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Passing same buffer as two method arguments does not read back modified content

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
java
Domain
tooling

Research direction

Start with the JNA call path for the do_something(ByteBuffer out, ByteBuffer in) entry point and the handling of heap-backed ByteBuffer arguments. Reproduce the call with the same buffer passed for both parameters on JNA 5.5.0, then verify that the modified output remains visible after the native function returns.

Written by the indexing model from the issue text.

Description

feature request

Assume the following native library function, which reads from in and writes to out:

int do_something(void *out, void *in);

mapped to the following interface in Jave:

public interface mylibrary extends Library {
	public int do_something(ByteBuffer out, ByteBuffer in);
}

When calling this function with the same buffer backed by a heap-allocated array as both output and input argument, i.e.:

public void process(ByteBuf buffer) {
    // buffer is backed by heap-allocated array
    mylibraryInstance.do_something(buffer, buffer);
}

then the buffer will not be updated to reflect the content that was written to out within the native function implementation.

I assume this is because each argument will be processed individually, the buffer will be copied two times into native memory, both instances will be read back after the function call and the second argument (in, unmodified) replaces the first argument (out, modified & expected content).

Workaround: Create a new buffer before calling the native function (and write it back afterwards):

public void process(ByteBuf buffer) {
    ByteBuffer outputBuffer = ByteBuffer.allocate(buffer.remaining());
    outputBuffer.limit(buffer.remaining());
    mylibraryInstance.do_something(outputBuffer, buffer);
    // here, outputBuffer has the expected (modified) content - write back to original buffer
}

Version of JNA and related jars: jna 5.5.0 and jna-platform 5.5.0
Version and vendor of the java virtual machine: Oracle JDK 12.0.2

Dominant language
Java
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