The setblock program

setblock [{+|-}fd ...] [: command arg ...]

setblock sets each given file descriptor to be blocking (for +) or nonblocking (for -). Then it execs the given command, if any. A colon separates the file descriptors from the command.

For example, this command sets file descriptor 3 to be nonblocking and file descriptor 4 to be blocking:

setblock -3 +4