xcall MIAMEX, MX_FLUSHBUF, channel

(UNIX only) MX_FLUSHBUF is similar to the previous one but instead allows you to flush (or "unbuffer") the data, which has been buffered but not yet written to the specified file channel.

channel (Num) is the file channel to flush the buffer of. The file must be open for output (or append). For the terminal device, use file channel 0 (which is automatically opened for you at the start of each program).

See MIAMEX 87: Disable stream buffering for more information about buffering.