open/4 option: record(MaxRecLen)
QP_stream: int max_reclen
This field indicates the maximum record length of the stream.
It is usually also the buffer length of the stream.
The value in this field is not changed once the stream is created.
The maximum record length can be bigger than the value store in the
max_reclen field for some operating systems allowing reading or
writing partial records, such as the UNIX operating system.
To create an unbuffered output stream, the value in max_reclen must
be set to 0. The bottom layer write function is then called for
each character placed in the output stream.
To create an unbuffered input stream, the value in
max_reclen can be either 0 or 1.