Difference between revisions of "File"
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Directory files are special files that contain the list of files in a directory and their FIDs. Directory files always have the extension of .DIR and version 1.
Directory filesare special files that contain the list of files in a directory and their FIDs. Directory files always have the extension of .DIR and version 1 .
Latest revision as of 04:58, 15 December 2019
A file is a data structure for storing information on a storage media, an ordered collection of logically related records treated as a unit. File operations are provided by the Record Management Services. Files are identified by file IDs which contain the location of the file header. File headers stored in INDEXF.SYS contain file metadata and pointers to where the file data is stored on disk.
RMS supports the following file organization types:
To find out the organization of a particular file, do a DIRECTORY/FULL.
- Random access by key value
- Random access by relative record number
- Random access by record file address (RFA)
Although you cannot change its file organization after you create a file, you can change the record access mode each time you access a record in the file. For example, a relative file can be processed in sequential access mode one time and in a random access mode the next time.
|Access Mode||File Organization|
|Random by relative record number||Permitted with fixed-length record format on disk devices only||Yes||No|
|Random by key value||No||No||Yes|
|Random by record file address||Permitted on disk devices only||Yes||No|
A file is idenitified by the file ID (for example, "277058,4,0") which consists of three numbers:
- the file number - the offset into INDEXF.SYS where the file header is stored
- the file sequence number specifying how many times this file number has been used
- the relative volume number specifying the number of the volume in the volume set where the file is stored
Several directory entries with different names possibly found in different directories can point to the same File_ID and thus same data on disk. This is called aliases or links.
In the following example, an alias is created for REPORT.DAT:
$ dir Directory DKA0:[JDOE] DATA.DIR;1 REPORT.DAT;1 Total of 2 files. $ set file/enter=report_alias.dat _File: report.dat $ dir Directory DKA0:[JDOE] DATA.DIR;1 REPORT.DAT;1 REPORT_ALIAS.DAT;1 Total of 3 files. $ dir /file_id Directory DKA0:[JDOE] DATA.DIR;1 (6170,17222,0) REPORT.DAT;1 (6169,17222,0) REPORT_ALIAS.DAT;1 (6169,17222,0) Total of 3 files.
Normally when a file that has an alias is deleted, a dangling directory entry appears - that is, the alias cannot be used to access the file header and the file is "lost":
$ delete report.dat;1 $ type REPORT_ALIAS.DAT;1 %TYPE-W-OPENIN, error opening DKA0:[JDOE]REPORT_ALIAS.DAT;1 as input -RMS-E-FNF, file not found
ODS-5 volumes with hardlinks enabled support hard links: that is, aliases that can act like files. On a volume with hard links enabled, you can delete the original file, and if there is at least one alias pointing to the file header, the file data is accessible.
Directory files are special files that contain the list of files in a directory and their FIDs. Directory files always have the extension of .DIR and version 1 (files with version numbers greater than one cannot be used as directory files). With sufficient privileges, the directory label can be removed from a file with SET FILE/NODIRECTORY. You can see if a file is a directory file with DIRECTORY/FULL; running F$FILE_ATTRIBUTES(filespec,"DIRECTORY") on a file returns "TRUE" if it is a directory file.
File metadata are stored in the file header in INDEXF.SYS. You can view it with DIRECTORY/FULL or DUMP/HEADER:
$ dir/full data.dir Directory DKA0:[JDOE] DATA.DIR;1 File ID: (6170,17222,0) Size: 1/16 Owner: [JDOE] Created: 29-MAR-2019 15:56:38.74 Modified: 29-MAR-2019 15:56:38.74 (0) Expires: <None specified> Backup: <No backup recorded> Effective: <None specified> Recording: <None specified> Accessed: <None specified> Attr Mod: <None specified> Data Mod: <None specified> Linkcount: 1 File organization: Sequential Shelved state: Online Caching attribute: Writethrough File attributes: Allocation: 16, Extend: 0, Global buffer count: 0 No default version limit, Contiguous, MoveFile disabled Directory file Record format: Variable length, maximum 512 bytes, longest 512 bytes Record attributes: No carriage control, Non-spanned RMS attributes: None Journaling enabled: None File protection: System:RWE, Owner:RWE, Group:RE, World:E Access Cntrl List: None Client attributes: None Total of 1 file, 1/16 blocks.