HP 3000 Manuals HP 3000 Manuals
Sharing the File Using FLOCK and FUNLOCK Sharing a file among two or more processes may be hazardous. When a file is being shared among two or more processes and is being written to by one or more of them, care must be taken to ensure that the processes are properly interlocked. For example, if process A is trying to read a particular record of the file, and at that time process B should execute and try to write that record, the results are not predictable. process A may see the old record or the new record, and not know whether it has read good data. If buffering is being done, please bear in mind that an output request (FWRITE) does not cause physical I/O to occur until a block is filled, which typically contains several records. A process trying to read such a file could, for example, read past the last record of the file which has been written on the disk because the end-of-file pointer is not kept in the file, but is kept in core where it can be updated quickly as writes occur. The necessary interlocking is provided by the intrinsics FLOCK and FUNLOCK, which use a resource identification number (RIN) as a flag to interlock multiple accessors. In the simple case of a file shared between a writer process and a reader process, where the writer is merely adding records to the file, the writer calls FLOCK prior to writing each record and FUNLOCK after writing. The reader calls FLOCK prior to reading each record, and FUNLOCK after reading. If the writing process should execute while the reader is in the middle of a read, the writer will be impeded on its FLOCK call until the reader signals that it is done by calling FUNLOCK. Similarly, if the reader should execute while the writer is performing a write, the reader will be impeded on its FLOCK call until the writer calls FUNLOCK. FUNLOCK ensures that all buffers are posted on the disk so that the reading processes can see all of the data. More complicated cases arise when a file has two or more writing processes, or when the writer may write more than one record at a time. If, for example, it should be necessary to write pairs of records, with read prohibited until both records of the pair are written, the writing process can call FLOCK before writing the first record of the pair, and FUNLOCK after writing the second. The shared file management scheme that MPE/iX provides you through the use of the FLOCK and FUNLOCK intrinsics guarantees you exclusive access to a file being shared by a set of processes that may be located in different jobs or sessions. MPE/iX also provides you with RINs you can use to manage anything you may consider a resource to your program, be it portions of a file, a device, or a segment of code in your program. Managing shared resources with RINs is described in Resource Management Programmer's Guide (32650-90024). For more information about the FLOCK and FUNLOCK intrinsics, consult the MPE/iX Intrinsics Reference Manual (32650-90028).