processes.html

---
layout: default
title: Processes
last-updated: February 2024
---

<div id="nav_bar_2" class="nav">
    <ul>
        <li><a href="#introduction"> Introduction </a></li>
        <li><a href="#actors"> Actors </a></li>
        <li><a href="#process-creation"> Process Creation and System Startup</a></li>
        <li><a href="#exec">Executing a Different Program</a></li>
        <li><a href="#file-descriptors"> File Descriptors </a></li>
        <li><a href="#termination">Process Termination</a></li>
        <li><a href="#attributes">Process Attributes</a></li>
        <li><a href="#what-is">Ya But What Is It?</a></li>
        <li><a href="#dr-fraser"> Dr. Brian Fraser </a></li>
        <li><a href="#references"> References </a></li>
    </ul>
</div>

<div id="introduction" class="content">
    <h1>Processes</h1>

    <div class="quote-text">
        "Understanding is the key to success with Linux."
    </div>
    <div class="quote-ref">
        <a href="https://www.tldp.org/LDP/sag/html/intro.html">Linux System Administrator's Guide</a>
    </div>

    <p>
        A <em>process</em> is an instance of an executing 
        program.
        This essay will chisel out the fundamentals of
        processes, as they exist on Unix-like platforms.
    </p>
</div>

<div class="content" id="actors">
    <h2> Actors </h2>

    <div class="aside-right">
        <h4>Note:</h4>
        <p>
            There is a close relationship between a <em>program</em> and a
            <em>process</em>&mdash; Namely, a program is a file
            containing the instructions that the process carries out.
        </p>
    </div>

    <p>
        In Unix-like operating systems, processes are the actors of the system:
        Left to its own devices, the kernel does nothing.
        The system depends entirely upon processes to direct 
        the kernel, and through the kernel, processes manipulate the machine.
    </p>

    <p>
        The kernel is responsible for:
    </p>

    <ol>
        <li>
            Creating new processes
        </li>
        <li>
            Allocating hardware resources to processes
        </li>
        <li>
            Maintaining information about each
        </li>
    </ol>

    <p>
        Per the Unix philosophy, each process has a narrowly defined role, 
        which it is expected to do well. It might copy a file, 
        display a window, provide audio services, or display the time.
        Many small programs are used together to accomplish
        a given task, thereby easing the maintenance and compounding 
        the utility of each.
    </p>

    <p>
        A given user may have more than one process acting on
        their behalf at a time, and there are usually hundreds
        of processes on the system at a time.
        Much like with files, the kernel maintains a large volume of
        information about each process on the machine.
    </p>

    <h3 id="priveledges"> Interacting with Files: Process Permissions</h3>

    <p>
        Every process is associated with some user, and we say that 
        it is executing with that user's <em>permissions.</em>
        A process can freely manipulate files owned by 
        its associated user, and 
        files created by a process inherit its associated user.
    </p>

    <p>
        Processes acting as user 
        <em><a href="http://www.linfo.org/root.html">root</a></em> 
        are special:
        They can change any file on the system, without qualification.
        That is, they by-default transcend the file 
        ownership scheme; we often use 
        <em>superuser</em> to describe this user.
    </p>
      
    <p>
        In practice, root is the administrative user.
        A typical use of superuser privileges is to 
        mount a physical device to prepare it for system-wide
        use.
        Superuser privileges are typically acquired
        through <code>sudo</code>.
    </p>
    
    <h3> See Also: </h3>
    <ul>
        <li><a href="https://www.sudo.ws/man/1.8.3/sudo.man.html"><code>sudo(8)</code></a> - Execute a command as another user</li>
        <li><a href="https://man7.org/linux/man-pages/man7/credentials.7.html"><code>credentials(7)</code></a> - Process identifiers</li>
    </ul>
</div>

<div class="content" id="process-creation">
    <h2 style="margin-bottom: 0;"> Process Creation</h2>
    <h4>and System Startup</h4>

    <p>
        At the end of its boot sequence, the kernel launches a single program,
        <code>/sbin/init</code>,<sup><a href="https://en.wikipedia.org/wiki/Init">[1]</a></sup>
        and in so doing creates the system's first process.
        While <code>init</code> schemes
        <a href="https://arxiv.org/abs/0706.2748">have evolved</a>
        over the years, its purpose has not changed:
        It is a process with superuser permissions that acts as a
        manager of the system and trustee among processes.
    </p>

    <div class="aside-left">
        <h4>Aside:</h4>
        <p>
            You can check your <em>init</em> program with
        </p>

        <div class="code">
            $ ps --pid 1<sup><a href="#references">[2]</a></sup>
        </div>

        <br>
    </div>

    <p>
        <code>init</code>'s first responsibility is to bring up userspace.
        It does this by launching a sequence of programs, most of which
        act in the background, some of which the user interacts with directly.
        After this, it manages available services
        and logs events for administrative purposes.
        Its final responsibility is to shut the system down cleanly.
    </p>

    <p>
        After the the kernel launches <code>/sbin/init</code>, process creation 
        is dictated by processes themselves, and is carried 
        out through the <em>fork()</em> system call.
    </p>

    <p>
        <em>fork()</em> instructs the kernel to duplicate 
        the current process,
        and the two resulting processes are very nearly identical.
        Of note, they both continue to execute the same program at the 
        same spot, have the same open files, and have duplicate memory spaces.
        The newly created process is referred to as the <em>child</em> process,
        the original is called the <em>parent,</em> and we say
        that the child process <em>inherits</em> its parent's attributes.
    </p>

    <p>
        Parent and child do differ in a few ways.
        First, upon birth, each process is given its own (unique)
        identification number by the kernel.
        Second, they each receive a different return value 
        from the <em>fork()</em> system call.
        This allows them to act differently within the same
        program:
    </p>

    <pre>
        pid_t rv = fork();      // One process executes this line
        if(rv == 0) {           // Two processes execute this line
            // I am child

        } else if (rv &gt; 0) {
            // I am parent

        }
    </pre>

    <p>
        Their executions thereby diverge, and each is free to act
        independently.
    </p>

    <h3>See Also:</h3>
    <ul>
        <li><a href="https://wiki.archlinux.org/title/systemd">systemd - ArchWiki</a></li>
        <li><code><a href="https://www.man7.org/linux/man-pages/man2/fork.2.html">fork(2)</code></a> - Create a child process</li>
    </ul>
</div>


<div class="content" id="exec">
    <h2>Executing a Different Program</h2>

    <p>
        The <em>exec()</em> family of functions replace the program
        currently being executed.  
        A simple invocation is:
    </p>

    <pre>
        char *args[] = {"ls", "-l", NULL};
        execv("/usr/bin/ls", args); 
            // Execution of *this* program stops here,
            // and the process continues executing at /usr/bin/ls
    </pre>

    <p>
        <em>exec()</em> simply loads a new program into
        the calling process's memory.
        During this operation, the old program and its
        memory (its "state") are discarded.
    </p>

    <p>
        <em>exec()</em> usually follows <em>fork()</em>:
    </p>

    <pre>
        pid_t rv = fork();
        if( rv == 0 ) {
            // I am child
            char *args[] = {"ls", "-l", NULL};
            execv("/usr/bin/ls", args); 

        } else if ( rv &gt; 0 ) {
            // Execution of parent process continues within 
            // this program
        }
    </pre>

    <p>
        This sequence results in one process launching another program,
        and is traditionally referred to as "fork and exec."
        Note that there is no need to adjust other 
        process attributes, and they are preserved across
        <em>exec()</em>.
        In particular, the PID of the calling process is left unchanged.
    </p>

    <h3>See Also:</h3>
    <ul>
        <li><a href="https://man7.org/linux/man-pages/man1/pstree.1.html"><code>pstree(1)</code></a> - Display a tree of processes</li>
    </ul>
</div>

<div id="file-descriptors" class="content">
    <h2> File Descriptors</h2>

    <p>
        Within a process, each open file is associated
        with a small nonegative integer.
        This number is called a <em>file descriptor;</em>
        it is returned by the kernel in response to a request to open 
        a file.
        The process uses this number to refer to the 
        file whenever it wishes to act upon it.
    </p>

    <p>
        In the following line, a process is instructing the kernel to
        open the file <em>rubber_ducky</em> for reading. 
        The process is storing the kernel's response in the 
        (integer) variable <em>fd:</em>
    </p>

    <pre>
        int fd = open("rubber_ducky", O_RDONLY);
    </pre>

    <p>
        If this call failed (which it would if <em>./rubber_ducky</em> did not
        exist, for example), then <em>fd</em> would be assigned <em>-1,</em> 
        an invalid file descriptor.
        We can therefore check with:
    </p>

    <pre>
        if (fd &lt; 0) {
                perror("open");     // Print error message
                exit(1);            // Terminate process 
        }
    </pre>

    <p>
        and then continue as though <em>open()</em> had succeeded.
    </p>

    <p>
        In the following line, the kernel is directed to read at-most 
        <em>buff_size</em> bytes from <em>fd</em>,
        and place the data into the memory denoted by <em>buff:</em>
    </p>

    <pre>
        int num_bytes_read = read(fd, buff, buff_size);
    </pre>

    <p>
        If all goes well, the system call will return with a nonnegative value,
        the number of bytes read;
        if the kernel encountered an error, it will return with a
        negative value.
        In either case, this value will be assigned to <em>num_bytes_read</em>,
        which may be checked similarly to <em>open().</em>
    </p>

    <p>
        <em>write()</em> is handled analogously, as is closing a file.
        Every system call has an entry in Section 2 of the man pages.
    </p>

    <h3>See Also:</h3>
    <ul>
        <li><code><a href="https://man7.org/linux/man-pages/man2/read.2.html">read(2)</a></code> - Read from a file descriptor</li>
        <li><a href="https://man7.org/tlpi/index.html">The Linux Programming Interface</a> by Michael Kerrisk</li>
    </ul>
</div>


<div class="content" id="termination">
    <h2> Process Termination </h2>
    <p>
        A process may terminate for a few reasons:
    </p>

    <ul class="big-list">
        <li>
            Explicitly terminates its own execution
            <p>
                This is the usual (and preferred) route of process
                termination:
                The program finishes its job or identifies an error, 
                and terminates itself.
                This may be done by either calling <em>exit()</em>, or 
                returning from <em>main()</em>.
            </p>
        </li>

        <li>
            It is told to terminate

            <p>
                A process may be told to terminate by another process through
                a <em>signal,</em> a primitive form of interprocess communication.
                This is abnormal process termination.
            </p>

            <p>
                Signals are brutal business.  
                For the purposes of process termination,
                <code>SIGINT</code> is preferred because it 
                affords the program the oportunity to exit 
                cleanly.<sup><a href="#references" title="Kerrisk, p389">[3]</a></sup>
                At the command line, it is created and sent 
                (to the foreground process) with CTRL+C.
            </p>

            <p>
                Command-line tools for sending signals to arbitrary 
                processes are <code><a href="https://www.man7.org/linux/man-pages/man1/kill.1.html">kill</a></code>,
                which refers to processes by PID, and
                <code><a href="https://www.man7.org/linux/man-pages/man1/killall.1.html">killall</a></code>,
                which refers to processes by program name.
            </p>
        </li>

        <li>
            It does something wrong
            <p>
                Almost always, this happens when the process tries
                to read or write a memory location not assigned to it,
                a condition known in Linux as <em>segmentation fault</em>.
                The kernel steps in, halts program execution and immediately
                terminates the process.
                Again, this is abnormal program termination.
            </p>
        </li>
    </ul>


    <h3>See Also:</h3>

    <ul>
        <li><a href="https://www.man7.org/linux/man-pages/man1/top.1.html"><code>top(1)</code></a> - Display Linux processes</li>
    </ul>
</div>


<div class="content" id="attributes">
    <h2>Process Attributes</h2>

    <p>
        The kernel stores a number of attributes
        for each process it is hosting; we will not exhaust them here.
        The process's PID is one such piece of information; it cannot
        be changed.
    </p>

    <p>
        Each process has a current working directory. 
        Relative filenames are resolved with respect to
        this path, and a process may change it via the 
        <em>chdir()</em> system call.
    </p>

    <p>
        Environment variables are <em>name=value</em> strings
        that are associated with each process.  
        The kernel does not interpret these variables directly,
        but instead simply allows processes to set them, and 
        retrieve them later.
        They are preserved across both <em>fork()</em>
        and <em>exec().</em>
    </p>

    <p>
        Environment variables allow a single process to communicate with
        all of its descendants.
        A good example of their use is <code>LANG</code>,
        which stipulates the current locale, including preferred language.
        We may view the shell's environment variables with 
        the <code>printenv</code> command:
    </p>

    <div class="code">
        $ printenv<br>
        SHELL=/bin/bash<br>
        COLORTERM=truecolor<br>
        XDG_CONFIG_DIRS=/etc/xdg<br>
        DESKTOP_SESSION=xfce<br>
        EDITOR=nvim<br>
        LANG=en_US.UTF-8<br>
        [...]
    </div>

    <h3>See Also:</h3>
    <ul>
        <li><a href="https://en.wikipedia.org/wiki/Environment_variable#True_environment_variables">Environment Variables</a> - Wikipedia</li>
    </ul>
</div>


<div class="content" id="what-is">
    <h2>Ya But What Is It?</h2>

    <p>
        The short answer is, a section of memory distinct from the
        rest of the machine, and some restricted access to the CPU, both
        granted by the kernel.
        The long answer is the primary abstraction presented to programmers,
        and the environment within which all programs execute.
        The process is one of the most important ideas
        of computer science, and we can only scratch the surface of
        what it is here.
    </p>
    
    <h3> Memory </h3>

    <p>
        Modern operating systems 
        provide each process with its own memory in the
        form of a distinct <em>address space.</em>
        On all modern computers, memory is byte-addressable, so that each
        address refers to exactly one byte.
        Addresses themselves are simply numbers, and 
        an address space is a collection of memory addresses.
    </p>

    <div class="img-right" style="background: #f1f1f1;">
        <a style="outline: solid black 2px; " title="Traced by User:Stannered, original by en:User:Dysprosia / BSD (http://opensource.org/licenses/bsd-license.php)" href="https://commons.wikimedia.org/wiki/File:Virtual_address_space_and_physical_address_space_relationship.svg"><img width="256" alt="Virtual address space and physical address space relationship" src="https://upload.wikimedia.org/wikipedia/commons/thumb/3/32/Virtual_address_space_and_physical_address_space_relationship.svg/512px-Virtual_address_space_and_physical_address_space_relationship.svg.png"></a>
    </div>

    <p>
        For each machine, there exists a unique physical address space.
        An address in this space refers to an actual byte of memory 
        (RAM); 
        only the kernel has access to this address space.
        The kernel, working in conjunction with hardware, 
        imposes a layer of indirection between each process's
        memory references and the physical address space.
        This layer is known as <em>virtual memory,</em>
        and consists of a collection of virtual addresses 
        mapped to physical ones.
    </p>

    <p>
        Virtual memory requires support from both the kernel and CPU:
        The kernel to assign address translations 
        (mappings from virtual addresses to physical addresses), 
        and the CPU to carry them out.
        Each time a process references an address, the
        CPU translates the virtual address to a physical address, so that for all
        intents and purposes, virtual addresses behave exactly
        as physical ones.
    </p>

    <p>
        Since each process operates within its own virtual address space, 
        each process is presented with the illusion
        that it has the entire physical address space to itself.
        By excluding physical addresses from translation,
        the rest of the machine is implicitly protected:
        Other sections of memory simply do not exist within the context of the
        currently running process, and therefore cannot be read from, 
        nor written to.
    </p>

    <h3> CPU </h3>

    <p>
        Modern CPU's support two distinct modes of operation.
        <em>Kernel mode,</em> sometimes called "supervisor mode," is
        the unrestricted mode.
        Here, all CPU instructions are available, and address translation
        is not performed, so that code running in this mode has
        unmediated access to the machine.
    </p>

    <p>
        <em>User mode</em> is a proper subset of kernel mode: 
        A program running
        in user mode does not have access to all instructions, and 
        its memory references are redirected through the virtual
        memory scheme mentioned above. 
        Executing programs in User Mode
        is known as <em>limited direct execution.</em><sup><a href="http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-mechanisms.pdf">[4]</a></sup>
        It is <em>limited</em> in the sense that each process has access to
        only a restricted subset of the CPU's instructions, and
        it is <em>direct</em> in the sense that the program runs
        directly on hardware.
    </p>

    <p>
        Virtual memory and limited direct execution are intended to 
        make safe and efficient use of available hardware.
        Together, they form a sandboxed environment within which a program 
        may safely be executed, separate from the rest of the machine.
    </p>

    <h3>Suggested Reading</h3>
    <ul>
        <li><a href="https://en.wikipedia.org/wiki/The_C_Programming_Language">The C Programming Language</a> by Brian Kernighan and Dennis Ritchie</li>
        <li><a href="https://pages.cs.wisc.edu/~remzi/OSTEP/">Operating Systems: Three Easy Pieces</a> by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau </li>
        <li><a href="https://csapp.cs.cmu.edu/">Computer Systems: A Programmer's Perspective</a> by Randal E. Bryant and David R. O'Hallaron</li>
    </ul>
</div>

<div class="content" id="dr-fraser">
    <h2 style="margin-bottom: 2pt;"> Dr. Brian Fraser: </h2>
    <h4 style="margin-bottom: 18pt;"> Fork and Exec Linux Programming </h4>
    <iframe class="video" src="https://www.youtube-nocookie.com/embed/l64ySYHmMmY" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>


<div id="references" class="content">
  <h2> References </h2>
  <ol>
    <li>
        init - Wikipedia (3 March 2021). Retrieved March 30, 2021, from <a href="https://en.wikipedia.org/wiki/Init">https://en.wikipedia.org/wiki/Init</a>
    </li>
    <li>
        Comparison of init systems - Gentoo wiki.  Retrieved February 26, 2024, from <a href="https://wiki.gentoo.org/wiki/Comparison_of_init_systems">https://wiki.gentoo.org/wiki/Comparison_of_init_systems</a>
    </li>
    <li>
        Kerrisk, Michael.  <em><a href="https://man7.org/tlpi/index.html">The Linux Programming Interface</a></em>. San Francisco, CA, No Starch Press, 2010.
    </li>
    <li>
        Arpaci-Dusseau, R. H., &amp; Arpaci-Dusseau, A. C. (2018). <em><a href="https://pages.cs.wisc.edu/~remzi/OSTEP/">Operating systems: Three easy pieces</a>.</em> Arpaci-Dusseau Books. 
    </li>
  </ol>
</div>