Problem:

Opening a line sequential file in EXTEND mode will put an exclusive lock on that file by default. In this case, you can't open that file with a Windows utility such as Notepad to view it until the file has been closed.

Resolution:

To get around this, you can code the "sharing with all other" phrase in the select statement:

select logfile assign to "logfile.txt"

    organization is line sequential

    sharing with all other.