This variable allows different logical files that access the same physical file to open the physical file only once. The default for this variable is 1 (on, true, yes). This variable is valid only for UNIX runtimes.

The default setting is recommended to close a potential flaw around file and record locking. When OPEN_FILES_ONCE=0 is in effect, and because the UNIX function used for locking (fcntl()) has the unfortunate behavior of removing all locks for a given file when any file descriptor for that file is closed, there is a potential window where other processes could access the file before locks are reapplied.