child, have the same <a
href="https://stackoverflow.com/questions/32655960/what-is-a-memory-image-in-nix-systems">memory
image</a><br> the same environment strings, and the same open
files.</strong><br></p></li>
<li><p><strong>After a process is created both the parent and child have
their own distinct address spaces.<br> In short, The child’s initial
address space is a copy of the parent’s,</strong> the child can read
parent spaces’ variables and program text, but no writable memory is
shared.</p></li>
</ul>
<p><strong>Example: <a
href="Codes/process.c">Codes/process.c</a></strong></p>
<div class="sourceCode" id="cb1"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">(</span><span class="dt">int</span> argc<span class="op">,</span> <span class="dt">char</span><span class="op">*</span> argv<span class="op">[])</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">int</span> x <span class="op">=</span> <span class="dv">2</span><span class="op">;</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a> <span class="dt">int</span> pid <span class="op">=</span> fork<span class="op">();</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span>pid <span class="op">==</span> <span class="op">-</span><span class="dv">1</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span>pid <span class="op">==</span> <span class="dv">0</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> x<span class="op">++;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a> printf<span class="op">(</span><span class="st">"Process id %d</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> getpid<span class="op">());</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> printf<span class="op">(</span><span class="st">"x is equal to %d</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> x<span class="op">);</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span> <span class="op">(</span>pid <span class="op">!=</span> <span class="dv">0</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a> wait<span class="op">(</span>NULL<span class="op">);</span></span>
<span id="cb1-15"><a href="#cb1-15" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb1-16"><a href="#cb1-16" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb1-17"><a href="#cb1-17" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<pre><code>❯ ./a.out
Process id 62895
x is equal to 2
Process id 62896
x is equal to 3</code></pre>
<h3 id="termination">Termination</h3>
<p><strong>There are four principal events that cause processes to be
created:</strong> 1. System initialization. 2. Execution of a process
creation system call by a running process. 3. A user request to create a
new process. 4. Initiation of a batch job.</p>
<h2 id="process-hierarchies">3. Process Hierarchies</h2>
<p><strong>A process, its children, and further descendants together may
form a process group.</strong><br> When a user sends a signal from the
keyboard, the signal may be delivered to all members of the process
group currently associated with the keyboard.</p>
<h2 id="process-states">4. Process States</h2>
<ol type="1">
<li>Running (actually using the CPU at that instant).</li>
<li>Ready (runnable; temporarily stopped to let another process
run).</li>
<li>Blocked (unable to run until some external event happens).</li>
</ol>
<figure>
<img src="Sources/status.png" alt="status" />
<figcaption aria-hidden="true">status</figcaption>
</figure>
<h3 id="process-scheduler">Process scheduler</h3>
<p><strong>Process scheduler is a part of the operating
system.</strong><br>In my opinion, The core of Processes in OS, is the
algorithms of process scheduler.<br> <br> The subject of
<strong>scheduling</strong>—deciding which process should run when and
for how long—is an important one.<br> How to <strong>balance</strong>
the competing demands of <strong>efficiency</strong> for the system as a
whole and fairness to individual processes?<br> Which is also the key of
this hole chapter notes, we will talk about in the following notes.</p>
</section>
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