[Chris Nelson] cites [W. Richard Stevens] as writing in his
''UNIX Network Programming'' that
   The process can let the system automatically assign a port.  For
   both the Internet domain and the XNS domain, specifying a port
   number of 0 before calling bind() requests the system to do this.

A variety of networking texts (most of them derivative of RFCs?) cite
"0-1023" as reserved port numbers, without more comment.

[CL] believes the following are true:
   * A correctly implemented stack responds to a request to use port 0 by selection of a not-currently-in-use port, generally as low as possible.
   * Tcl is "passive", that is, simply inherits the behavior of the C API over which it is implemented.
   * All recent Tcl versions for Unix and Win* behave this way for port 0 requests.
We hope someone will test out a few Tcl versions under MacOS during
October 2001.

----



Our experience on 'Windows isn't encouraging of using this technique.  I'd forgotten why but poked around and found this in the bowels of one of our libraries:

    if { [string match $tcl_platform(platform) unix] } {
        # Specifying 0 as the port number tells socket to find a free
        # one
        if { [catch {socket -server $newClientCmd 0} sock] } {
            return 0
        } else {
            # -sockname returns address, hostname, and port number
            set port [lindex [fconfigure $sock -sockname] 2]
            ...
        }
    } else {
        # On Microsoft Windows, dial-up networking may be invoked by
        # [fconfigure -sockname] so we iterate looking for a free port
        # rather than use 0 to have [socket] find one for us.

        for { set port 2000 } {$port < 32767 } {incr port } {
            if {![catch {socket -server $newClientCmd $port} sock]} {
                ...
            }
        }
        return 0
    }

This code is 2-3 years old and stable.  Does anyone know if the situation has changed? Has fconfigure -sockname been reimplemented in a friendlier fashion? -- CLN











----
Daniel Steffen reports that the MacOS is as it should be:
    "seems to work ok:

    % set tcl_platform(platform)
    macintosh
    % info patchlevel
    8.4a4
    % fconfigure [socket -server blah 0] -sockname
    xxx.xxx.xxx.xxx xxx.xxx.xxx.xxx 53433"