Tcl8.6.14/Tk8.6.14 Documentation > Tcl Commands, version 8.6.14 > fcopy

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fcopy — Copy data from one channel to another


fcopy inchan outchan ?-size size? ?-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 or characters have been transferred; size is in bytes if the two channels are using the same encoding, and is in characters otherwise. 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 or characters (using the same rules as for the -size option) 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 input operations with inchan, or output operations with outchan, during a background fcopy. The converse is entirely legitimate, as exhibited by the bidirectional fcopy example below.

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 wrong-sided I/O attempted (by a fileevent handler or otherwise) 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 manual entry 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.

Fcopy obeys the encodings and character translations configured for the channels. This means that the incoming characters are converted internally first UTF-8 and then into the encoding of the channel fcopy writes to. See the manual entry for fconfigure for details on the -encoding and -translation options. No conversion is done if both channels are set to encoding “binary” and have matching translations. If only the output channel is set to encoding “binary” the system will write the internal UTF-8 representation of the incoming characters. If only the input channel is set to encoding “binary” the system will assume that the incoming bytes are valid UTF-8 characters and convert them according to the output encoding. The behaviour of the system for bytes which are not valid UTF-8 characters is undefined in this case.


The first example transfers the contents of one channel exactly to another. Note that when copying one file to another, it is better to use file copy which also copies file metadata (e.g. the file access permissions) where possible.

fconfigure $in -translation binary
fconfigure $out -translation binary
fcopy $in $out

This second 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 third 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 -size $chunk \
                -command [list CopyMore $in $out $chunk]
set in [open $file1]
set out [socket $server $port]
set chunk 1024
set total 0
fcopy $in $out -size $chunk \
        -command [list CopyMore $in $out $chunk]
vwait done

The fourth example starts an asynchronous, bidirectional fcopy between two sockets. Those could also be pipes from two [open "|hal 9000" r+] (though their conversation would remain secret to the script, since all four fileevent slots are busy).

set flows 2
proc Done {dir args} {
     global flows done
     puts "$dir is over."
     incr flows -1
     if {$flows<=0} {set done 1}
fcopy $sok1 $sok2 -command [list Done UP]
fcopy $sok2 $sok1 -command [list Done DOWN]
vwait done


eof, fblocked, fconfigure, file


blocking, channel, end of line, end of file, nonblocking, read, translation
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Copyright © 1994-1997 Sun Microsystems, Inc.