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Search - "memory alignment"
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I found something really interesting depicted in the image.
It is because of memory alignment / padding.
I could explain it to you guys, but this url would do a far better job:
https://kalapos.net/Blog/ShowPost/...8 -
Help.
I'm a hardware guy. If I do software, it's bare-metal (almost always). I need to fully understand my build system and tweak it exactly to my needs. I'm the sorta guy that needs memory alignment and bitwise operations on a daily basis. I'm always cautious about processor cycles, memory allocation, and power consumption. I think twice if I really need to use a float there and I consider exactly what cost the abstraction layers I build come at.
I had done some web design and development, but that was back in the day when you knew all the workarounds for IE 5-7 by heart and when people were disappointed there wasn't going to be a XHTML 2.0. I didn't build anything large until recently.
Since that time, a lot has happened. Web development has evolved in a way I didn't really fancy, to say the least. Client-side rendering for everything the server could easily do? Of course. Wasting precious energy on mobile devices because it works well enough? Naturally. Solving the simplest problems with a gigantic mess of dependencies you don't even bother to inspect? Well, how else are you going to handle all your sensitive data?
I was going to compare this to the Arduino culture of using modules you don't understand in code you don't understand. But then again, you don't see consumer products or customer-specific electronics powered by an Arduino (at least not that I'm aware of).
I'm just not fit for that shooting-drills-at-walls methodology for getting holes. I'm not against neither easy nor pretty-to-look-at solutions, but it just comes across as wasteful for me nowadays.
So, after my hiatus from web development, I've now been in a sort of internet platform project for a few months. I'm now directly confronted with all that you guys love and hate, frontend frameworks and Node for the backend and whatever. I deliberately didn't voice my opinion when the stack was chosen, because I didn't want to interfere with the modern ways and instead get some experience out of it (and I am).
And now, I'm slowly starting to feel like it was OKAY to work like this.10 -
Interesting bug hunt!
Got called in because a co-team had a strange bug and couldn't make sense of it. After a compiler update, things had stopped working.
They had already hunted down the bug to something equivalent to the screenshot and put a breakpoint on the if-statement. The memory window showed the memory content, and it was indeed 42. However, the debugger would still jump over do_stuff(), both in single step and when setting a breakpoint on the function call. Very unusual, but the rest worked.
Looking closer, I noticed that the pointer's content was an odd number, but was supposed to be of type uint32_t *. So I dug out the controller's manual and looked up the instruction set what it would do with a 32 bit load from an unaligned address: the most braindead thing possible, it would just ignore the lowest two address bits. So the actual load happened from a different address, that's why the comparison failed.
I think the debugger fetched the memory content bytewise because that would work for any kind of data structure with only one code path, that's how it bypassed the alignment issue. Nice pitfall!
Investigating further why the pointer was off, it turned out that it pointed into an underlying array of type char. The offset into the array was correctly divisible by 4, but the beginning had no alignment, and a char array doesn't need one. I checked the mapfiles and indeed, the old compiler had put the array to a 4 byte boundary and the new one didn't.
Sure enough, after giving the array a 4 byte alignment directive, the code worked as intended.8 -
It was my first month as a driver developer fresh out of grad school, at a semiconductor company. There was an elusive memory alignment bug that had eluded the experts on the team. Of course, they had to assign it to me.
I was looking at the code where alignment was being calculated for nested structures case. I saw something funny: "size_t(object)". I asked the expert where it should have been "sizeof(object)". And, sure enough it was!!
I was super-thrilled to have fixed a bug in my first month, which had troubled the team for a long time! ^_^6 -
We often rant about people who think that because we can program we can do everything with computers.
But I have to admit that when I get asked what I do I often only say that I program or do something with computers. I usually don't get more specific because it's so hard to explain to someone who doesn't know anything about the subject that I would have to explain the basics each time. And I'm just to lazy for that.
It's nice when people ask me how it is going at work but I probably won't say anything more than ok or fine because my day was fucked up by a memory alignment bug in the chainloader and I now don't have the patience to explain what these both things are and why they fuck up my day. -
I know this topic is tired and this isn't supposed to be a pure "REEEE SPACES BAD" kinda rant but I still don't understand why people would ever use spaces over tabs for indentation. I'm genuinely curious so please give me your arguments in favor of spaces because I just don't understand
So here's my position:
Tabs are objectively better than spaces in every single way
(I know that IDEs also do some of these for spaces, more on that later)
1. They are typed with one key press
2. They can be removed with one keypress
3. They allow for individually configurable width (some people prefer 2 and some 4 width)
4. They take up less memory (kinda irrelevant, but still)
5. You can properly navigate your code using the arrow keys which is much faster than using the mouse while typing
6. You don't have problems with accidentially having one too much or one too little
7. You don't have problems when copy pasting or moving code around (e.g. refactoring)
8. Code is much easier to select with the mouse, and
9. it's much easier clicking the right spot with the mouse where you want to continue typing, which is often at the start of a line
Apart from specific alignment, where spaces are fine (but which also almost never comes up), I just can't see a single thing where spaces are better at. So much so that most IDEs have to *pretend* that they're tabs when typing and removing them. It's so ironic yet people still defend it and big companies still use them.
I feel like I'm going mad 😨56 -
Hello C++ / C programmers. I've noticed my professor putting the ASCII code of a character into an int instead of just using a char to store it. When he does this he's not doing math or anything with them, so is there any advantage to it? My TA mentioned something about memory alignment, but I'm not experienced enough to know how something being aligned differently in memory would help or hurt a program.5
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Oh god, structure alignement, why you do this... You might be interested if you do C/C++ but haven't tried passing structures as binary to other programs.
Just started working recently with a lib that's only a DLL and a header file that doesn't compile. So using python I was able to use the DLL and redefined all of the structures using ctypes, and the nice thing is: it works.
But I spent the whole afternoon debugging why the data in my structures was incoherent. After much cussing, I figured out that the DLL was compiled with 2 bytes packing...
Packing refers to how structures don't just have all the data placed next to each other in a buffer. Instead, the standard way a compiler will allocate memory for a structure is to ensure that for each field of the structure, the offset between the pointer to the structure and the one to the field in that structure is a multiple of either the size of the field, or the size of the processor's words. That means that typically, you'll find that in a structure containing a char and a long, allocated at pointer p, the double will be starting at p+4 instead of the p+1 you might assume.
With most compilers, on most architectures, you still have the option to force an other alignment for your structures. Well that was the case here, with a single pragma hidden in a sea of ifdefs... Man that took some time to debug...2