Do you mean fork is a blocking operation? That's surprising but hmm ok. I didn't realize that. I thought that the process's pages all became copy-on-write, so that page updates could trigger faults handled by in-memory operations. Maybe those could be considered blocking (I hadn't thought of it like that) or maybe you mean something different?
It does... and it's a blocking operation (Bad). It's used when storing data to a file to allow restarts w/o losing the said data.