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The Art of Unix Programming
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Unix Programming - Studying Cases - Case Study: The Terminfo Database

Case Study: The Terminfo Database

The terminfo database is a collection of descriptions of video-display terminals. Each entry describes the escape sequences that perform various manipulations on the terminal screen, such as inserting or deleting lines, erasing from the cursor position to end of line or screen, or beginning and ending screen highlights such as reverse video, underline, or blink.

The terminfo database is primarily used by the curses(3) libraries. These underlie the “roguelike” interface style we discuss in Chapter11, and some very widely used programs such as mutt(1), lynx(1), and slrn(1). Though the terminal emulators such as xterm(1) that run on today's bitmapped displays all have capabilities that are minor variations on those of the ANSI X3.64 standard and the venerable VT100 terminal, there is still enough variation that hardwiring ANSI capabilities into applications would be a bad idea. Terminfo is also worth studying because problems that are logically similar to the one it addressed arise constantly in managing other kinds of peripheral hardware that doesn't have a standard way to report their own capabilities.

The design of terminfo benefits from experience with an earlier capability format called termcap. The database of termcap descriptions lived in a textual format in one big file, /etc/termcap; though this format is now obsolete, your Unix system almost certainly includes a copy.

Normally, the key used to look up your terminal type entry is the environment variable TERM, which for purposes of this case study is set by magic.[61] Applications that use terminfo (or termcap) pay a small penalty in startup lag; when the curses(3) library initializes itself, it has to look up the entry corresponding to TERM and load the entry into memory.

Experience with termcap showed that the startup penalty was dominated by the time required to parse the textual representation of capabilities. Accordingly, terminfo entries are binary structure dumps that can be marshaled and unmarshaled more quickly. There is a master textual format for the entire database, the terminfo capability file. That file (or individual entries) can be compiled to binary form with the terminfo compiler tic(1); binary entries can be decompiled to the editable text format by infocmp(1).

The design superficially contradicts the advice we gave in Chapter5 against binary caches, but this is actually the extreme case in which that's a good tactic. Edits to the text masters are very rare — in fact, Unixes normally ship with the terminfo database precompiled and the text master serving primarily as documentation. Thus, the synchronization and inconsistency problems that would normally militate against this approach almost never arise.

The designers of terminfo could have optimized for speed in a second way. The entire database of binary entries could have been put in some kind of big opaque database file. What they actually did instead was more clever and more in the Unix spirit. Terminfo entries live in a directory hierarchy, usually on modern Unixes under /usr/share/terminfo. Consult the terminfo(5) man page to find the location on your system.

If you look in the terminfo directory, you'll see subdirectories named by single printable characters. Under each of these are the entries for each terminal type that has a name beginning with that letter. The goal of this organization was to avoid having to do a linear search of a very large directory; under more modern Unix file systems, which represent directories with B-trees or other structures optimized for fast lookup, the subdirectories won't be necessary.

I found that even on a fairly modern Unix, splitting a big directory up into subdirectories can improve performance substantially. It was tens of thousands of files, an authorized-user database for a big educational institution, on a late-model DEC Alpha running DEC's Unix. (Subdirectories named by first and last letter of name — e.g., "johnson" would be in directory "j_n" — worked best of the schemes we tested. Using the first two letters wasn't nearly as good, because there were a lot of systematically-generated names which differed only toward the end.) This may just say that sophisticated directory indexing is still not as common as it should be... but even so, that makes an organization which works well without it more portable than one which requires it.

-- Henry Spencer

Thus, the cost of opening a terminfo entry is two file system lookups and a file open. But since mining the same entry from one big database would have required a lookup and open for the database, the incremental cost for terminfo's organization is at most one file system lookup. Actually, it's less than that; it's the cost difference between one file system lookup and whatever retrieval method the one big database would have used. This is probably marginal, and quite tolerable once per application at startup time.

Terminfo uses the file system itself as a simple hierarchical database. This is a superb bit of constructive laziness, obeying the Rule of Economy and the Rule of Transparency. It means that all the ordinary tools for navigating, examining and modifying the file system can be used to navigate, examine, and modify the terminfo database; no special ones (other than tic(1) and infocmp(1) for packing and unpacking the individual records) need to be written and debugged. It also means that work on speeding up database access would be work on speeding up the file system itself, tuning that would benefit many more applications than just users of curses(3).

There is one additional advantage of this organization that doesn't come up in the terminfo case; you get to use Unix's permissions mechanism rather than having to invent your own access-control layer with its own bugs. This falls out as a consequence of adopting the “everything is a file” philosophy of Unix rather than trying to fight it.

The terminfo directory layout is rather space-inefficient on most Unix file systems. The entries are usually between 400 and 1400 bytes long, but file systems normally allocate a minimum of 4K for every nonempty disk file. The designers accepted this cost for the same reason they chose a packed binary format, to cut the startup latency of terminfo-using programs to a minimum. Disk capacity for constant price has exploded over a thousandfold since, tending to vindicate that decision.

The contrast with the formats used by the Microsoft Windows registry files is instructive. Registries are property databases used by both Windows itself and applications. Each registry lives in one big file. Registries contain a mix of text and binary data that requires specialized editing tools. The one-big-file approach leads, among other things, to the notorious ‘registry creep’ phenomenon; average access time rises without bound as new entries are added. Because there is no standard API for editing the registry provided by the system, applications use ad-hoc code to edit it themselves, making it notoriously subject to corruption that can lock up the entire system.

Using the Unix file system as a database is a tactic other applications with simple database requirements might do well to emulate. Good reasons not to do it are more likely to have to do with the database keys not naturally looking like filenames than they are with any performance problems. In any case, it's the sort of good fast hack that can be very useful in prototyping.


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The Art of Unix Programming
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