Advanced Programming in the UNIX Environment

Week 3: umask(2)

In this video lecture, we'll take a look at what file ownership and permissions are applied when we create a new file. In the process, we'll learn about the concept of a process's "umask", and look at how the 'umask' shell built-in is implemented.

https://youtu.be/bRAR2bv2HSM

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Advanced Programming in the UNIX Environment

An illustration of how permission octal modes and umasks work.

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Advanced Programming in the UNIX Environment

Week 3: Union Mounts and Whiteout Files

One of the file types we've encountered are those of type S_IFWHT, so called "whiteout" files. But what the hell is that?? Let's illustrate! We describe the concept of union mounts and see what happens when a file in the upper layer is removed while the same file still exists in the lower layer: a whiteout file is created to cover up the lower file.

https://youtu.be/MkFExG9jhEE

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Advanced Programming in the UNIX Environment

Week 4: The Unix Filesystem

In this video lecture, we begin a closer look at the Unix Filesystem (UFS). We visualize how the filesystem structures the disk and organizes files and directories. We illustrate how directory entries are created and moved around.

https://youtu.be/kY4JAXYyByQ

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Advanced Programming in the UNIX Environment

Week 4: Links

In this video lecture, we'll take a look at the system calls used to create, remove, and rename both hard- and symbolic links. We'll compare expected output against trivial implementations of the ln(1) and mv(1) commands.

https://youtu.be/0l8o2goJ1kc

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Advanced Programming in the UNIX Environment

Week 4: Directories

In this video lecture, we take a look at how directories are created and removed, as well as how to move around the filesystem hierarchy. We also learn why the 'cd' command must be a shell builtin in order to work.

https://youtu.be/xZ7dNXZ58G8

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Advanced Programming in the UNIX Environment

Week 4: Directory Size

In this video lecture, we dive deep into the structure of the directory on a traditional Unix File System and see how its size is independent of the file sizes of its entries, but dependent on the filename lengths. We'll also use hexdump(1) to cheat a bit and look at the directory structure on disk.

https://youtu.be/gY0SE-71LZA

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Advanced Programming in the UNIX Environment

Week 4: /etc/passwd

In this short video, we try to answer the awkward question "Mommy, where do UIDs come from?". We look at the /etc/passwd user database and identify the various fields before we look at how things can get weird.

https://youtu.be/fv16TWDnLYM

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Advanced Programming in the UNIX Environment

Week 4: getpwuid(2) and /etc/groups

In this video lecture, we look at the library functions used to look up account information. We learn about how primary and supplementary groups are handled as well as how the hashed passwords are stored outside of the world readable /etc/passwd file.

https://youtu.be/aomkx6_aWpc

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Advanced Programming in the UNIX Environment

Week 4: atime, mtime, ctime

In this video lecture, we start looking a little bit closer into what our systems think of "time", something that experts most accurately describe as a big ball of wibbly, wobbly, timey, wimey... stuff. In short, analyze how the atime, mtime, and ctimes of a file connect.

https://youtu.be/ffYEEj5vnlw

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Advanced Programming in the UNIX Environment

Week 4: time(3) is an illusion

Having revisited the atime, mtime, and time in our last video segment, we now have to come to terms with time(3) itself. We'll discuss managing the data structures and handling arbitrary concepts such as leap seconds, timezones, and (ugh) Daylight Savings Time. It gets silly real quick.

https://youtu.be/3N2aH1vUacQ

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Related to our discussion of time(3), here's a deeper dive into the Unix epoch, its origins, and how the Y2K38 issue actually affects different unix systems well before the heat-death of the universe.

https://www.netmeister.org/blog/epoch.html

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Advanced Programming in the Unix Environment

Week 5: The Unix Development Environment

This week, we cover the concept of the Unix userland as an Integrated Development Environment. This introduction outlines the different tools we will cover in subsequent videos, including the editor, the compiler chain, make(1), the debugger, and code revision and management tools. No vibe coding here.

https://youtu.be/QYwYLR3UD2w

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Advanced Programming in the Unix Environment

Week 5: The Editor

In this video lecture, we look at the required feature for a full-fledged programmer's editor and illustrate some of the core functionality by example of vim(1). This includes basic motion commands, setting and moving to markers, using folds, and the use of the ':make' and quick fix lists to address compiler errors efficiently.

(Don't worry, we'll talk about ed(1) later.)

https://youtu.be/DdaJ87G9Kes

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Advanced Programming in the Unix Environment

Week 5: The Compiler Chain, Part I

In this video lecture, we begin our discussion of compilers as part of the Unix programming environment. We provide a high-level overview of how compilers process input source code and turn it into an executable.

https://youtu.be/SOeo4zLBqdI

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Advanced Programming in the Unix Environment

Week 5: The Compiler Chain, Part II

In this video lecture, we look at the preprocessing stage of the compiler and observe how it invokes the cpp(1) command to include header files or replace macro definitions in place.

https://youtu.be/8qPxQ3g8bso

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Advanced Programming in the Unix Environment

Week 5: The Compiler Chain, Part III

In this next-to-last part on the compiler chain, we look at the compilation proper and the optimization and assembly of the intermediate code into an object file. (We will revisit some of this when we take a closer look at shared libraries in week 11.)

https://youtu.be/ddvYL-G3oew

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Advanced Programming in the Unix Environment

Week 5: The Compiler Chain, Part IV

In this video lecture, we conclude our discussion of the compiler chain, analyzing the last step in the process: linking the object files into the executable. We identify aspects of the C Runtime libraries as well as the standard C library that we need to include and cover a few of the most important flags to ld(1).

https://youtu.be/8LCyPmjknBY

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Advanced Programming in the Unix Environment

Week 5: make(1)

Moving on from the compiler, we now look at how the make(1) utility can be used to help us selectively build our code project.

https://youtu.be/WLTuUtj7LPw

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Advanced Programming in the Unix Environment

Week 5: Debugging your code

Next in our discussion of the Unix _development_ environment, we start a short series on the use of a debugger by example of gdb(1). First, we illustrate just why exactly we might want to use a separate tool instead of sprinkling "printf" statements all over our code...

https://youtu.be/61YwbTQy2G0

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Advanced Programming in the Unix Environment

Week 5: Using gdb(1)

Alright, let's look at gdb(1)! We begin by using it to troubleshoot our failing programs and immediately identify the location of our program where it segfaults. We also see how to inspect variables and call functions from within the debugger.

https://youtu.be/Miw0XLzHCws

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Advanced Programming in the Unix Environment

Week 5: Using gdb(1), part II

In this video lecture, we continue to explore the capabilities of the debugger. We show how you can set a breakpoint to pause the program and how to step through the program while watching our code.

https://youtu.be/fOp4Q4mnTD4

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Advanced Programming in the Unix Environment

Week 5: Using gdb(1), part III

In this video lecture, we'll use our newly learned debugging skills to analyze and fix yet another flawed program. In the process, we learn to debug code from multiple source files and find out how to change the value of variables while the program is running.

https://youtu.be/hgcj7iAxhhU

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Advanced Programming in the Unix Environment

Week 5: Using gdb(1), part IV

In our last segment on gdb(1), we use the debugger to examine memory locations in a running program and illustrate how pointers and arrays work in the C programming language.

https://youtu.be/hr-rn0yUhAw

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Advanced Programming in the Unix Environment

Tool Tip: ed(1) is the standard text editor

As part of our discussion of unix development tools, we take a short detour to provide a quick walkthrough of ed(1), the standard text editor. This is not done merely to up your geek creds, but understanding ed(1) will also helps us better understand other tools like vi(1), sed(1), and ultimately even things like diff(1) and patch(1) better.

https://youtu.be/mRZsV7aMK0I

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Advanced Programming in the Unix Environment

Week 6: Memory Layout of a Process

Let's talk about the details of a process. We start with a visual representation of a process in memory and create a program that prints out the addresses of different elements of a program and observe the alignment of the text, data, and bss segments as well as the placement of the heap and stack. We also illustrate what a stack overflow looks like.

https://youtu.be/aHmcowhoOzU

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Advanced Programming in the Unix Environment

Week 6: Program Startup

In this video lecture, we learn how a program starts. "What, you write 'main()' and off you go!" you say? Not so fast! Let's look at the program entry point '_start' and see if we can change it, and in the process find out that the function prototype of 'main' may not be what you think it is. Use your debugger to better understand how the process begins and ends its life.

https://youtu.be/Yul4d-BDdx8

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Advanced Programming in the Unix Environment

Week 6: Process Termination

After we talked about program startup in our previous video, we now look at how programs terminate. We observe the return status and how exit handlers are registered.

https://youtu.be/xNeEw1f_js8

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