Chapter 17: Data Files

Many programs need to read and write data files. A program might read data files to initialize its configuration or to receive data from another program; a program might write data files to save its state or to send data to another program. In this chapter we'll explore techniques for reading and writing data files, and for designing data file formats so that they are functional, useful, and convenient.

What makes a good data file? There are many desirable attributes which we might want to achieve or to trade off against one another. We might want data files to be small (to save disk space) or to be efficient to read and write (to save computer time) or to easy to read and write (that is, easy to implement, to save programmer time). We might want them to be human readable, to make them easier to debug or modify, or so that they could be created ``by hand'' (i.e. all using standard file-manipulation tools). On the other hand, if the files are to contain sensitive data, we might prefer that they not be human-readable. We might want the files to be portable across different machine architectures (if we will be moving data files from machine to machine). We might want to ensure that if the data file format ever changes (perhaps to add new information), newer versions of our software (that is, the software that reads and writes the data files) can still read the old files, and perhaps even that old versions of the software can at least partially read the new files. We'll see ways of achieving all of these attributes.

Roughly speaking, there are two large classes of data file formats: ``text'' and ``binary''. Text files, as their name implies, contain human-readable text; that is, if you were to read one into a text editor or dump one to your screen, it would consist of strings of printable characters, arranged into lines. (By ``printable characters'' we mean characters which display nicely on the screen, as opposed to ``control characters.'' Generally speaking, the only control characters a text file will contain will be CR or LF or CRLF combinations to mark the ends of lines, and perhaps horizontal tabs. C represents the end-of-line character(s) by \n, and tabs by \t.)

Binary files, on the other hand, contain arbitrary patterns of bits and bytes, arranged for the computer's convenience, not the human's. The bytes making up a binary file are not intended to be interpreted as characters or text; if you dump one to the screen, you get all sorts of garbage. Some of the bit patterns will happen to represent printable characters, but others will be control characters, others may be special graphics characters, and still others may end up representing sequences which will switch the display into inverse video, clear the screen, etc. (Depending on your display environment, printing arbitrary binary characters may confuse the display so badly that it becomes unusable and must be reset.)

In a text file, we might represent the integer 12345 as the five characters 1 2 3 4 5 (that is, as the text string "12345"). In a binary file, on the other hand, we might represent it as two bytes with values 0x30 and 0x39, since 12345 base 10 is 3039 base 16. (Just to confuse the issue, it happens that in the ASCII character set the values 0x30 and 0x39 represent the characters '0' and '9', but this is sheer coincidence; the character values 0 and 9 of course have no meaningful relationship to the value 12345 that we're storing.)

17.1: Text Data Files

17.2: Binary Data Files

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