stdio

Also spelled -. Note that tocat’s stdout carries relayed data when stdio is the sink. Logs and dumps go to stderr for this reason, and tocat refuses to open stdout as a dump target at all.

$ dd if=/dev/zero | tocat - tcp:localhost:9000

Both descriptors are resized when they turn out to be pipes. Nobody declares them as such, but tocat … | pv makes fd 1 one, and its 64 KiB default would otherwise cap every write. See Buffers.

On the synchronous path tocat reads and writes the descriptors directly rather than going through std::io::stdin/stdout: the standard handles bring a LineWriter that scans every byte for newlines and an 8 KiB BufReader, both of which are wrong for binary payload.

Three things contend for stderr: log records, a tee pointed there, and the --progress line. Logs and the progress line are coordinated under one lock; a stderr dump is not, and tocat warns about that combination.