Copy data from one channel to another 

Tclsh Built-In Commands


fcopy inchan outchan ?-sizesize? ?-command callback?


The fcopy command copies data from one I/O channel, inchan to another I/O channel, outchan. The fcopy command leverages the buffering in the Tcl I/O system to avoid extra copies and to avoid buffering too much data in main memory when copying large files to slow destinations like network sockets.

The fcopy command transfers data from inchan until end of file or size bytes have been transferred. If no -size argument is given, then the copy goes until end of file. All the data read from inchan is copied to outchan. Without the -command option, fcopy blocks until the copy is complete and returns the number of bytes written to outchan.

The -command argument makes fcopy work in the background. In this case it returns immediately and the callback is invoked later when the copy completes. The callback is called with one or two additional arguments that indicates how many bytes were written to outchan. If an error occurred during the background copy, the second argument is the error string associated with the error. With a background copy, it is not necessary to put inchan or outchan into non-blocking mode; the fcopy command takes care of that automatically. However, it is necessary to enter the event loop by using the vwait command or by using Tk.

You are not allowed to do other I/O operations with inchan or outchan during a background fcopy. If either inchan or outchan get closed while the copy is in progress, the current copy is stopped and the command callback is not made. If inchan is closed, then all data already queued for outchan is written out.

Note that inchan can become readable during a background copy. You should turn off any fileevent handlers during a background copy so those handlers do not interfere with the copy. Any I/O attempted by a fileevent handler will get a "channel busy" error.

fcopy translates end-of-line sequences in inchan and outchan according to the -translation option for these channels. See the reference page for fconfigure for details on the -translation option. The translations mean that the number of bytes read from inchan can be different than the number of bytes written to outchan. Only the number of bytes written to outchan is reported, either as the return value of a synchronous fcopy or as the argument to the callback for an asynchronous fcopy.


This first example shows how the callback gets passed the number of bytes transferred. It also uses vwait to put the application into the event loop. Of course, this simplified example could be done without the command callback.

proc Cleanup {in out bytes {error {}}} {
	global total
	set total $bytes
	close $in
	close $out
	if {[string length $error] != 0} {
		# error occurred during the copy
set in [open $file1]
set out [socket $server $port]
fcopy $in $out -command [list Cleanup $in $out]
vwait total

The second example copies in chunks and tests for end of file in the command callback

proc CopyMore {in out chunk bytes {error {}}} {
	global total done
	incr total $bytes
	if {([string length $error] != 0) || [eof $in] {
		set done $total
	close $in
	close $out
 } else {
	fcopy $in $out -command [list CopyMore $in $out $chunk] \
	    -size $chunk }
set in [open $file1]
set out [socket $server $port]
set chunk 1024
set total 0
fcopy $in $out -command [list CopyMore $in $out $chunk] -size $chunk
vwait done


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


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eof, fblocked, fconfigure

PTC MKS Toolkit 10.4 Documentation Build 39.