Tcl中利用open函数创建pipe (about key word:open/exec/flush)

 Running other programs from Tcl - exec, open
[from: http://www.tcl.tk/man/tcl/tutorial/Tcl26.html]
So far the lessons have dealt with programming within the Tcl interpreter. However, Tcl is also useful as a scripting language to tie other packages or programs together. To accomplish this function, Tcl has two ways to start another program:
open ...... run a new program with I/O connected to a file descriptor
exec ...... run a new program as a subprocess
The open call is the same call that is used to open a file. If the first character in the file name argument is a "pipe" symbol (|), then open will treat the rest of the argument as a program name, and will run that program with the standard input or output connected to a file descriptor. This "pipe" connection can be used to read the output from that other program or to write fresh input data to it or both.

If the "pipe" is opened for both reading and writing you must be aware that the pipes are buffered. The output from a puts command will be saved in an I/O buffer until the buffer is full, or until you execute a flush command to force it to be transmitted to the other program. The output of this other program will not be available to a read or gets until its output buffer is filled up or flushed explicitly.

(Note: as this is internal to this other program, there is no way that your Tcl script can influence that. The other program simply must cooperate. Well, that is not entirely true: the expect extension actually works around this limitation by exploiting deep system features.)

The exec call is similar to invoking a program (or a set of programs piped together) from the prompt in an interactive shell or a DOS-box or in a UNIX/Linux shell script. It supports several styles o