Version 18 of pipeline

Crucial to "How Tcl is special" is the pipeline. Through the syntax

  set channel [open |$external_command r+]

Tcl opens an external process, and connects to the stdin and stdout of $external_command. A special syntax is needed in external_command to get stderr to be concantenated to stdout. Reading and writing to stdin and stdout are with the usual Tcl I/O commands: gets, puts, and read. (It may be necessary to flush resp. fflush to prevent deadlocks caused by buffering).

This makes for exceptionally handy glue for many situations.

The concept of pipeline, however, requires the external command to be designed to interact with data either on its stdin or stdout. Many GUI applications (and occasionally non-graphical applications) do not do so. In those cases, the pipeline provides little benefit.

The concept of the pipeline is best used when writing tools using a filter paradigm.

The pipeline is also used, in a slightly different way, by Tcl's exec command, and by the bgexec command that is provided by the BLT extension.

Many Wiki pages and other references obliquely discuss or illustrate Tcl process pipelines. While the construction is crucial to a proper understanding of Tcl's capabilities, many newcomers take quite a while to come across an explanation that "gets through" to them. Among the pertinent available writings are

"Concepts of Architectural Design for Tcl Applications"
"Scripted wrappers for legacy applications", a series of articles from ITworld, which appears to be gone. Temporarily at least, check out google's cached version at [L1 ] and part 2 of the series at [L2 ].
"client/server with fileevent"
Pipe servers in C from Tcl
VFS, exec and command pipelines
...

This transcript of an interaction session illustrates a simple pipeline with Unix's bc executable:

    % set channel [open |bc r+]
    file3
    % puts $channel "1234567890 * 987654321"
    % flush $channel
    % puts [gets $channel]
    1219326311126352690

with winnt cmd:

    % set channel [open |cmd r+]
    file3
    % gets $channel
    % gets $channel
    % puts $channel "hostname"
    % flush $channel
    % gets $channel
    % gets $channel
    % puts [gets $channel]
    % close $channel

Can someone look at this example and explain where the writer went wrong, if anywhere?

 # Goal - to eventually return the filetype of a list of files;

 set aid [open "|file --brief --files-from -" r+]
 fconfigure $aid -buffering line
 fileevent $aid readable {puts [gets $aid]}
 puts $aid "/win/d/movies/mp3/en/Giganten CD1/track01.cdda.ogg"
 puts $aid "/etc/motd"
 vwait forever

CL's guess: put

    flush $aid

after the puts-s.

While (classic) MacOS supports no Tcl pipelines, see "Inventory of IPC methods" for generalizations that apply there and elsewhere.

[Explain "named pipe" as specialization of concept.]

idiom

Category Glossary