最后，关闭输入流，清空输出流，如果输出流是管道，则用pclose函数关闭它。

Finally, close the input stream, flush the output stream, and if it was a pipe, close it with the pclose function.

youdao

实际上，您仍然可以对标准错误使用重定向或管道命令，但不能对标准输出使用，因为没有任何标准输出—正在内部使用popen函数由管道将它输送至lp进程。

Actually, you can still redirect or pipe stderr, but not stdout, since there will not be any standard output — it's being internally piped to the lp process using the popen function.

youdao

但是一个纯管道必须总是用纯函数实现。

But a pure pipe must always be implemented with a pure function.

youdao