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Additional Background

The GNU/Linux view of files can be surprising for programmers with a background that focuses on mainframe Z/OS or Windows. This is additional background information for programmers who are new to the POSIX use of the file abstraction. This POSIX view informs how Python works.

In the Z/OS world, files are called data sets, and can be managed by the OS catalog or left uncataloged. While this is also true in the GNU/Linux world, the catalog (called a directory) is seamless, silent and automatic, making files far easier to manage than they are in the Z/OS world. In the GNU/Linux world, uncataloged, temporary files are atypical, rarely used, and require special API's.

In the Z/OS world, files are generally limited to disk files and nothing else. This is different from the GNU/Linux use of file to mean almost any kind of external device or service.

Block Mode Files. File devices can be organized into two different kinds of structures: block mode and character mode. Block mode devices are exemplified by magnetic disks: the data is structured into blocks of bytes that can be accessed in any order. Both the media (disk) and read-write head can move; the device can be repositioned to any block as often as necessary. A disk provides direct (sometimes also called random) access to each block of data.

Character mode devices are exemplified by network connections: the bytes come pouring into the processor buffers. The stream cannot be repositioned. If the buffer fills up and bytes are missed, the lost data are gone forever.

Operating system support for block mode devices includes file directories and file management utilities for deleting, renaming and copying files. Modern operating systems include file navigators (Finders or Explorers), iconic representations of files, and standard GUI dialogs for opening files from within application programs. The operating system also handles moving data blocks from memory buffers to disk and from disk to memory buffers. All of the device-specific vagaries are handled by having a variety of device drivers so that a range of physical devices can be supported in a uniform manner by a single operating system software interface.

Files on block mode devices are sometimes called seekable. They support the operating system seek function that can begin reading from any byte of the file. If the file is structured in fixed-size blocks or records, this seek function can be very simple and effective. Typically, database applications are designed to work with fixed-size blocks so that seeking is always done to a block, from which database rows are manipulated.

Character Mode Devices and Keyboards. Operating systems also provide rich support for character mode devices like networks and keyboards. Typically, a network connection requires a protocol stack that interprets the bytes into packets, and handles the error correction, sequencing and retransmission of the packets. One of the most famous protocol stacks is the TCP/IP stack. TCP/IP can make a streaming device appear like a sequential file of bytes. Most operating systems come with numerous client programs that make heavy use of the netrowk, examples include sendmail, ftp, and a web browser.

A special kind of character mode file is the console; it usually provides input from the keyboard. The POSIX standard allows a program to be run so that input comes from files, pipes or the actual user. If the input file is a TTY (teletype), this is the actual human user's keyboard. If the file is a pipe, this is a connection to another process running concurrently. The keyboard console or TTY is different from ordinary character mode devices, pipes or files for two reasons. First, the keyboard often needs to explicitly echo characters back so that a person can see what they are typing. Second, pre-processing must often be done to make backspaces work as expected by people.

The echo feature is enabled for entering ordinary data or disabled for entering passwords. The echo feature is accomplished by having keyboard events be queued up for the program to read as if from a file. These same keyboard events are automatically sent to update the GUI if echo is turned on.

The pre-processing feature is used to allow some standard edits of the input before the application program receives the buffer of input. A common example is handling the backspace character. Most experienced computer users expect that the backspace key will remove the last character typed. This is handled by the OS: it buffers ordinary characters, removes characters from the buffer when backspace is received, and provides the final buffer of characters to the application when the user hits the Return key. This handling of backspaces can also be disabled; the application would then see the keyboard events as raw characters. The usual mode is for the OS to provide cooked characters, with backspace characters handled before the application sees any data.

Typically, this is all handled in a GUI in modern applications. However, Python provides some functions to interact with Unix TTY console software to enable and disable echo and process raw keyboard input.

File Formats and Access Methods. In Z/OS (and Open VMS, and a few other operating systems) files have very specific formats, and data access is mediated by the operating system. In Z/OS, they call these access methods, and they have names like BDAM or VSAM. This view is handy in some respects, but it tends to limit you to the access methods supplied by the OS vendor.

The GNU/Linux view is that files should be managed minimally by the operating system. At the OS level, files are just bytes. If you would like to impose some organization on the bytes of the file, your application should provide the access method. You can, for example, use a database management system (DBMS) to structure your bytes into tables, rows and columns.

The C-language standard I/O library (stdio) can access files as a sequence of individual lines; each line is terminated by the newline character, \n . Since Python is built in the C libraries, Python can also read files as a sequence of lines.


 
 
  Published under the terms of the Open Publication License Design by Interspire