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The code: const size_t buffer_size = 5; int buffer[buffer_size]; compiles fine in c, but not for the same reason. C99/C11 has dynamic arrays https://en.wikip

The code: const size_t buffer_size = 5; int buffer[buffer_size]; compiles fine in c, but not for the same reason. C99/C11 has dynamic arrays https://en.wikipedia.org/wiki/Variable-length_array#C99 https://en.wikipedia.org/wiki/Variable-length_array#C99

lochsh 7y ago
Yes, I think I made an embarrassing mess up here by using a compiler that doesn't support VLAs, and not fully understanding VLAs. Thanks for pointing it out.
eMSF 7y ago
To be exact, it doesn't compile fine outside of functions (which the sample code didn't have) because file scope arrays can't be VLAs.
- lochsh 7y ago
  This is what I was going for in the blog post, but I got confused after seeing these comments as Clang _does_ compile this when the buffer is of static storage class. Which I don't think is standard -- but it's not using VLAs. I wondered if it just has constant expression semantics for const variables. Weirdly, adding a _Static_assert to test this theory proves it for c99 but not c11 :/ https://godbolt.org/z/q-bb-n https://godbolt.org/z/q-bb-n c99 with clang https://godbolt.org/z/ad14Ah https://godbolt.org/z/ad14Ah c11 with clang https://godbolt.org/z/xJSDQa https://godbolt.org/z/xJSDQa c11 with gcc (which is the only one giving the output I'd expect)
  - eMSF 7y ago
    >I wondered if it just has constant expression semantics for const variables. That would be my guess also, for applicable const variables. File scope const variables are quite constexpr-y in C anyway, since C requires all file scope variable initializers to be constant expressions (C++ only requires that for constexpr variables). Toying around with clang in Godbolt, it seems that there are some quirks regarding this. The following is accepted: static const int n = 0; int buf[n]; // invalid zero-length array is accepted // probably another non-standard extension But the following is not, despite n having the same zero value: static const int n; int buf[n]; // complains about a file scope VLA
  - _kst_ 7y ago
    The C standard permits implementations to support additional forms of constant expressions. Try "clang -std=c11 -pedantic-errors".

lochsh 7y ago

Yes, I think I made an embarrassing mess up here by using a compiler that doesn't support VLAs, and not fully understanding VLAs. Thanks for pointing it out.

eMSF 7y ago

To be exact, it doesn't compile fine outside of functions (which the sample code didn't have) because file scope arrays can't be VLAs.

lochsh 7y ago
This is what I was going for in the blog post, but I got confused after seeing these comments as Clang _does_ compile this when the buffer is of static storage class. Which I don't think is standard -- but it's not using VLAs. I wondered if it just has constant expression semantics for const variables. Weirdly, adding a _Static_assert to test this theory proves it for c99 but not c11 :/ https://godbolt.org/z/q-bb-n https://godbolt.org/z/q-bb-n c99 with clang https://godbolt.org/z/ad14Ah https://godbolt.org/z/ad14Ah c11 with clang https://godbolt.org/z/xJSDQa https://godbolt.org/z/xJSDQa c11 with gcc (which is the only one giving the output I'd expect)
- eMSF 7y ago
  >I wondered if it just has constant expression semantics for const variables. That would be my guess also, for applicable const variables. File scope const variables are quite constexpr-y in C anyway, since C requires all file scope variable initializers to be constant expressions (C++ only requires that for constexpr variables). Toying around with clang in Godbolt, it seems that there are some quirks regarding this. The following is accepted: static const int n = 0; int buf[n]; // invalid zero-length array is accepted // probably another non-standard extension But the following is not, despite n having the same zero value: static const int n; int buf[n]; // complains about a file scope VLA
- _kst_ 7y ago
  The C standard permits implementations to support additional forms of constant expressions. Try "clang -std=c11 -pedantic-errors".