Writing to stdin

Overview

System.in or stdin is usually used as an InputStream. However on Linux you can access this stream in other ways.

Accessing file descriptor 0

In linux, each file descriptor is accessible via /proc/{process-id}/fd/{file-id} You can use this to see what files a process has open but also see the contents of a file.

Writing and memory mapping

Getting the process id in Java is obscure, but once you have this you can re-open existing file descriptors such as stdin, file descriptor 0.

int processId = Integer.parseInt(ManagementFactory.getRuntimeMXBean().getName().split("@")[0]);
RandomAccessFile raf = new RandomAccessFile("/proc/" + processId + "/fd/0", "rw");
final FileChannel fc = raf.getChannel();
final MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size());
bb.putLong(System.nanoTime());
bb.force();
raf.close();

This opens the System.in is a read-write mode and memory maps it before changing its contents.

When you run this program it looks like

$ echo "        " > file.dat
$ od -xc file.dat
0000000    2020    2020    2020    2020    000a
                                         \n
0000011
$ java -cp . Main < file.dat
$ od -xc file.dat
0000000    0000    5522    11a9    79c6    000a
         \0  \0   "   U 251 021 306   y  \n
0000011

At first is rather surprising you can write to stdin or even memory map it. What happens if stdin is not a real file.

$ echo "        " | java -cp . Main
Exception in thread "main" java.io.FileNotFoundException: /proc/7935/fd/0 (Text file busy)
	at java.io.RandomAccessFile.open(Native Method)
	at java.io.RandomAccessFile.(RandomAccessFile.java:233)
	at java.io.RandomAccessFile.(RandomAccessFile.java:118)
	at Main.main(Main.java:10)