如何在不读取IO的情况下知道IO是否为空？ [英] How can I know if an IO is empty without reading it?

问题描述

我有一个 IO 对象（比如 $ stdout ， $ stderr ）。我可以在它上面执行 read.empty？来查看它是否为空，但是如果没有读取它是否可以知道它是否为空？我知道文件有大小，但是 IO 没有。

I have an IO object (like $stdout, $stderr). I can do read.empty? on it to see if it is empty, but is it possible to know if it is empty without reading it? I know that File has size, but IO does not.

编辑
如果我的问题不够明确，很抱歉。我从 Open3.popen3 获取这些对象作为返回值。我想看看是否有任何内容写入标准错误，例如。如果可能的话，我想在不阅读的情况下这样做。

Edit Sorry if my question was not clear enough. I get these objects as return values from Open3.popen3. I want to see if anything was written to the standard error, for example. And I want to do it without reading it if possible.

推荐答案

这是 IO :: select 方法：

编辑问题后的更新示例：

Updated example after question edited:

require 'open3'

select_timeout = Rational(1,10) # optional

Open3.popen3('ls /') do
  |stdin, stdout, stderr, w_thread|

  rdin, rdout, rderr = IO.select([stdin], [stdout], [stderr], select_timeout)
  if rdin
    p 'no data on standard input' unless rdin.member? stdin
    p 'no data on standard output' unless rdout.member? stdout
    p 'no data on standard error' unless rderr.member? stderr
  else
    p 'none of streams has data available'
  end

  retval = w_thread.value
end
# "no data on standard output"
# "no data on standard error"

而只有标准输入包含数据来自衍生线程命令 ls /

Whereas only standard input contains data from spawned thread command ls /

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