Flexible Array Members | C Programming Tutorial

[@gpythona]

professor, thank you very much for all these precious tutorials, in addition to the fact that you are a great lecturer, this and many more corner cases which are not covered very much on the internet what separates you from the whole internet. I appreciate each and every video you've published, they're all just great. Thank you for everything, greetings from Turkey 🇹🇷

[@PortfolioCourses]

You're very welcome and thank you for this kind feedback! :-) I really have fun covering topics like this that are more niche and aren't often covered by other sources online.

[@starc0w]

Very good tutorial! I love your stil! Thank you so much! One thing I have missed: The flexible array member does not need any space itself. So far so good. However, more space can be incurred by the alignment. struct data { char c; int array[]; }; Sizeof struct data now results in 4 bytes and not 1 byte, which one might have expected. This, although the flexible array member does not need any space. But since the elements of the flexible array member must always come directly to the end of the structure and these in turn must be aligned according to their types, the compiler inserts padding within the structure depending on the situation (alignment of int is "normally" a multiple of 4). The following structure has now a sizeof of 16 bytes (alignment of long long is "normally" a multiple of 8): struct data { char elements[9]; long long array[]; }; Maybe a topic for another video (This and the topic of alignment in general)? Cheers

[@PortfolioCourses]

Wow thanks for the 'super thanks'! :-) And yes structure padding can come into play with structs, this video covers the concept: kzbin.info/www/bejne/n3WnZ6Shbaagotk. Maybe one day I can do a video on structure padding and flexible array members specifically. :-)

[@nuligebla1173]

typedef struct { int id; int salary; char name[]; char nickname[]; //error: flexible array member not at end of struct } Employee; typedef struct { int id; int salary; char name[50]; char nickname[]; //works! no warnings or issues! } Employee; printf("sizeof (struct employee): %zu ", sizeof(Employee)); // sizeof (struct employee): 60 // 4 + 4 + 50 + ? // (2 extra bytes: offset? not a pointer, that would be 4...) just an experiment... great lesson. thanks!

[@PortfolioCourses]

Nice experimenting! :-) I suspect the extra two bytes is structure padding: kzbin.info/www/bejne/n3WnZ6Shbaagotk

[@nuligebla1173]

that makes sense, thanks! these videos are really helping!

[@PortfolioCourses]

@@nuligebla1173 You're welcome, glad to hear the videos are helping! 🙂

[@ple9652]

Thank you so much! You just saved me hours of debugging!

[@logos_42]

Thanks a lot. Your exercises are really valuable for a coding student like me🙏

[@PortfolioCourses]

You’re welcome David, I’m glad you’re finding the exercises valuable! :-)

[@Algebraiic]

unrelated but currently learning RV32I and i couldn’t help to imagine how goated portfolio courses would be at teaching the logic of it 🔥

[@PortfolioCourses]

Hmmm, I don't have any plans to cover that topic, but you never know, maybe one day. :-)

[@fifaham]

It would be nice to have the compiler assume a default size for the "name" if placed as first element in the struct. Later it will change the addresses accordingly after determaining the desired size. I wonder if this is something the compiler designers have thought of! The issue is as a programmers at times we get so many bugs errors (LOL) that need to be fixed, the less the errors the more we can focus on develpment. Great presentation. Thank you Kevin.

[@PortfolioCourses]

I'm glad you enjoyed it, and you're welcome! :-)

[@JamBadguy]

fantastic vid, subbed and liked, was super helpful for understanding :)

[@togosakutaro5882]

Great 2 hear from u again. Are u feeling better now?

[@PortfolioCourses]

Yes, I'm feeling much better now and hoping to get back to regularly posting videos. :-)

[@yogeshchauhan9401]

Will that flexible member follow structure padding ?

[@PortfolioCourses]

That's a good question! Ultimately the compiler has some say in how structure padding will work. And I'm not 100% sure myself. But my understanding is that the structure padding will be done *before* the flexible array member, and then after that, we just have an array accessed via struct_variable->array[3], etc. This article seems to confirm that understanding... "the flexible array member will not participate in the trailing padding of the structure": sites.google.com/site/embeddedmonologue/home/c-programming/data-alignment-structure-padding-and-flexible-array-member

[@dimitrioskalfakis]

interesting and strange feature. since one uses malloc anyway why not define *name instead of name[] and proceed with pointer allocation as usual?

[@PortfolioCourses]

That’s a really good question! :-) We could do that too, but the block of memory allocated for name could be “somewhere else” in memory and name would be a pointer to that block of memory. So there would be a block of memory for the struct and another one for the array. More blocks of memory could be worse for heap fragmentation: chortle.ccsu.edu/assemblytutorial/chapter-33/ass33_3.html. Also if the struct is dynamically allocated we would then have to use free() twice, once for the struct and once for the array.

[@awekeningbro1207]

so we need to somehow hardcode the length of the data before the compilation? how is this method any useful?

[@PortfolioCourses]

No, that could come from user input. We’re using dynamic memory allocation so we can make the length whatever we like at execution time.

[@starc0w]

10:36 Obviously you are creating the struct on the stack here. Is there any case where it makes sense to create a struct with a flexible array member on the stack? If this struct is on the stack, any access to the flexible array is UB, because it is not clear what follows behind this struct on the stack. Or am I perhaps missing something? Shouldn't it be consequently impossible to create a struct with a flexible array member on the stack?

[@PortfolioCourses]

I’m doing that for the reason stated in the video, to demonstrate that we cannot statically initialize flexible array members. If it wasn’t on the stack it couldn’t be statically initialized because we cannot statically initialize things on the heap. It’s something that is specifically not allowed in C so I’m demonstrating a specific example of that.

[@starc0w]

@@PortfolioCourses Ok great, thanks for the explanation! Would you also advise against creating such a struct on the stack in any case? Or am I missing a scenario where this could make sense?

[@PortfolioCourses]

If it is a struct that contains different members that are useful in different related contexts someone might declare one on the stack and use it in contexts where the flexible array member isn’t used. I’m not sure if a specific situation off the top of my head, maybe the struct stores “results” in an array along with statuses, someone might make a struct on the stack just for storing statuses maybe for the sake of outputting a report or something. As long as they don’t try to initiate the flexible array member, they could still use the struct on the stack and the other members.

[@starc0w]

@@PortfolioCourses Ok, thank you very much!

[@keithg8875]

'promo sm'

[@PortfolioCourses]

I don't get it? :-)