Why does reading from Process' InputStream block altough data is available

Why does reading from Process' InputStream block altough data is available

Java:
Process p = Runtime.getRuntime().exec("myCommand");
final InputStream in = p.getInputStream();
new Thread()
{
public void run()
{
int b;
while ((b = in.read()) != -1) // Blocks here until process
terminates, why?
System.out.print((char) b);
}
}
CPP:
#include <stdio.h>
#include <unistd.h>
int main(int argc, char** argv)
{
printf("round 1\n");
// At this point I'd expect the Java process be able
// to read from the input stream.
sleep(1);
printf("round 2\n");
sleep(1);
printf("round 3\n");
sleep(1);
printf("finished!\n");
return 0;
// Only now InputStream.read() stops blocking and starts reading.
}
The documentation of InputStream.read() states:
This method blocks until input data is available, the end of the stream is
detected, or an exception is thrown.
And yes, I'm aware of this (thus linux related?):
java.lang.Process: Because some native platforms only provide limited
buffer size for standard input and output streams, failure to promptly
write the input stream or read the output stream of the subprocess may
cause the subprocess to block, or even deadlock.
My questions are:
Why is InputStream.read() blocking although I should already have data
available right after the process starts? Am I missing something on either
side?
If it's linux related, is there any way to read from the process' output
stream without blocking?