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Tell HN: C Experts Panel – Ask us anything about C

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Re: Tell HN: C Experts Panel – Ask us anything about C

#641

Post by pascal_cuoq »

ori_b wrote: post #491

What do you think of a variant on this? https://blog.regehr.org/archives/1180

I still want to write at least one sequel to that post, on the theme “Alright, can we make a Friendly C Compiler by disabling the annoying optimizations, then?”.

Obviously the people who want a Friendly C Compiler do not want to disable all optimizations. This would be easy to do, but these users do not want the stupid 1+2+16 expressions in their C programs, generated through macro-expansion, to be compiled to two additions with each intermediate result making a round-trip through memory.

So the question is: can we get a Friendly C Compiler by enabling only the Friendly optimizations in an unfriendly compiler?

And for the answer to that, I had to write an entire other blog post as preparation, to show that there are some assumptions an optimizing compiler can do:

- that may be used in one or several optimizations, but the compiler authors did not really keep track of where they were used,

- that cannot be disabled and that the compiler maintainers will not consider having an option to disable,

- and that are definitely unfriendly.

Here is the URL of the blog post that I had to write in preparation for the upcoming blog post about getting ourselves a Friendly C Compiler: https://trust-in-soft.com/blog/2020/04/06/gcc-always-assumes... . I recommend you take a look, I think it is interesting in itself.

You will have guessed that I'm not optimistic about the approach. We can try to maintain a list of friendly optimizations for ourselves, though, even if the compiler developers are not helping. This might still be less work that maintaining a C compiler.

Re: Tell HN: C Experts Panel – Ask us anything about C

#642

Post by DougGwyn »

graycat wrote: post #284

(1) Explain just how malloc() and free() work under the covers and the implications for multi-threading, memory leaks , virtual memory paging, etc. Maybe also cover some means, algorithms, and code for reporting on the state , status, etc. of the memory use by malloc() and free(). By the way, I know and have known well for longer than most C programmers have lived JUST what the heap data structure, as used in "heap s…

(1) There are several implementations; most are based on Knuth's "boundary tag" algorithms. As to "heap", a stack has one accessible end, a heap is essentially random-accessible. Nothing to do with the heap data structure. (2) Stack overflow can occur even early within a program. I've campaigned for a requirement that such overflows be caught and integrated into a standard exception handler, to no avail. (3) Why not code your own, so there won't be arguments about it. (4) There are lots of tools for program development, but it's not standardized by WG14. (5) Use wider integer types. (6) Use wider floating representations. (7) Standard C doesn't specify such a facility, but it has occasionally be suggested. (8) There were a lot of books, e.g. on structured system analysis, during the 1970s trying to apply lessons learned. C isn't special in that regard, as many of the big problems don't involve syntax. (9) C++ is now a big language and it takes a lot of work to master its internals.

Re: Tell HN: C Experts Panel – Ask us anything about C

#643

Post by AaronBallman »

pascal_cuoq wrote: post #438
AaronBallman wrote: post #404

Earlier quoted context omitted.

I think that may be inaccurate -- IIRC, in C, you can do type punning via a union but not memcpy, and in C++ you can do type punning via memcpy but not a union and this incompatibility drives me nuts because it makes inline functions in a header file shared between C and C++ really messy. (Moral of the story: don't pun types.)

The C standard also allows to use memcpy to do type punning: If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one Simply memcpy into a variabl…

I must be remembering incorrectly then, thank you!

Re: Tell HN: C Experts Panel – Ask us anything about C

#644

Post by clarry »

quelsolaar wrote: post #627
clarry wrote: post #616

Earlier quoted context omitted.

> Why not mandate a warning every time the compiler detects and makes use of UB? It would solve SO many issues. Because that's hardly ever what happens, except when it actually does, and compilers do an increasingly good job of issuing diagnostics in that case. If you actually mandated it, no compiler today would come close to being standards compliant. This comes close to making the language unimplementable. The mos…

They need to do better then remove NULL checks silently. You can read all about Linus rants on this. Every time the compiler breaks things they blame the C standard for letting them do what ever. Thats whats wrong with C today. The C standard hasn't put its foot down.

I want my compiler to remove redundant checks (without any noise), and that is why I pass it an optimization flag. If you don't want such optimizations, then maybe you should not ask the compiler to make them.

Re: Tell HN: C Experts Panel – Ask us anything about C

#645

Post by pascal_cuoq »

dang wrote: post #532
ori_b wrote: post #491

What do you think of a variant on this? https://blog.regehr.org/archives/1180

pascal_cuoq cowrote it. Maybe we should ask him if his views have changed since then. Btw, there was a thread about it at the time: https://news.ycombinator.com/item?id=8233484 .

Thanks Dan, I missed this question in the heat of the moment.

Re: Tell HN: C Experts Panel – Ask us anything about C

#646

Post by quelsolaar »

jmckinley wrote: post #366

It is 2020. You are looking at a series of projects your company has teed up. All are greenfield efforts - no legacy. What would be the attributes of a project that would have you recommend C as the programming language?

Anything high performance: game engine, scientific computation, deep packet inspection, image analysis, machine learning, rendering engines, high frequency trading.... The list is long!

Re: Tell HN: C Experts Panel – Ask us anything about C

#647

Post by ATsch »

kkdwivedi wrote: post #620
ATsch wrote: post #409

Earlier quoted context omitted.

typedef struct {uint8_t *data; size_t len;} ByteBuf; is the first line of code I write in a C project.

Another option is a struct with a FAM at the end. typedef struct { size_t len; uint8_t data[]; } ByteBuf; Then, allocation becomes ByteBuf *b = malloc(sizeof(*b) + sizeof(uint8_t) * array_size); b->len = array_size; and data is no longer a pointer.

Well, your ByteBuf is still a pointer. You also now need to dereference it to get the length. It also can't be passed by value, since it's very big. You can also not have multiple ByteBufs pointing at subsections of the same region of memory.

Thing is, you rarely want to share just a buffer anyway. You probably have additional state, locks, etc. So what I do is embed my ByteBuf directly into another structure, which then owns it completely:

    typedef struct {
        ...
        ByteBuf mybuffer;
        ...
    } SomeThing;
So we end up with the same amount of pointers (1), but with some unique advantages.

Re: Tell HN: C Experts Panel – Ask us anything about C

#648

Post by begriffs »

AaronBallman wrote: post #638
begriffs wrote: post #626

What does the presence or absence of __STDC_ISO_10646__ indicate exactly? I found this part of the C99 spec obscure. For instance, the macOS clang environment does not define this symbol. Is their implementation of wchar_t or lacking some aspect of Unicode support?

If that macro is defined, then wchar_t is able to represent every character from the Unicode required character set with the same value as the short code for that character. Which version of Unicode is supported is determined by the date value the macro expands to. Clang defines that macro for some targets (like the Cloud ABI target), but not others. I'm not certain why the macro is not defined for macOS though (it m…

Would the following be a correct way to determine whether there's a problem?

* First call setlocale(LC_CTYPE, "en_US.UTF-8")

* Next feed the UTF-8 string representation of every Unicode codepoint one at a time to mbstowcs() and ensure that the output for each is a wchar_t string of length one

* If all input codepoints numerically match the output wchar_t UTF-32 code units, then the implementation is officially good, and should define __STDC_ISO_10646__?

Re: Tell HN: C Experts Panel – Ask us anything about C

#649

Post by DougGwyn »

quelsolaar wrote: post #619
throw_m239339 wrote: post #201

Earlier quoted context omitted.

> It's a simple language with only a few concepts to learn I mean by that logic, Assembly could be deem even simpler, yet writing OR reading programs in Assembly is absolutely not simple at all. At the end of day, one has to write programs that solve (complicated) problems, and learning how to do that in C is difficult, thus the learning curve deemed higher when it comes to writing professional C. I can guarantee you…

Modern assembly language has a huge set of instructions, that make them hard to learn, but the concept is still easy to learn.

Many antique computers are simulated by SIMH. If you have the corresponding software, you can operate on your desktop a simulated computer's software development system. For example, DEC VAX (VMS or Unix) has a relatively simple and sane assembly language.

Re: Tell HN: C Experts Panel – Ask us anything about C

#650

Post by zokier »

twic wrote: post #608
kazinator wrote: post #424

Earlier quoted context omitted.

The C family has already evolved in this direction decades ago. Have you heard of C++ (Cee Plus Plus)? It is production-ready; if you want a dialect of C with arrays that know their length, you can use C++. If you wanted a dialect of C in 1993 with arrays that know their length for use in a production app you could also have used C++ then. The problem with all these "can we add X to C" is that there is always an impl…

> if you want a dialect of C with arrays that know their length, you can use C++ C++ doesn't have arrays which know their length.

What's std::array then?

> combines the performance and accessibility of a C-style array with the benefits of a standard container, such as knowing its own size

https://en.cppreference.com/w/cpp/container/array

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