The reason why XML, CSV, and other similar file formats were created was to address the inherent problems with fixed with formats. THe first formats were fixed width because they are very simple to work with. In essence, they are the serialization of an array of structs in C. So, marshalling one of those in C is really simple. Finding a given record when you know its index (10th, 1024th, etc) is very simple. Overwriting a given record is very simple. It's the ultimate in RAM-backed-to-disk. The only problem is that you have to know the mapping. If you don't know what a fixed-width format means, you're out of luck.

And, furthermore, many fixed-width files have a header and, possibly, a footer. DBM::Deep's file format is a record-based format with a two headers (first is fixed, second is variable). Good luck detecting that it's a DBM::Deep file without recognizing the first four bytes.

Frankly, I'd do the following:

  1. Is it XML, CSV, HTML, etc?
  2. Is it a fixed-width format I recognize (PNG, JPG, DOC, XLS, etc)?
  3. Punt.
Which, essentially, is what the file utility does.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re: how to identify a fixed width file by dragonchild
in thread how to identify a fixed width file by ftumsh

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