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I owe my career to Beej. I was getting burnt out of web development and his guides along with Jesse Storimer's books made programming fun for me again. I highly recommend learning how Unix systems work.
It's a lot of fun and opens up a whole new world of programming! I'm feeling the same way about web, I'd like to transition to something similar.
I quite enjoy programming in C, so hopefully that helps. What are you doing at your job now? How did you go about making the switch from web to systems? Has it paid off with an increase in enjoyment? Truth be told, a significant portion of my day job is still web application stuff. Only the lucky few get to hack on databases, hypervisors and file systems for a living! But, armed with the knowledge of sockets, processes, etc. But even before I was able to work the knowledge into my day job, nothing beats the dopamine rush of learning things that fascinate you!
The rabbit holes that learning the basics of Unix have opened feel like they could occupy my hobbyist hours for the rest of my life. Its amazing how we think the grass is greener. I have been doing systems programming for the past 7 years and have been thinking about moving over to JS based web development for the past few months.
Actually both of you are right. You need a break from what you have been doing from a long time. Perhaps exchanging your jobs with one another might help. I think we humans just need change every now and then. Even when the problems and tools we use to solve those problems never change, we still somehow get upset with the tools or the problems.
Thank you for your efforts, Brian! Something I've struggled to implement on Linux is cross-process multicast notifications, where any process can post and multiple subscribed processes receive it. On OS X there's notify 3 which is very nice. Any good options on Linux, other than writing my own socket server? The solution for this kind of problem will depend heavily on what type of messages you're trying to send. When messaging becomes that complex, there are often other things that impact the overall design in important ways that need need to be considered.
For example, if I was implementing something that is usually associated with user events rare-ish, basically zero bandwidth, complex signal with stateful messaging semantics , I would probably just write a simple server to manage it all.
This has an advantage of centralizing any messaging complexity and lets you manage any multi-message state easily. Rebroadcasting messages to allow peer-to-peer messaging would be a trivial addition.
For something that requires very low latency e. Here, the need for low latency dictated the design. Of course, this means managing locks. Not fun, but a cost that is sometimes worth paying.
There really isn't a one-size-fits all solution. I keep forgetting that we now have it has an option. Thanks for the reply. My use case is very simple: a stateless "something happened" notification, which can be delivered asynchronously.
Coalescing or even occasional drops are fine. I did originally use a Unix domain socket server, but that added a lot of complexity: one has to arrange for it to be launched, guard against the possibility that it gets stuck, version it, deal with permissions, etc. My new solution on Linux is a total hack: there's a FIFO, and to post a notification, you write to it. Clients see that the FIFO became readable, and that change represents the notification. The sender then drains the data it wrote, so that the FIFO becomes unreadable again.
This is a total abuse of FIFOs, but it's proven to be much simpler than trying to manage a separate server. I've never heard of TIPC. From a little searching it looks like it's very capable but geared towards clusters, and is overkill for my use case. What do you think? Beware: I tried that once, and it was unreliable. Only some clients woke up. Ha ha ha, that's great! If we are sent to programmer purgatory, at least we'll have each other. If drops are fine, TIPC is probably overkill.
I would probably just wrap something generic using UNIX domain sockets up into a library and re-use that as needed.
Depending on your permissions requirements, and if you really only need a signaling flag, have you considered the filesystem? Just touch a file in a well-defined directory named after the event that happened, and poll it periodically.
Removing the file clears the flag. Signals can coalesce, but you should never drop any. You can poll a directory that will normally be empty without much CPU load the directory inode will be cached most of the time.
You could setup a multiple listeners by giving them their own "inbox" directory, like. Your signals also persist across programs shutdowns and crashes - you can send and receive even when the other side isn't running - and your state can persist across reboots.
Also, you can leverage some of the guarantees provided by the kenel's vfs layer. For example, rename 2 is atomic, so you can send small data payloads by writing to a different name first.
Those events can be received either in a simple blocking style by letting poll 2 wake-up your process. The man oage inotify 7 should have an example. Making a group specific to the message sending can help. Thank you for this thoughtful reply.
There's a variety of options if I'm willing to poll, including shared memory or the filesystem idea you outline, but I hope to avoid polling for hygienic reasons.
That's why I like inotify in blocking mode - the call to poll is just to wake up the process I think you could just blocking-read the inotify file handle? I haven't tried it directly. The point of using inotify is that you don't need to poll, because the kernel send your process reliable events instead over a file handle.
The use of poll 2 is just a consequence of the interface using a file handle. As, I originally said, though, there is certainly no one-size-fits-all solution, these are just a few of the available options, which may not be apropriate for your situation.
I like blocking inotify in principle - the problem is that it just didn't work! I think there is a gap in the Linux APIs in this area. Its multicast IPC mechanisms are just too heavyweight.
You can do it with shared memory. One writer writes and multiple readers can observe. I did this for both low latency and throughput reasons. In general you have to be very careful how you handle it and consider various consistency and failures scenarios.
You'd probably have to declare this using the 'volatile' keyword. Otherwise compilers will optimize away access to part of this seemingly unused variables. Data gets written to buffer.
Also, note these counters also function as total counts of items written and so each reader cand determine how far ahead the writer is. Another note: depending on the sizes of your counter it will not necessarily be updating atomically. Compiler could separate the update as multiple instructions and say, increment the lower part of the value, then the upper part.
Writer could get pre-empted between those two instructions, so you could get this strange torn value. But, you might be reading data you didn't expect. Whether that works for your use case or not you'll have to see. EDIT: Don't also forget about the slow and stupid multicast mechanism -- writing to a file. Some file operations can be atomics renaming a file. And some operating systems let you watch the files for changes. Classic signals are tricky Specifically, dbus signals without a destination are routed to all connections with match rules added with org.
AddMatch on the message bus which match the signal. I'll add to the chorus of praise for Beej's work. I've often consulted this particular guide and the companion piece for networks as well. Helped me write web and other servers, a great way to learn about important technologies, providing knowledge that stays useful even if the production server runs on Node. Besides, I'm biased.
She's a remarkably smart person, so I was predisposed to think "BeeJ" would know what he's talking about and it turns out he did.
Beej's Guide to Unix Interprocess Communication