Using threads with MVS files

MVS™ files that are opened by data-set names or ddnames are thread-specific in the following ways:

MVS files opened in update mode where repositioning functions are used after the files have been extended are also restricted to the owning thread. This is because a reposition might need to reopen the read DCB in order to be able to see the new EOF marker. The runtime library does not enforce this restriction.

Multivolume data sets, files that are part of a concatenated ddname, and hiperspace memory files are further restricted in multithreaded applications. All I/O operations are restricted to the thread on which the file is opened.

When standard streams are directed to MVS files, they are governed by the previous restrictions. Standard streams are directed to MVS files in one of two ways:

By default when a main() program is run from the TSO ready prompt or by a JCL EXEC PGM= statement, that is, whenever it is not initiated by the exec() function. This is regardless of whether you are running with POSIX(ON) or POSIX(OFF). In these cases, the owning thread is the initial processing thread (IPT), the thread on which main() is executed.
By explicit action when the user redirects the streams by using command line redirection, fopen(), or freopen(). The thread that is redirected (the IPT, if you are using command line redirection) becomes the owning thread of the particular standard stream. The usual MVS file thread affinity restrictions outlined above apply until the end of program or until the stream is redirected to the UNIX file system.

Any operation that violates these restrictions causes SIGIOERR to be raised and errno to be set with the following associated message:

EDC5024I:  An attempt was made to close a file that had been
opened on another thread.

All MVS files opened from a given thread and still open when the thread is terminated are closed automatically by the library during thread termination.

Having more than one writer use separate file pointers to a single data set or ddname is prohibited as always, regardless of whether the file pointers are used from multiple threads or a single thread.

Note: These restrictions specifically do not apply to UNIX file system. All opens and closes by the C library that result in calls to an underlying access method for a given MVS file must occur on the same thread. Therefore, the following specific functions are prohibited from any thread except the owning thread (the one that does the initial fopen()) of the file:

fclose()
freopen()
rewind()