HP 3000 Manuals HP 3000 Manuals
Simultaneous Access of Files
When an HPFOPEN/FOPEN request is issued for a file, that request is
regarded as an individual accessor of the file and a unique file number
and other file control information is established for that file. Even
when the same program issues several different HPFOPEN/FOPEN 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 presently in use, the access restrictions that apply to this
file for other accessors depend upon the access mode requested by this
initial accessor:
* If the first accessor opens the file for Read-only access, any
other accessor can open it for any other type of access (such as
Write-only or Append), except that other accessors are prohibited
Exclusive access.
* If the first accessor opens the file for any other access mode
(such as Write-only, Append, or Update), this accessor maintains
Exclusive access to the file until it closes the file; no other
accessor can access the file in any mode.
Programs can override these defaults by specifying other options in
HPFOPEN/FOPEN intrinsic calls. Users running those programs can, in
turn, override both the defaults and programmatic options through the
FILE command. The options are listed in Table 12-1 . The actions
taken by MPE/iX when these options are in effect, and simultaneous access
is attempted by other HPFOPEN/FOPEN calls, are summarized in Table 12-2
. The action taken depends upon the current use of the file versus
the access requested.
Table 12-1. File Sharing Restriction Options
-----------------------------------------------------------------------------------------------
| | | |
| ACCESS | FILE | DESCRIPTION |
| RESTRICTION | PARAMETER | |
| | | |
-----------------------------------------------------------------------------------------------
| | | |
| Exclusive Access | EXC | After file is opened, prohibits concurrent access in any |
| | | mode through another HPFOPEN/FOPEN request, whether |
| | | issued by this or another program, until this program |
| | | issues FCLOSE or terminates. |
| | | |
| Exclusive Write | SEMI | After file is opened, prohibits concurrent Write access |
| Access | | through another HPFOPEN/FOPEN request, whether issued by |
| | | this or another program, until this program issues |
| | | FCLOSE or terminates. |
| | | |
| Shareable Access | SHR | After file is opened, permits concurrent access to file |
| | | in any mode through another HPFOPEN/FOPEN request issued |
| | | by this or another program, in this or any session or |
| | | job. |
| | | |
-----------------------------------------------------------------------------------------------
Exclusive access
This option is useful when you wish to update a file and wish to prevent
other users or programs from reading or writing on the file while you are
using it; thus, no user can read information that is about to be changed,
nor can he alter that information. To override the programmatic option
under which the file would be opened and request exclusive access, you
could use the EXC keyword parameter in the FILE command:
FILE DATALIST;EXC <---- Requests exclusive access
RUN FLUPDATE
NOTE In all cases, when the first accessor to a file opens it with
Exclusive (EXC) access, all other attempts to open the file fails.
Table 12-2. Actions Resulting from Multiaccess of Files
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Semi-exclusive access
This option allows other accessors to read the file, but prevents them
from altering it. When appending new part numbers to a file containing a
parts list, for instance, you might use this option to allow other users
to read the current part numbers at the same time that you are adding new
ones to the end of the file. You could request this option as follows:
FILE PARTSLST;SEMI <---- Requests semi-exclusive access
RUN FLAPPEND
Shared access
When opened with the share option, a file can be shared (in all access
modes) among several HPFOPEN/FOPEN requests, whether they are issued from
the same program, different programs within the same job or session, or
programs running under different jobs or sessions. Each accessor
transfers its input/output to and from the file with its own unique
buffer, using its own set of file control information and specifying its
own buffer size and blocking factor. Effectively, each accessor accesses
its own copy of that portion of the file presently in its buffer. Thus,
share access is useful for allowing several users to read different parts
of the same file. It can, however, present problems when several users
try to write to the file. For instance, if two users are updating a file
concurrently, one could easily overwrite the other's changes when the
buffer content from the first user's output is overwritten on the file by
the buffer content from the second user's output. To use Write access
most effectively with shared files, specify the multiaccess option as
discussed below.
To request share access for a file, use the SHR parameter in the FILE
command, as follows:
FILE RDFILE;SHR <---- Requests shared access
RUN RDPROG
Multiaccess
This option extends the features of the share access option to allow a
deeper level of multiple access. Multiaccess not only makes the file
available simultaneously to other accessors (in the same job or session),
but permits them to use the same data pointers, blocking factor, and
other file-control information. Thus, transfers to and from the file
occur in the order they are requested, regardless of which program in
your job or session does the requesting. 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; truly
sequential access by several concurrently running programs.
NOTE Multiaccess allows the file to be shared (in all access modes)
among several HPFOPEN/FOPEN requests from the same program, or from
different concurrently running programs in the same job or session.
Unlike share, access, however, multiaccess does not permit the file
to be shared among different sessions and jobs.
Global multiaccess
This option extends the features of the multiaccess option to permit
simultaneous access of a file by processes in different jobs or sessions.
As in multiaccess, accessors use the same data pointer, blocking factor,
and other file-control information. You can request this option as
follows:
FILE GFILE;GMULTI <---- Requests global multi access
RUN GPROG
NOTE To prohibit the use of MULTI or GMULTI access, use the NOMULTI
keyword in a FILE command. When the NOMULTI keyword is used,
different processes may share the data in a file, but they maintain
separate buffers and pointers.
Note that it is the first accessor to a file that sets the allowable
access to a file. For example, if the first accessor specifies share
access, that is, the access that will be allowed to all future accessors.
However, if a subsequent accessor specifies an access option that is more
restrictive than the first opener's access option, it remains in effect
until the user that requested it closes the file.