chan - Manipulate channels
http://purl.org/tcl/home/man/tcl8.5/TclCmd/chan.htm
chan option ?arg arg ...?
This command (introduced in Tcl 8.5) provides several operations on a channel, including many that have been available using a mix of other commands. Option indicates what to do with the channel. Any unique abbreviation for option is acceptable. The valid options are: (see the man page listed above for more details; on this web page, notes on selected options have been contributed).
Why was this thing called 'chan' instead of 'channel'? schlenk: It's mentioned in the TIP [L1 ], basically the same naming style as interp for interpreter.
I see. Thank you for pointing that out. Pretty lame reasoning when "channel" is much more descriptive. But I'm not the one doing all the work so I'm not complaining!
JYL If you dont like chan you can do 'interp alias {} channel {} chan'
AMG: chan aggregates all the commands you'll need for working with chan blocked channelId - like fblocked
chan close channelId - like close
chan configure channelId ?optionName? ?value? ?optionName value?... - like fconfigure
chan copy inputChan outputChan ?options...? - same as fcopy
chan create mode cmdPrefix
chan event channelId event ?script? - like fileevent
chan flush channelId - like flush
chan gets channelId ?varName? - like gets
chan names ?globPattern? - like file channels
chan pending mode channelId - returns the number of bytes (of input or output, depending on mode) buffered internally
chan pop channelId - pop one transform off a stacked channel
chan postevent channelId eventSpec
chan push channelId cmdPrefix - stack a transform on top of the channel
chan pipe - create a standalone pipe and return a pair of file descriptors for its read and write ends, in this order (TIP #304 [L2 ], available in Tcl >= 8.6).
chan puts ?-nonewline? ?channelId? string - like puts
chan read channelId ?numChars? - like read
chan read ?-nonewline? channelId -like read
chan seek channelId offset ?origin? - like seek
chan tell channelId - like tell
chan truncate channelId ?length?
TIP 287 proposes a [chan available channelId] command to find out how much available input is currently being buffered that could be chan read in. (It would make it safe to use chan gets with sockets since you could introspect and detect someone sending an excessively long line rather than running out of memory first.)
See also chan mode for an example how to extend that ensemble.
RS History sometimes runs in circles... Tcl 2.1 didn't have commands dealing with channels. Peter da Silva added the "stream" extension, where one could write
stream fp open $filename r set x [stream fp gets] stream fp close
Later, the parts of "stream" went into the core as separate commands. Still later (in 8.5), they get reunited again in chan, which arguably makes the command set leaner, but scripts wordier...
MG has just needed to find out how long the eol sequence in a particular file was. Checking fconfigure $fid -translation didn't help (as it was just 'auto' on the read, and always crlf (Win XP) on the write, even if the file used Unix line-endings). Came up with this (which is possibly obvious, but I'm rather pleased with myself anyway;)
proc lineEndingSize {file} { set fid [open $file r] chan gets $fid line set size [expr { [chan tell $fid] - [string bytelength $line] }] close $fid return $size; }
DKF: That can go wrong. The problem is that you need to use the encoded length of the string read, and some encodings have multiple ways of encoding a particular character. This is all rather nasty; even guessing based on the value of fconfigure -encoding can go wrong! So instead try this:
proc lineEndingSize file { set f [open $file] gets $f line set after [tell $f] seek $f 0 read $f [string length $line] set before [tell $f] close $f return [expr {$after - $before}] }
MG I'll try that instead, thanks :)
CRC I'm just starting out with Tcl, and happened to come across this while looking up some info on fconfigure and fileevent. The TIP mentions chan as sort of an aggregation of the file/channel-related commands, to make things easier and more consistent for new users (like me).
It also mentions a recommendation to deprecate some of the old commands, for consistency's sake. My question is, is that recommendation going to be adopted? Should I as a new user (with no legacy code or burned-in Tcl habits) do "chan <everything>" rather than fconfigure, fileevent, etc? Thanks.
Category Command of Tcl 8.5 | Category Channel |
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