rpcgen(1): Four output files are generated by default
manpage - manual pages
The rpcgen(1) man page says "three", but four files are generated (proto.h, proto_xdr.c, proto_svc.c, proto_clnt.c):
The rpcgen utility is normally used as in the first synopsis where it takes an input file and generates three output files. If the infile is named proto.x, then rpcgen generates a header in proto.h, XDR routines in proto_xdr.c, server- side stubs in proto_svc.c, and client-side stubs in proto_clnt.c. With the -T option, it also generates the RPC dispatch table in proto_tbl.i.