From: Eric Wong <normalperson@...>
Date: 2013-02-26T15:39:24+09:00
Subject: Re: STDIN inheritance during exec and difference between getc and sysread

Dmitry Bolshakov <lists@ruby-forum.com> wrote:
> Hi all!
> 
> I've accidentally found that "getc" and "sysread" behave differently
> after "exec"
> getc does not get remaining data from the STDIN but sysread does it
> 
> bdimych@mint-vbox ~ $
> bdimych@mint-vbox ~ $
> bdimych@mint-vbox ~ $ echo -n ab | ruby -e "p STDIN.getc; exec 'ruby',
> '-e', 'p STDIN.getc'"
> "a"
> nil
> bdimych@mint-vbox ~ $
> bdimych@mint-vbox ~ $ echo -n ab | ruby -e "p STDIN.sysread(1); exec
> 'ruby', '-e', 'p STDIN.sysread(1)'"
> "a"
> "b"
> bdimych@mint-vbox ~ $
> bdimych@mint-vbox ~ $
> bdimych@mint-vbox ~ $
> 
> why so? where is "b" in the first case?

getc uses Ruby I/O buffering in user space, so the process read
more bytes than it needed the first time getc was called.

Buffered I/O leads to surprises like this behavior across fork/exec.