[foreign-memaccess+abi] RFR: 8268230: Foreign Linker API & Windows user32/kernel32: String conversion seems broken [v3]

[Jorn Vernee]

On Fri, 9 Jul 2021 07:42:14 GMT, Sebastian Stenzel <github.com+1204330+overheadhunter at openjdk.org> wrote:

> If I call `CLinker.toCString(new String(bytes, UTF-16BE))`, it'll still be converted to UTF-8 internally.

Note that the char set that is passed to the `String` constructor only indicates the encoding of the `bytes` that are being passed. These bytes are then converted to one of the internal encodings used by Java strings to produce the new `String`, and then `toCString` converts from that `String` back to a UTF-8 encoded memory region.

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For encoding to a native representation, here is an example that assumes the native representation of the string should be terminated with 2 null bytes (the exact format the native string should have depends ultimately on the domain, and what the library expects):


String input = foo(); // get a Java string from somewhere
// Using a standard null terminated format here, but the library could also expect a different format
byte[] bytes = (input + '\0').getBytes(UTF_16BE);
MemorySegment text = MemorySegment.allocateNative(bytes.length);
text.copyFrom(MemorySegment.ofArray(bytes));
// use 'text'


For converting from a native string to a Java string a way to determine the length of a string is needed. Examples are `strlen` and `wcslen`, but it depends on the encoding the string is in, and again the domain (e.g. the library might expose a function to determine the length of the strings it returns).

Then something like this could be used:


MemoryAddress input = foo(); // get a native string from somewhere
int length = utf16be_string_length(input);
byte[] bytes = new byte[length];
MemorySegment inputSegment = input.asSegment(length, ResourceScope.newImplicitScope());
MemorySegment.ofArray(bytes).copyFrom(inputSegment);
String text = new String(bytes, UTF_16BE);
// use 'text'

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PR: https://git.openjdk.java.net/panama-foreign/pull/554