I've been trying out the Distributed Code Jam testing tool, and I noticed an
apparent bug when running Java code due to the use of the "char" type in the
API. This program sends the char with value 255 from node 0 to node 1, but node
1 receives the char value 65535:
class Main {
public static void main(String[] args) {
if (message.MyNodeId() == 0) {
char outC = 255;
System.out.printf("Sending %d%n", (int)outC);
message.PutChar(1, outC);
message.Send(1);
} else if (message.MyNodeId() == 1) {
message.Receive(0);
char inC = message.GetChar(0);
System.out.printf("Received %d%n", (int)inC);
}
}
}
Here's what it prints:
$ dcj test --source Main.java --library dummy_library.java --nodes 2
...
STDOUT 0: Sending 255
STDOUT 1: Received 65535
Duration: 87ms (longest running instance: 1)
The trouble happens because "char" in Java is a 16-bit unsigned type, but it's
being sent as a signed 8-bit signed value (-1 in this case). When it gets
turned back into a char, sign extension fills in the higher-order bits as 1s,
so the value received is different from the value sent (even though the value
sent was in the range 0 to 255).
To avoid this, you could mask off the lower-order bits (e.g. "value & 0xff") so
that received characters are always in the 0-255 range. Alternatively, since
the intention of PutChar/GetChar is to send a single byte, it seems like the
Java "message" class should really be using the "byte" type instead of the
"char" type for GetChar and PutChar.
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