What's the difference between CLongDouble and Double

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?

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

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.


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

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Quinn “The Eskimo!” @ DTS @ Apple