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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

#311

Post by loeg »

cesarb wrote: post #134
nicoburns wrote: post #81

Are there any plans to "clean up C"? A lot of effort has been put into alternative languages, which are great, but there is still a lot of momentum with C, and it seems that a lot of improvements that could be done in a backwards compatible way and without introducing much in the way of complexity. For example: - Locking down some categories of "undefined behaviour" to be "implementation defined" instead. - Proper ar…

> Proper array support (which passes around the length along with the data pointer). I second this one. One of the best things from Rust is its "fat pointers", which combine a (pointer, length) or a (pointer, vtable) pair as a single unit. When you pass an array or string slice to a function, under the covers the Rust compiler passes a pair of arguments, but to the programmer they act as if they were a single thing (…

Fat pointers in C would involve an ABI break for existing code, in that uintptr_t and uintmax_t would probably need to double in size.

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

#312

Post by toasted_flakes »

AvImd wrote: post #165
klodolph wrote: post #136

Earlier quoted context omitted.

This can only be done at compile time in very specific cases. The huge problem here is the compiler has no way of knowing which cases of undefined behavior are bugs in the program and which cases of undefined behavior are just examples of unreachable code. If the compiler aborted compilation when it detected undefined behavior, you’d be getting a lot of false positives for unreachable code, and you’d need to solve th…

> If the compiler aborted compilation when it detected undefined behavior, you’d be getting a lot of false positives for unreachable code Could you please provide an example of this?

Overflow of signed integers is undefined.

    int add(int a, int b) { return a + b; }
Unless the compiler can prove that `add` is never called with a and b values resulting in an overflow, this code can lead to UB, and, under your rules, the compilation aborts.

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

#313

Post by kzrdude »

klodolph wrote: post #136
AvImd wrote: post #95

Is there a possibility there will be introduced a new rule saying "if the compiler detects an UB it should abort the compilation instead of breaking the code in the most incomprehensible way possible"? Right now it's just scary to start a new project in C. It would be really great if there was more emphasis on correctness of the produced code instead of the insane optimizations.

This can only be done at compile time in very specific cases. The huge problem here is the compiler has no way of knowing which cases of undefined behavior are bugs in the program and which cases of undefined behavior are just examples of unreachable code. If the compiler aborted compilation when it detected undefined behavior, you’d be getting a lot of false positives for unreachable code, and you’d need to solve th…

Good point, but could it not be required that the unreachable code would be annotated to be unreachable? It could even have a (development only) assertion in the location.

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

#315

Post by emreiyican »

I know this opinion is unpopular and contradict with a core value of the C standardization committee but I personally think at some point, C standard should abandon supporting the legacy codebase. I think bool and stdint definitions should be available as part of the standard feature set and shouldn't need including their respective headers. These and some other features are available at the core of every modern language but C, and C has to provide them via other means. Is the sentiment of discontinuing legacy support shared within the committee, by any proportion?

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

#316

Post by rseacord »

DougGwyn wrote: post #191
pcr910303 wrote: post #148

As there are a lot of C-masters lurking in this thread: How can one process unicode (UTF-8) properly in C? As a CJK person, I wish there was a robust solution. Are there any standardized ways or proposals? (Using wchar doesn't count.)

UTF-8 encoding works "as is" based on byte strings (char[]). The latest versions of the draft standard provide somewhat more support. I recommend heading toward a future where only UTF-8 encoding is used for multibyte characters and UCS-2 or similar for wchar_t. There is no need to support several different encodings.

Aaron Ballman even got a u8 character prefix added to C2x:

N2198 2018/01/02 Ballman, Adding the u8 character prefix

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2198.pdf

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

#317

Post by hsivonen »

emilfihlman wrote: post #254
Tronic2 wrote: post #113

char effectively behaves as a signed type, making it unsuitable for binary operations (e.g. UTF-8 manipulation). I/O functions deal with char pointers, so using unsigned type like uint8_t requires casting back and forth. Is there any way out of this problem, and am I already breaking the aliasing rules with that cast?

There are no aliasing differences between uint8_t and char as far as I know.

In practice not. In theory, it’s implementation-defined whether yhere are differences.

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

#318

Post by ancarda »

As a C newbie, will there ever be "safe" C, i.e. no undefined behavior and help with writing code that has less memory related crashes/bugs? For comparison, Rust has the `unsafe { }' block which lets you mark regions of code as being able to do funky stuff. Could we get the opposite for C, i.e. `safe { }' and for an entire file, `#pragma safe'?

I have a love-hate relationship with C - I like it for small projects, but anything serious I really need to write it in a more safe language. I think GCC has some flags that can help, and I've been using tools like splint, but something baked into the standard would be amazing.

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

#319

Post by JoeAltmaier »

rbultje wrote: post #300
cataphract wrote: post #109

Earlier quoted context omitted.

Signed overflow being undefined behavior allows optimizations that wouldn't otherwise be possible Quoting http://blog.llvm.org/2011/05/what-every-c-programmer-should-... > This behavior enables certain classes of optimizations that are important for some code. For example, knowing that INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". Knowing the multiplication "cannot" overflow (because doing so would be…

> for (i = 0; i The worst thing is that people take it as acceptable that this loop is going to operate differently upon overflow (e.g. assume N is TYPE_MAX) depending on whether i or N are signed vs. unsigned.

Is this a real concern, beyond 'experts panel' esoteric discussion? Do folks really put a number into an int, that is sometimes going to need to be exactly TYPE_MAX but no larger?

I've gone a lifetime programming, and this kind of stuff never, ever matters one iota.

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