Using file locks

Programs using files can specify locks. Locks are used to lock byte ranges in files and are intended for use by cooperating application programs. For information about how systems participating in a shared file system handle file locks, see Locking files in the sysplex.

Programs can use file locks via fcntl().

For more information about the fcntl callable service, seez/OS UNIX System Services Programming: Assembler Callable Services Reference.
z/OS XL C/C++ Runtime Library Reference has details about the fcntl() function.

Locks are advisory. (Advisory locking is used in UNIX systems.) Consequently, more than one program can update a file at the same time. Keep advisory locking in mind during problem determination.