Tcl Source Code

Manpage for chan copy and fcopy both state 

"returns the number of bytes written to outputChan." 

and then later

"Only the number of bytes written to outputChan is reported, "

dgp on the chat states:
22:15]	dgp	it's the number of chars attempted to read from inchan
[22:17]	dgp	I don't think I changed this in my 2014 work, but check it.
[22:19]	dgp	I think you don't want to copy a partial char

Experimentation indicates dgp is correct. It appears to be number of chars, not bytes, and on input, not output.

Well, that's messy. The units are the same as for the -size option, and it's bytes when it isn't a transforming copy and characters when it is a transforming copy. Well, I've noted that this is the case in the docs; I'm not going to attempt to make this any clearer.