An MPE Power Tool: Byte Stream Files
July 17, 2013
These fundamental files are a lot like those used in Windows and Linux and Unix, Fairchild explained. HP engineered "emulation type managers" into MPE/iX, an addition that became important once the 3000 gained an understanding of Posix. In 1994, MPE XL became MPE/iX when HP added this Unix-like namespace.
It's a rare gift to see a primer on 3000 file types emerge from HP. Understanding the 3000 at this level is important to the customer who wants 3000 third party companies to take on the tasks HP has dropped. Although migration advocates point out that HP support is long gone for MPE, the knowledge remains intact among independent providers. Fairchild explained the basics of this basic file type.
Fairchild detailed how HP gave MPE's byte stream files the knowledge of "organization of data" for applications.
The underlying properties of a byte stream file is that each byte is considered its own record. In MPE file system terms, a record is the smallest unit of IO that can be performed on a file. (You can write a partial record fixed length record, but the file system will pad it to a full record.) Since the smallest unit of IO that can be performed on a byte stream file is a single byte, that becomes its MPE record size. In the MPE file system, the EOF tracks the number of records that are in a file. Since the record size of a byte stream file is one byte, the EOF of a byte stream file is also equal to the number of bytes in the file. This is why one 4-byte variable sized record is equal to 5 byte stream records (4 bytes of data + 1 \n character).
It's also worth noting that any file can be in any directory location and will behave the same way. (Well, almost. CM KSAM files are restricted to the MPE namespace. And of course the special files (that you don't normally see) that make up the file system root, accounts and groups are also restricted... one root, accounts as children of the root, groups as children of accounts. And lockwords aren't allowed outside the MPE namespace. But other than that the first sentence is true.)
The general model that we had in architecting the whole Posix addition was that behavior of a file does not change regardless of where it is located. This was summed up in the saying, "A file is a file." So there are no such things as "MPE files" and "Posix files." There's just files.
What does change is the way you name that file. Files in the MPE namespace can be named either through the MPE syntax (FILE.GROUP.ACCOUNT), or through the HFS syntax (/ACCOUNT/GROUP/FILE). You can also use symbolic links to create alternate names to the same file. This was summed up as a corrallary to the first saying, "But a name is not a name."