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