What's the difference between CLongDouble and Double

Using Swift
#1

What's the difference between CLongDouble and Double?
According to Apple document, CLongDouble is an alias of Double. However, when I run the following code, I get different max number results.

print(Double.greatestFiniteMagnitude)        // 1.7976931348623157e+308
print(CLongDouble.greatestFiniteMagnitude).  // 1.189731495357231765e+4932

Are there really the same type? Also, which type is more precise?

#2

That could be a documentation problem. CLongDouble is defined in CTypes.swift as Double or as Float80, depending on the platform.

In a macOS command line project I get this:

print(CLongDouble.self)               // Float80
print(MemoryLayout<CLongDouble>.size) // 16
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#3

Thank you very much. Hope the document will be fixed soon

#4

Right. CLongDouble should be whatever type long double is in clang on the target platform. In particular, when targeting x86_64 on Linux or macOS, it is Float80, but when targeting arm64 on macOS, it is Double. The documentation site that you linked to doesn't handle per-arch differences in definitions or availability (see also CFloat16, which doesn't exist on macOS/x86_64), but this is documented (somewhat tersely) in the source.

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#5

Just for the record, I filed a documentation bug about this (r. 92595826).

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Quinn “The Eskimo!” @ DTS @ Apple

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