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Giving C a superpower: custom header file (safe_c.h)

(hwisnu.bearblog.dev)
271 points mithcs | 2 comments | 17 Nov 25 10:40 UTC | HN request time: 0.018s | source
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woodruffw ◴[17 Nov 25 13:30 UTC] No.45953391[source]▶
Intentionally or not, this post demonstrates one of the things that makes safer abstractions in C less desirable: the shared pointer implementation uses a POSIX mutex, which means it’s (1) not cross platform, and (2) pays the mutex overhead even in provably single-threaded contexts. In other words, it’s not a zero-cost abstraction.

C++’s shared pointer has the same problem; Rust avoids it by having two types (Rc and Arc) that the developer can select from (and which the compiler will prevent you from using unsafely).

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accelbred ◴[17 Nov 25 16:49 UTC] No.45955366[source]▶
Unfortunately, for C++, thats not true. At least with glibc and libstdc++, if you do not link with pthreads, then shared pointers are not thread-safe. At runtime it will do a symbol lookup for a pthreads symbol, and based off the result, the shared pointer code will either take the atomic or non-atomic path.

I'd much rather it didnt try to be zero-cost and it always used atomics...

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TuxSH ◴[17 Nov 25 18:42 UTC] No.45956553[source]▶
True, but that's a fault of the implementation, which assumes POSIX is the only thing in town & makes questionable optimization choices, rather that of the language itself

(for reference, the person above is referring to what's described here: https://snf.github.io/2019/02/13/shared-ptr-optimization/)

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1. wyldfire ◴[17 Nov 25 19:46 UTC] No.45957423[source]▶
> the language itself

The "language" is conventionally thought of as the sum of the effects given by the { compiler + runtime libraries }. The "language" often specifies features that are implemented exclusively in target libraries, for example. You're correct to say that they're not "language features" but the two domains share a single label like "C++20" / "C11" - so unless you're designing the toolchain it's not as significant a difference.

We're down to ~three compilers: gcc, clang, MSVC and three corresponding C++ libraries.

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2. TuxSH ◴[19 Nov 25 11:02 UTC] No.45978142[source]▶
I agree with what you said, however neither libc++ nor MS-STL have this "optimization" to my knowledge