HP 3000 Manuals HP 3000 Manuals
Sharing a File
Accessing and controlling a file that is open only to you is a relatively
simple matter. However, when several users are simultaneously accessing
a file, each user must be aware of special considerations for sharing the
file:
* How will others be allowed concurrent access to the file?
* Will concurrent access require special management?
When an HPFOPEN or FOPEN intrinsic call is issued for a file, the File
System regards the request as an individual accessor of the file and
establishes a unique file number and other file control information for
the file. Even when a program issues several different HPFOPEN or FOPEN
intrinsic calls for the same file, each call is treated as a separate
accessor. Under the normal (default) security provisions of MPE/iX, when
an accessor opens a file not currently in use, the access restrictions
that apply to this file for other accessors depend on the access mode
this first accessor requested:
* If the first accessor opens a file for read-only access, then any
other accessor can open it for any other type of access (for
example, write-only or append). However, the other accessors are
prohibited exclusive access.
* If the first accessor opens a file for any other access mode (for
example, write-only, append, or update), this accessor maintains
exclusive access to the file until it closes it; no other accessor
can access the file in any mode.
A program can override the defaults by specifying other options in
HPFOPEN and FOPEN intrinsic calls. A user running this program can, in
turn, override both the defaults and programmatic options by using the
:FILE command. Table 6-7 describes these options and the actions
MPE/iX takes when the options are in effect and simultaneous access is
attempted by other HPFOPEN or FOPEN intrinsic calls. The action depends
on the current use of the file and the access requested.
Table 6-7. File Sharing Restriction Options
----------------------------------------------------------------------------------------------
| | | |
| Access Restrictions | :FILE Parameters | Description |
| | | |
----------------------------------------------------------------------------------------------
| | | |
| | | |
| Exclusive Access | EXC | After you open a file, |
| | | prohibits concurrent access |
| | | in any mode through another |
| | | HPFOPEN/FOPEN request by any |
| | | program (including this |
| | | one), until this program |
| | | issues FCLOSE or terminates. |
| | | |
- -
| | | |
| Exclusive Write Access | SEMI | After you open a file, |
| | | prohibits concurrent write |
| | | access through another |
| | | HPFOPEN/FOPEN request by any |
| | | program (including this |
| | | one), until this program |
| | | issues FCLOSE or terminates. |
| | | |
- -
| | | |
| Sharable Access | SHR | After you open a file, |
| | | allows concurrent access in |
| | | any mode through another |
| | | HPFOPEN/FOPEN request by any |
| | | program (including this |
| | | one), in any session or job |
| | | (including this one). |
| | | |
- - - -
----------------------------------------------------------------------------------------------
The Exclusive Access option is useful to update a file and prevent other
users or programs from reading, writing, or altering information that is
about to be changed.
The Exclusive-write Access option allows other accessors to read the file
but prevents them from altering it. For example, it is useful for
updating a parts list, where you want to append new part numbers, without
prohibiting other users from reading current part numbers.
The Sharable Access option allows a file to be shared in all access modes
by requests from multiple programs. It is useful for allowing several
users to read different parts of the same file. Effectively, each
accessor accesses its own copy of the portion of the file in the
accessor's buffer.
For detailed information on Exclusive, Exclusive-write, and Sharable
Access, refer to Accessing Files Programmer's Guide (32650-90017).
The multi-access option extends the features of Sharable Access to allow
a deeper level of multiple access. It makes the file simultaneously
available to other accessors in the same job or session and permits them
to use the record pointer and other file-control information. When
several concurrently running programs (processes) are writing to the
file, the effect on the file is the same as if one program were
performing all output. Multi-access provides truly sequential access by
several concurrently running programs.
Global multi-access permits simultaneous access to a file by processes in
different jobs or sessions. Figure 6-15 shows the action resulting
from multi-access of files.
For detailed information on the multi-access and global multi-access
options, refer to Accessing Files Programmer's Guide (32650-90017).
![[]](../../pic3k/M012013/FFN513c50.gif)
Figure 6-15. Actions Resulting from Multiaccess of Files
When a file is shared among two or more processes and one or more of the
processes is writing to it, the processes must be properly interlocked to
prevent undesirable results. The FLOCK and FUNLOCK intrinsics provide
the necessary interlocking by using a Resource Identification Number
(RIN) as a flag to interlock multiple accessors. For detailed
information on this topic, refer to Accessing Files Programmer's Guide
(32650-90017) and MPE/iX Intrinsics Reference Manual (32650-90028).