http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/tell.htm
-- tell and flush
are friends: if you are using tell and you do reads and writes in between, you usually have to do a flush or your tell will fail with a -1 value. This is because the Tcl Channel code cannot
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command