Simultaneous Access of Files

• 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.

Table 12-1 File Sharing Restriction Options

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

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.