get the file name from the file handle value

Question

let's say I have a filehandle. Is there a way for me to know the name of the file it is writing to? I.e., you usually use:

set fh [ open "fname" "w" ]

I want a proc that would have the output:

puts [ getFHFName $fh]
>fname

Any ideas?

Answer 1

Tcl doesn't provide any built-in mechanism to provide that. (It'd be something you could find out with fconfigure if we did do it… but it isn't.)

The easiest workaround is to keep a global array that maps from file handles to filenames. You can either keep this yourself, or you can override open and close to maintain the mapping.

rename open _open
rename close _close
array set fhmap {}

proc open {filename args} {
    global fhmap
    set fd [_open $filename {*}$args]
    # Note! Do not normalise pipelines!
    if {![string match |* $filename]} {
        set filename [file normalize $filename]
    }
    set fhmap($fd) $filename
    return $fd
}
# Probably ought to track [chan close] too.
proc close {filehandle args} {
    global fhmap
    # Note that we use -nocomplain because of sockets.
    unset -nocomplain fhmap($filehandle)
    tailcall _close $filehandle {*}$args
}

proc getFHFName {filehandle} {
    global fhmap
    return $fhmap($filehandle)
}