Speaker 1: Welcome to our deep dive into the world of Linux
Speaker 1: system programming.
Speaker 2: Yeah, we're going to be exploring the core the Linux
Speaker 2: operating system. Okay, using this book is our guide. O'Reilly
Speaker 2: dot Linux dot system dot Programming, dot second edition dot
Speaker 2: pdf as our guide.
Speaker 1: So we're really getting down into the nuts and.
Speaker 2: Bolts we are. We're going to be uncovering any gritty Yeah,
Speaker 2: how things work at the lowest level.
Speaker 1: Awesome.
Speaker 2: And you know, it's not just about code, it's about understanding,
Speaker 2: like those fundamental building blocks that make Linux tick. Okay,
Speaker 2: So think system calls. Yeah, those are the direct lines
Speaker 2: to the heart of the OS, to the kernel, to
Speaker 2: the kernel exactly, okay, and the logic that drives it all.
Speaker 1: So we're not just driving the car. We're actually seeing
Speaker 1: how the engine works exactly.
Speaker 2: And here's a fun fact for you to kick things off.
Speaker 2: Are you ready?
Speaker 1: I'm ready?
Speaker 2: Okay. Linux actually uses way fewer system calls than something
Speaker 2: like Windows.
Speaker 1: Really.
Speaker 2: Yeah, we're talking hundreds versus potentially thousand.
Speaker 1: Wow. I would have thought a complex system like Linux
Speaker 1: would need way more ways to talk to its kernel.
Speaker 2: That's where the elegance of Linux comes in.
Speaker 1: Okay.
Speaker 2: It achieves a lot with a surprisingly streamlined set of tools.
Speaker 1: That's pretty cool.
Speaker 2: Yeah.
Speaker 1: So before we get too deep into the weeds, though,
Speaker 1: let's take a step back and defind something. Okay, what
Speaker 1: exactly is system programming?
Speaker 2: Oh that's a great question.
Speaker 1: When I think about using a computer, I don't think
Speaker 1: about this level.
Speaker 2: Right, You're not really meant to You're using all the
Speaker 2: nice applications and things, exactly. The system programming is kind
Speaker 2: of like, think of it like this. It's like being
Speaker 2: the architect of a building.
Speaker 1: Okay.
Speaker 2: So you're working with the raw materials, the foundation, the
Speaker 2: structural framework.
Speaker 1: Not just like decorating the rooms exactly.
Speaker 2: Yeah, you're building the load bearing walls.
Speaker 1: Ooh, I like that metaphor.
Speaker 2: Yeah. So we're writing software that interacts directly with the
Speaker 2: core of the operating system, the kernel. Okay, and it's
Speaker 2: closest partners okay, like the C library known as glib online.
Speaker 2: It's the bedrock upon which all those user friendly applications
Speaker 2: are built.
Speaker 1: So if I'm writing a game or a web browser,
Speaker 1: that's like decorating the rooms. But system programming is making
Speaker 1: the rooms.
Speaker 2: That's a great way to put it.
Speaker 1: Okay, I like it. I'm with you.
Speaker 2: And just like a good architect needs the right tools, right,
Speaker 2: absolutely so do system programmers of course. So the g
Speaker 2: and UC compiler okay, where GCC is one of our
Speaker 2: main tools in this world.
Speaker 1: So that's the blueprint and our hammer.
Speaker 2: Yes, like that our tool belt.
Speaker 1: And let's not forget about system calls. Ooh, yes, these
Speaker 1: are like the language we use to communicate with the kernel. Okay,
Speaker 1: So need to open a file, yep, send data over
Speaker 1: the network, even just allocate some memory. It all starts
Speaker 1: with a system call, got it. So you're directly asking
Speaker 1: the kernel to do something for you.
Speaker 2: Those are those direct lines of communication we talked about exactly.
Speaker 1: And the book also spends a lot of time talking
Speaker 1: about files.
Speaker 2: Yes it does.
Speaker 1: Why are files so central to Linux? Yeah? I was
Speaker 1: wondering that too, because I know, I know we have them.
Speaker 2: But well this might surprise you, but in Linux, pretty
Speaker 2: much everything is represented as a file.
Speaker 1: Wait a minute, what everything is a file?
Speaker 2: We're talking devices, directories, even your terminal window.
Speaker 1: Hold on, So my keyboard is a file. Yep, my
Speaker 1: hard drive is a file.
Speaker 2: It is.
Speaker 1: That's kind of blowing my mind a little bit.
Speaker 2: It is. It's a beautifully unifying concept it is.
Speaker 1: But how does the kernel keep track of all of that.
Speaker 2: That's where inodes and file descriptors come in.
Speaker 1: Okay, I'm intrigued, tell me more.
Speaker 2: So. Think of an inode like a file's unique ID card.
Speaker 1: Okay.
Speaker 2: It holds all this essential information, size, permissions, ownership, time stamps.
Speaker 1: It's like the file's permanent record.
Speaker 2: It's it's the file's permanent record. A file descripture, on
Speaker 2: the other hand, is like a temporary handle your program
Speaker 2: gets to interact with that file.
Speaker 1: Okay. So the inode is the files master record, and
Speaker 1: the descriptor is my program's key to access it precisely.
Speaker 2: I like it. Okay. Now, all these files they live
Speaker 2: in a file system, right, But unlike some other systems
Speaker 2: where you might have different drives and they have their
Speaker 2: own separate spaces, like your C drive and your D drive.
Speaker 2: Yeah yeah, yeah, Linux uses a unified name space.
Speaker 1: So it's all just one big, happy family exactly.
Speaker 2: It's like having one giant filing cabinet for everything, okay.
Speaker 2: And this makes life much easier for programs. They don't
Speaker 2: need to worry about where a file is physically located.
Speaker 2: That makes sense now, Historically, this name space was shared
Speaker 2: by everyone. Okay, but Linux has evolved.
Speaker 1: It always does, it always does.
Speaker 2: It's always changed to support per process name spaces.
Speaker 1: Poopresss name spaces. Now, what is that?
Speaker 2: It means that each process can have its own customized
Speaker 2: view of the file system ooh if needed. So it's
Speaker 2: like having your own personal filing cabinet within that larger one.
Speaker 1: Okay, that's a cool feature. Yeah, all right, so we've
Speaker 1: got files, we've got this unified name space. But what
Speaker 1: about processes?
Speaker 2: Ah? Yes, processes.
Speaker 1: Those are those mysterious things that make our programs actually
Speaker 1: come to life.
Speaker 2: You're exactly right. A process is essentially a running instance
Speaker 2: of your program. Imagine it's like this little virtualized world
Speaker 2: with its own memory space, got it resources and the
Speaker 2: illusion that it has the whole computer to itself.
Speaker 1: So each program gets its own little sandbox to play in.
Speaker 2: Yes, and it's unaware of the other kids playing in
Speaker 2: the other sandboxes. Right, and that isolation is key for
Speaker 2: stability and security.
Speaker 1: Makes sense.
Speaker 2: Now, A process goes through a whole life cycle, right,
Speaker 2: it's born, It lives its life executing your code.
Speaker 1: It has a purpose.
Speaker 2: It has a purpose, and eventually it meets its end. Okay,
Speaker 2: Sometimes it leaves behind a temporary ghost known as a
Speaker 2: zombie process.
Speaker 1: A zombie process that sounds a little ominous.
Speaker 2: It does sound ominous, but don't worry. They're usually harmlessness.
Speaker 1: Okay, good.
Speaker 2: It just means that the process is finished running, but
Speaker 2: its parent hasn't yet acknowledged its demise.
Speaker 1: So it's still kind of hanging around.
Speaker 2: Yeah, it's just waiting for that final goodbye.
Speaker 1: Interesting.
Speaker 2: But during its life, a process has certain permissions that
Speaker 2: dictate what it can and can't do. It receive signals, okay,
Speaker 2: those are like interprocess messages, and it has to handle
Speaker 2: errors gracefully.
Speaker 1: So every program needs some rules in some ways to
Speaker 1: communicate exactly.
Speaker 2: It needs some etiquette, some manners. Yeah, out reading and
Speaker 2: writing data.
Speaker 1: That's a great question, because, yeah, how do they do that.
Speaker 2: Well, that's where input output are IO comes in the
Speaker 2: art of reading and writing data, and in Linux this
Speaker 2: relies heavily on those system calls we talked about.
Speaker 1: Earlier, those direct lines to the kernel.
Speaker 2: Exactly.
Speaker 1: Awesome.
Speaker 2: So imagine you're downloading a movie. Okay, the first thing
Speaker 2: you do is you find the right url.
Speaker 1: I find it on my favorite streaming service, Yes.
Speaker 2: Exactly, that's like the open system call.
Speaker 1: Okay.
Speaker 2: Then you start receiving those data packets yep, that's read it.
Speaker 2: And finally you save those packets to your hard drive.
Speaker 2: That's right, right, okay, and when you're done, you close
Speaker 2: the connection. That's closed.
Speaker 1: Okay.
Speaker 2: That makes a lot more sense now, Yeah, it's nice
Speaker 2: little analogy.
Speaker 1: Yeah, I like it.
Speaker 2: So those are the basics of got it. What about
Speaker 2: this buffer io i've heard about?
Speaker 1: Yeah? Is that just like a fancy name for the
Speaker 1: same thing.
Speaker 2: Not exactly. Buffer io is all about performance.
Speaker 1: Ooh performance. We always like that.
Speaker 2: We always like that. So remember each system call has
Speaker 2: a bit of overhead.
Speaker 1: Okay.
Speaker 2: So if you're reading a file like one bite at
Speaker 2: a time, yeah, you're making a lot of system calls.
Speaker 1: That sounds inefficient, it is.
Speaker 2: That's where buffering comes in.
Speaker 1: We're bundling those calls together.
Speaker 2: They get it, Okay. The c standard io library, which
Speaker 2: you access using stideo, right, it uses this technique. Ok
Speaker 2: So it uses functions like fopen, fred, fright and f
Speaker 2: close to work with files. But it does so in
Speaker 2: chunks okay, and it reduces the number of times it
Speaker 2: needs to talk directly to the kernel.
Speaker 1: That makes sense. It's just being a little bit smarter
Speaker 1: about it.
Speaker 2: Exactly. It's being efficient.
Speaker 1: It's like ordering in bulk exactly. I like it.
Speaker 2: But what about when we have multiple threads uh oh
Speaker 2: accessing the same file.
Speaker 1: Yeah, that sounds like it could get a little messy.
Speaker 2: You could get messy. That's where thread safety comes in. Okay,
Speaker 2: So when you have multiple threads accessing shared resources like files,
Speaker 2: you need mechanisms like locking to ensure that they don't
Speaker 2: corrupt data, of course, by trying to write at the
Speaker 2: same time.
Speaker 1: Yeah, you don't want them to step in on each
Speaker 1: other's toes exactly.
Speaker 2: That's like having a traffic light system.
Speaker 1: Oh. I like that.
Speaker 2: Yeah, to control access to a busy intersection, keep.
Speaker 1: Things flowing smoothly exactly. Okay, so buffer io is great,
Speaker 1: but we need to be careful when multiple threads are involved.
Speaker 2: We do, we do.
Speaker 1: This is getting pretty deep. Yeah, what about techniques that
Speaker 1: go beyond the basics of io Ooh okay, it's lurking
Speaker 1: in those advanced waters.
Speaker 2: Get ready to level up your IO skills.
Speaker 1: I'm ready. Let's go.
Speaker 2: We're going to talk about techniques like scatter gather io,
Speaker 2: event pulling, and even memory mapping, each with their own superpowers.
Speaker 1: Okay, I'm ready for those superpowers. All right, let's start
Speaker 1: with scatter gather io. What kind of magic does that involve?
Speaker 2: Imagine you have data coming from three different sources, okay,
Speaker 2: and you need to write it all to a single
Speaker 2: file instead of making three separate right calls.
Speaker 1: Yeah, that sounds inefficient.
Speaker 2: It is scatter gather io. Let's you do it in
Speaker 2: one shot, okay, combining those data chunks into a single
Speaker 2: efficient operation.
Speaker 1: So it's like optimizing a delivery route by combining multiple
Speaker 1: packages exactly.
Speaker 2: I like it. Linux provides system calls like rehobar and
Speaker 2: writeev and they use a special structure called iovec to
Speaker 2: define those vectors of buffers.
Speaker 1: Got it?
Speaker 2: Making this magic happen.
Speaker 1: That's awesome.
Speaker 2: But what if you're dealing with a lot of files
Speaker 2: and you need to be notified when something happens, like
Speaker 2: new data arriving?
Speaker 1: Ooh yeah, that sounds like a lot to keep track of.
Speaker 2: Is that's where event polling comes in, like having a
Speaker 2: surveillance system for your files.
Speaker 1: I like it.
Speaker 2: So the older system calls select and poll Okay. There
Speaker 2: were early attempts at this okay, but they had limitations
Speaker 2: when you had a huge number of files to watch.
Speaker 1: Yeah, that makes sense.
Speaker 2: Thankfully, Linux introduced the epole facility, okay, which is much
Speaker 2: more effe at monitoring multiple files.
Speaker 1: So it's like upgrading from a security guard watching grainy
Speaker 1: CCTV footage, yes, to like a high tech system with
Speaker 1: AI powered alerts.
Speaker 2: That's a great analogy. And finally, we have memory mapping,
Speaker 2: which is where things get really interesting.
Speaker 1: Okay, I'm intrigued.
Speaker 2: Tell me mar Imagine you have this massive file I
Speaker 2: think a huge database or a high resolution image. With
Speaker 2: memory mapping, you can map that entire file into your
Speaker 2: process's address space. Oh wow, you can treat it as
Speaker 2: if it were already loaded into memory. No need to
Speaker 2: read it in chunks.
Speaker 1: So I have instant access.
Speaker 2: You have direct access.
Speaker 1: Why wouldn't we always do this.
Speaker 2: It's a fantastic tool for performance, especially with large files.
Speaker 2: The system calls map and mun map are your keys
Speaker 2: to this kingdom.
Speaker 1: I'm ready to rule.
Speaker 2: But it's important to understand that there are different ways
Speaker 2: memory mapping can work, and you can use flags like
Speaker 2: map shared and map private.
Speaker 1: Interesting, what's the difference So with.
Speaker 2: Map shared If multiple processes map the same file, any
Speaker 2: changes one process makes are visible to the others. Okay,
Speaker 2: it's like a shared Google doc got it map Private,
Speaker 2: on the other hand, creates a private copy for each process,
Speaker 2: so changes are isolated.
Speaker 1: So it's like working on my own personal copy exactly. Okay,
Speaker 1: that makes sense.
Speaker 2: Yeah, so memory mapping incredibly powerful.
Speaker 1: Wow, very cool.
Speaker 2: But all this talk about memory makes me realize we
Speaker 2: haven't really talked about how Linux actually manages memory.
Speaker 1: Yeah. That's a big one.
Speaker 2: Yeah, it's a big one.
Speaker 1: How does it ensure everything runs smoothly?
Speaker 2: Well? Memory management is crucial, I would think. So. Yeah,
Speaker 2: it's the unsung hero of any operating system. Okay, making
Speaker 2: sure that each process gets fair share of memory. Okay,
Speaker 2: and that everything plays nicely together.
Speaker 1: So it's like a master conductor.
Speaker 2: It's orchestrating this complex symphony.
Speaker 1: I like that.
Speaker 2: So first we need to understand the process address space.
Speaker 2: Remember those isolated sandboxes we talked about earlier. Yes, well,
Speaker 2: the sandboxes are further divided into pages, which are these
Speaker 2: chunks of memory. Okay, not all pages are created equal, though, Oh,
Speaker 2: you have valid pages that correspond to actual data in
Speaker 2: RAM okay, or on disc and you have invalid pages
Speaker 2: that represent unallocated regions.
Speaker 1: So accessing an invalid page is like falling into a
Speaker 1: trapdoor in the sandbox.
Speaker 2: That's a great way to put it. And trying to
Speaker 2: access memory that's not yours is a big no no. Yeah,
Speaker 2: that's bad and usually results in a segmentation violation, a
Speaker 2: programmer's nightmare.
Speaker 1: Oh, segmentation violation. I've heard that term.
Speaker 2: It's not good. That's bad, it's not good.
Speaker 1: I don't like that.
Speaker 2: But thankfully, goodness, we have the memory management unit or MMU.
Speaker 1: Okay, the MMU.
Speaker 2: The MMU to keep things in check. It acts as
Speaker 2: a translator, converting those virtual addresses used by our program
Speaker 2: into physical addresses okay, where that data actually resides in RAM.
Speaker 1: So the MMU is like a postal worker who knows
Speaker 1: the real address behind every po box.
Speaker 2: Another perfect analogy.
Speaker 1: I'm falling.
Speaker 2: So the MMU also plays a vital role in memory protection,
Speaker 2: making sure that processes don't wander into memory territory that
Speaker 2: doesn't belong to them.
Speaker 1: They stay in their own lanes.
Speaker 2: They stay in their own lane. Okay, but what about
Speaker 2: when a program needs to allocate memory on the fly.
Speaker 1: Yeah, like when I'm adding things to my shopping cart online.
Speaker 2: Exactly, and the website needs to store that information somewhere
Speaker 2: right exactly. That's where dynamic memory allocation comes in, okay,
Speaker 2: and it's handled through a special region of memory called
Speaker 2: the heap. The heap, you use functions like malik, calic,
Speaker 2: realic and free to request and release memory from this
Speaker 2: heap as needed.
Speaker 1: So the heap is like a self storage facility. It
Speaker 1: is where programs can rent units of various sizes.
Speaker 2: That's a great way to visualize it.
Speaker 1: Okay, I'm picturing it.
Speaker 2: But managing the heap can get tricky. You can end
Speaker 2: up with something called fragmentation. Fragmentation okay, where you have
Speaker 2: enough free space overall, ok but it's scattered around small chunks.
Speaker 1: You can't fit that big couching.
Speaker 2: You can't fit that big couchin exactly.
Speaker 1: Now you technically have enough space exactly. That's frustrating.
Speaker 2: It is frustrating. But Linux has some clever ways to
Speaker 2: deal with that, okay. Good. For larger chunks of memory,
Speaker 2: it can use something called anonymous memory mapping.
Speaker 1: Anonymous memory mapping.
Speaker 2: Yes, okay, reserving a contiguous block of memory. It's like
Speaker 2: renting a whole warehouse.
Speaker 1: Unit instead of trying to squeeze into those few small lockers.
Speaker 2: Exactly, that makes sense.
Speaker 1: Yeah, now, I remember you mentioned memory locking earlier. Yes,
Speaker 1: when would we want to use that?
Speaker 2: Memory locking is particularly useful when you have a critical
Speaker 2: piece of code that needs to access memory with absolutely
Speaker 2: no delays.
Speaker 1: Of course, you.
Speaker 2: Wouldn't want the kernel to swap that memory.
Speaker 1: Out to disc right, No, that would be terrible.
Speaker 2: It'd be like pausing a movie right in the middle
Speaker 2: of an action scene. So memory locking done through the
Speaker 2: lock system call okay. It ensures that those critical memory
Speaker 2: pages stay in.
Speaker 1: Rank they're locked in.
Speaker 2: They're locked in guaranteeing predictable performance.
Speaker 1: So we're putting a do not disturb sign on those
Speaker 1: memory pages, you got.
Speaker 2: It, preventing the kernel from relocating them. Awesome. It's a
Speaker 2: powerful tool. It sounds like it for performance sensitive applications. Yeah,
Speaker 2: but it's important to use it judiciously, as over using
Speaker 2: it can have drawbacks.
Speaker 1: Of course, too much of a good thing.
Speaker 2: Exactly, especially in multi threaded.
Speaker 1: Environments right where things are already complicated.
Speaker 2: Exactly.
Speaker 1: This whole deep dive into memory management has been fascinated.
Speaker 2: It is a fascinating topic.
Speaker 1: It really is. I'm starting to realize just how much
Speaker 1: complexity is hidden beneath the surface of something as seemingly
Speaker 1: simple as running a program.
Speaker 2: And we've only just begun. Oh No, there's a whole
Speaker 2: universe of fascinating concepts and techniques to explore. Bring it on.
Speaker 2: I'm real, but I'm glad you're starting to appreciate the
Speaker 2: elegance and power of Linux system programming.
Speaker 1: Absolutely i am. I'm already looking forward to uncovering more
Speaker 1: of these hidden gems right as we continue our deep dive.
Speaker 2: Awesome, Now that we've explored memory management, let's shift gears
Speaker 2: a bit, okay and talk about something a bit more tangible.
Speaker 1: All right, I'm with you.
Speaker 2: Files and directories.
Speaker 1: Okay, I can definitely relate to those.
Speaker 2: Yeah, we all have to deal with those.
Speaker 1: I spend way too much time organizing files on my computer. So, right,
Speaker 1: what kind of insights can system programming give us about
Speaker 1: files and directories?
Speaker 2: Well, first of all, we need a way to gather
Speaker 2: information about a file. Okay, think of it like a
Speaker 2: detective doing some reconnaissance. That's where the STAT family system
Speaker 2: calls comes in. They give you all sorts of details
Speaker 2: about a file, file sized, permissions, timestamps.
Speaker 1: So it's like a file's fingerprint.
Speaker 2: Exactly. It's a file's fingerprint, okay. So you can find
Speaker 2: out if it's a regular file or a directory, when
Speaker 2: it was last modified, who owns it, and even what
Speaker 2: permissions are set, so.
Speaker 1: Who can ReadWrite, or execute it?
Speaker 2: Exactly.
Speaker 1: Wow, that's a lot of information.
Speaker 2: It's a treasure trove of data, it is.
Speaker 1: So once we've done our investigation, how do we actually
Speaker 1: manipulate files, like changing permissions or ownership?
Speaker 2: Well, Linux provides these handy system calls like shab and
Speaker 2: choun Okay, So schmad lets you tweak those permissions. Okay,
Speaker 2: who can do what with a file?
Speaker 1: Got it?
Speaker 2: Choun, on the other hand, lets you change the owner
Speaker 2: and group, essentially deciding who's in control.
Speaker 1: It's like being the administrator of your own little file kingdom.
Speaker 2: Exactly. You get to decide who has access and what
Speaker 2: they're allowed to do.
Speaker 1: I like it. I have the power.
Speaker 2: Now. Sometimes you need to store extra information about a file, okay,
Speaker 2: something beyond those basic attributes we get from stack. Yeah,
Speaker 2: that's where extended attributes come in.
Speaker 1: Extended attributes, that sounds interesting. What are we talking about here.
Speaker 2: Think of them like custom labels.
Speaker 1: Okay.
Speaker 2: You can attach to files so you can store things
Speaker 2: like author information, version numbers, even application specific data.
Speaker 1: So it's like adding notes to a file that only
Speaker 1: certain programs or users can see.
Speaker 2: That's a good way to think about it, okay. And
Speaker 2: Linux provides system like get center, set sadder, and list
Speaker 2: sadder to manage these extended attributes.
Speaker 1: Okay, so we've got our files all figured out, but
Speaker 1: what about directories?
Speaker 2: Ah? Yes, directory?
Speaker 1: How do we navigate those and keep everything organized?
Speaker 2: Well, first, we need to know where we are in
Speaker 2: this vast file system, right right. Of course, that's where
Speaker 2: get SETI comes in.
Speaker 1: Handy, gainst SETI.
Speaker 2: It tells you the absolute path to your current working directory.
Speaker 2: Think of it like a GPS for the file system.
Speaker 1: So, no, we're getting lost in a maze.
Speaker 2: Of folders exactly. You always know where you are.
Speaker 1: I like it.
Speaker 2: And when it comes to actually creating and managing directories,
Speaker 2: we have m deer to create new ones and room
Speaker 2: deer to remove empty ones. Okay, those are familiar, Yeah,
Speaker 2: those are the classics. Of course, we can't forget about links.
Speaker 1: Links, Yes, those are always.
Speaker 2: A bit tricky for me, maybe be a little bit tricky.
Speaker 1: I always get hard links and symbolic links confused, right,
Speaker 1: what's the difference again?
Speaker 2: So think of a hard link like an alias for
Speaker 2: a file.
Speaker 1: Okay.
Speaker 2: It points directly to the files in.
Speaker 1: Ode, right, that unique idea, That unique.
Speaker 2: Idea we talked about earlier. So it's like having multiple
Speaker 2: entryways leading to the same room.
Speaker 1: Okay.
Speaker 2: A symbolic link, on the other hand, is more like
Speaker 2: a signpost okay, pointing to the actual file. It stores
Speaker 2: the path to the target file okay, and can even
Speaker 2: point to files on different file systems.
Speaker 1: So a hard link is like having multiple copies of
Speaker 1: the same key, yes, while a symbolic link is like
Speaker 1: having a note that says the key is under the
Speaker 1: flower pot.
Speaker 2: I love that analogy. That's perfect.
Speaker 1: Okay. Good, I'm glad I got that one right.
Speaker 2: Yeah, And Linux gives us system calls like link for
Speaker 2: hard links, some link for symbolic links, and read link
Speaker 2: to find out where a symbolic link is pointing.
Speaker 1: Very cool. This whole world of files and directories is
Speaker 1: starting to make a lot more sense now.
Speaker 2: Good. I'm glad to hear that.
Speaker 1: But I'm curious what about when we want to do
Speaker 1: things like copy or move files. Okay, do we need
Speaker 1: to write a bunch of code to handle that ourselves.
Speaker 2: Here's where things get really clever.
Speaker 1: Okay, I'm intrigued.
Speaker 2: Linux doesn't have specific system calls for copying or moving files.
Speaker 2: It doesn't, no, but it gives us the building blocks
Speaker 2: to do it ourselves.
Speaker 1: Okay.
Speaker 2: And it all comes back to that core concept. Everything
Speaker 2: in Linux is a file right right, even devices.
Speaker 1: Okay, I'm following along so far. But how do devices
Speaker 1: help us with copying files?
Speaker 2: So some devices act as data sources or sinks. Okay. So,
Speaker 2: for example, there's the null device devnyl, which is like
Speaker 2: a black hole that discards anything you write to it.
Speaker 1: Okay, So it's like a digital trash can exactly.
Speaker 2: And then there's the zero device dev zero, which provides
Speaker 2: an endless stream of zeros.
Speaker 1: An endless stream of zeros okay, useful for certain things.
Speaker 2: I can see where this is going. Yeah, we can
Speaker 2: use these devices as intermediaries to copy data exactly.
Speaker 1: So to copy a file, you can open the source
Speaker 1: file for reading, okay, open the destination file for writing okay,
Speaker 1: and then just stream the data from the source to
Speaker 1: the destination.
Speaker 2: Okay, that makes sense.
Speaker 1: Moving a file can be as simple as renaming it.
Speaker 2: Which essentially changes its location within the file system.
Speaker 1: So it's all about leveraging those existing file io mechanisms
Speaker 1: in clever ways.
Speaker 2: It is. It's very elegant.
Speaker 1: It's amazing how much you can accomplish with those basic
Speaker 1: building blocks.
Speaker 2: It is. It's the power of abstraction.
Speaker 1: Now, before we move on, there's one more powerful tool
Speaker 1: I want to introduce you to. Okay, hit me in
Speaker 1: notify and notify. The name sounds familiar, but I can't
Speaker 1: quite place it. What is it used for?
Speaker 2: So and Notify is like a real time surveillance system
Speaker 2: for your files and directories.
Speaker 1: Okay.
Speaker 2: Imagine you want to be notified whenever a file is
Speaker 2: modified or a new file is added to a directory. Yeah,
Speaker 2: and Notified can do that for you.
Speaker 1: Wow, that sounds incredibly useful. I can imagine using that
Speaker 1: for all sorts of things like backing up files or
Speaker 1: keeping track of changes made by other users.
Speaker 2: Exactly. It's very versatile.
Speaker 1: Okay.
Speaker 2: And it uses this file descripture based interface okay, so
Speaker 2: you can integrate it seamlessly into your application.
Speaker 1: Awesome.
Speaker 2: You can add watches for specific files or directories and
Speaker 2: then receive events whenever those watches are triggered.
Speaker 1: So it's like setting up trip wires around my file
Speaker 1: alerting me to any changes.
Speaker 2: That's a great analogy. And it can monitor a wide
Speaker 2: range of events from file access and modification to creation, deletion, renaming. Wow,
Speaker 2: it even provides details about what changed and who made
Speaker 2: the change.
Speaker 1: That's pretty powerful it is.
Speaker 2: It's a powerful tool.
Speaker 1: So in Notifies sounds like a must have tool for
Speaker 1: any serious system programmer.
Speaker 2: I would agree with that.
Speaker 1: This whole deep dive into file and directory management has
Speaker 1: been really eye opening.
Speaker 2: It's a fascinating area.
Speaker 1: I'm starting to realize just how much goes on behind
Speaker 1: the scenes to keep everything organized and running smoothly.
Speaker 2: There's a lot happening under the hood.
Speaker 1: There is so much complexity. Yeah, but it's also really elegant.
Speaker 2: It is when you start to understand the principles behind it. Yes, now,
Speaker 2: let's shift our focus to another crucial aspect of system programming. Okay, signals.
Speaker 1: Signals. It sounds like we're about to enter the world
Speaker 1: of inner process communication, where we're.
Speaker 2: Going to explore how processes talk to each other.
Speaker 1: Okay, So what exactly are signals and how do they
Speaker 1: work in Linux?
Speaker 2: So, signals are the way processes talk to each other.
Speaker 2: Think of them like text messages, okay, short asynchronous bursts
Speaker 2: of information.
Speaker 1: So they're like little pings between processes letting them know
Speaker 1: what's going on.
Speaker 2: That's a good way to put it.
Speaker 1: Yeah.
Speaker 2: Yeah. For example, let's say you're running a program and
Speaker 2: you press futrol plus c to stop it. What's happening
Speaker 2: is that you're sending a signal to that process, okay,
Speaker 2: telling it to terminate.
Speaker 1: So that's what's going on behind the scenes. When I
Speaker 1: hit ut trol plus c, it is. I never realized
Speaker 1: it was a signal being sent.
Speaker 2: It is, and the kernel is like the postal service
Speaker 2: delivering these signals to the right processes.
Speaker 1: Okay, So what happens when a process receives a signal?
Speaker 1: Does it have to like constantly check for them like
Speaker 1: we check our phones for messages.
Speaker 2: Not quite No. Linux provides a mechanism for processes to
Speaker 2: register signal handlers. Signal handlers these are special functions that
Speaker 2: are executed when a specific signal arrives.
Speaker 1: So it's like setting up a voicemail box. For specific
Speaker 1: types of messages.
Speaker 2: That's a great analogy. And just like with voicemail, a
Speaker 2: process can choose to handle a signal, ignore it, or
Speaker 2: let the default action happen.
Speaker 1: And what's the default action? Hopefully it's not always terminating
Speaker 1: the process.
Speaker 2: Well, it depends on the signal. Okay, Some signals are
Speaker 2: designed to be fatal, like the infamous sigsahv.
Speaker 1: Oh yeah, I've heard of that.
Speaker 2: One, which indicates a segmentation fault ray, meaning your program
Speaker 2: tried to access memory it wasn't supposed to ouch. That
Speaker 2: sounds painful, it's not good. And in that case, terminating
Speaker 2: the process is probably the best course of action.
Speaker 1: Right, stop it before it causes any more trouble.
Speaker 2: Exactly, it prevents further damage. But for many signals, the
Speaker 2: default action might be something less drastic, like pausing the
Speaker 2: process or to simply ignoring the signal.
Speaker 1: So there's a whole range of responses depending on the
Speaker 1: type of signal.
Speaker 2: Exactly. It's a rich vocabulary.
Speaker 1: This is pretty intricate. But what happens if a process
Speaker 1: is in the middle of something important and it doesn't
Speaker 1: want to be interrupted by signals?
Speaker 2: Ah, good question. Linux has a solution for that too, okay,
Speaker 2: I was hoping. So signal masking.
Speaker 1: Signal masking, you.
Speaker 2: Can temporarily block specific signals from being delivered, okay, essentially
Speaker 2: putting them on hold.
Speaker 1: So it's like setting your phone to do not disturb mode.
Speaker 2: Exactly when you need to focus.
Speaker 1: Okay.
Speaker 2: The sigproc mask system call okay is your tool for
Speaker 2: controlling which signals get through and which ones are blocked.
Speaker 1: I like it. We have the power. You have the power,
Speaker 1: so we can block signals. But how do we retrieve
Speaker 1: those pending signals once we're ready to deal with them.
Speaker 2: There are a couple of options, okay. One is to
Speaker 2: use the SIG suspend system.
Speaker 1: Call sixty cent.
Speaker 2: This allows you to temporarily unblock a set of signals
Speaker 2: while simultaneously suspending the process waiting for one of those
Speaker 2: signals to arrive.
Speaker 1: So it's like saying, wake me up when one of
Speaker 1: these specific messages.
Speaker 2: Arrives, precisely. Another option is to use sigpending okay, to
Speaker 2: peak at the queue of pending signals without actually unblocking them.
Speaker 2: It's like checking your voicemail without listening to any messages.
Speaker 1: Okay, so just seeing if anything's there exactly. This whole
Speaker 1: signal handle system is surprisingly sophisticated.
Speaker 2: Is there's a lot of thought that goes into it.
Speaker 1: I never realized how much thought goes into managing these
Speaker 1: interprocess messages.
Speaker 2: Yeah, it's a crucial part of making sure that everything
Speaker 2: runs smoothly and that processes can cooperate effectively.
Speaker 1: Wow, this deep dive has been packed with information.
Speaker 2: It has. We've covered a lot of ground, from.
Speaker 1: Those fundamental concepts of files, processes, and signals to those
Speaker 1: advanced techniques we've just touched upon.
Speaker 2: It's been quite a journey.
Speaker 1: It feels like we've uncovered a whole hidden layer of
Speaker 1: the operating system.
Speaker 2: And that's the beauty of Linux system programming. There's always
Speaker 2: something new to discover, something deeper to explore. It's a
Speaker 2: journey that never really ends.
Speaker 1: I'm definitely feeling that sense of wonder. This deep dive
Speaker 1: has not only deepened my understanding of Linux, but has
Speaker 1: also sparked a real desire to keep learning and experimenting.
Speaker 2: That's the best outcome we could hope for, and to
Speaker 2: our listeners, we encourage you to continue your own explorations
Speaker 2: into the fascinating world of Linux system programming. There's a
Speaker 2: universe of no waiting to be discovered, and who knows
Speaker 2: what amazing things you might create.
Speaker 1: Well said, Until next time, Happy coding everyone,