Low-level z/OS® UNIX I/O is the POSIX.1-defined I/O method. All input and output is processed using the defined read(), readv(), write(), and writev() functions.
For application programmers used to a UNIX environment, z/OS UNIX behaves in familiar and predictable ways. Standard UNIX programming practices for shared resources, along with designing applications to respect locks put on files by multiple threads running in a process, will ensure that data is handled predictably.
For a discussion of POSIX.1-defined low-level I/O and some of the practical considerations to take into account when designing an application, see The POSIX.1 Standard: A Programmer's Guide, by Fred Zlotnick (Redwood City, CA: The Benjamin/Cummings Publishing Company, Inc., 1991).