ndelay_on man page
ndelay_on — put file descriptor in non-blocking mode
extern int ndelay_on(int fd);
ndelay_on puts a file descriptor in non-blocking mode. Reading from and writing to non-blocking network sockets, FIFOs or device files will return immediately, even if not as many bytes have been processed as were specified.
If an error occurred, ndelay_on will return nonzero and set errno accordingly.