Why do I have to press Ctrl+D twice to close stdin?
In Python 3, this was due to a bug in Python's standard I/O library. The bug was fixed in Python 3.3.
In a Unix terminal, typing Ctrl+D doesn't actually close the process's stdin. But typing either Enter or Ctrl+D does cause the OS read
system call to return right away. So:
>>> sys.stdin.read(100)xyzzy (I press Enter here) (I press Ctrl+D once)'xyzzy\n'>>>
sys.stdin.read(100)
is delegated to sys.stdin.buffer.read
, which calls the system read() in a loop until either it accumulates the full requested amount of data; or the system read() returns 0 bytes; or an error occurs. (docs) (source)
Pressing Enter after the first line caused the system read() to return 6 bytes. sys.stdin.buffer.read
called read() again to try to get more input. Then I pressed Ctrl+D, causing read() to return 0 bytes. At this point, sys.stdin.buffer.read
gave up and returned just the 6 bytes it had collected earlier.
Note that the process still has my terminal on stdin, and I can still type stuff.
>>> sys.stdin.read() (note I can still type stuff to python)xyzzy (I press Enter) (Press Ctrl+D again)'xyzzy\n'
OK. This is the part that was busted when this question was originally asked. It works now. But prior to Python 3.3, there was a bug.
The bug was a little complicated --- basically the problem was that two separate layers were doing the same work. BufferedReader.read()
was written to call self.raw.read()
repeatedly until it returned 0 bytes. However, the raw method, FileIO.read()
, performed a loop-until-zero-bytes of its own. So the first time you press Ctrl+D in a Python with this bug, it would cause FileIO.read()
to return 6 bytes to BufferedReader.read()
, which would then immediately call self.raw.read()
again. The second Ctrl+D would cause that to return 0 bytes, and then BufferedReader.read()
would finally exit.
This explanation is unfortunately much longer than my previous one, but it has the virtue of being correct. Bugs are like that...
I wrote an explanation about this in my answer to this question.
How to capture Control+D signal?
In short, Control-D at the terminal simply causes the terminal to flush the input. This makes the read
system call return. The first time it returns with a non-zero value (if you typed something). The second time, it returns with 0, which is code for "end of file".