Question

When I’m generating a text file programatically, should I insert the ASCII EOF marker (decimal value 26) at the end of the file?

Do the .NET Programming Languages do this automatically?

Answer 1

There’s no reason for the ^Z EOF marker anymore (and there hasn’t been for a long time) – it’s a hold over from CP/M which did not support exact lengths for file sizes in the directory – file sizes were in terms of the number of 128 byte blocks, so to end a file on a non-128 byte boundary you had to use an EOF character.

Since early versions of MS-DOS were heavily influenced by CP/M (and Microsoft wanted CP/M programs to port easily), the convention stuck.

Your program should open text files with the appropriate attributes so the OS and/or language runtime will signal an EOF when it sees a ^Z in case you come across a file that uses the convention. But there’s no need to write one anymore.

One possible exception is if you have a binary file, but want to put some text at the start of it, then a ^Z, then your data. If someone dumps it to the console it’ll say something intelligent instead of spewing garbage. There’s not a whole lot of reason to do this, but I’ve seen that done rarely.

From Wikipedia (http://en.wikipedia.org/wiki/CP/M):

File size was specified as the number
of 128-byte records (directly
corresponding to disk sectors on
8-inch drives) occupied by a file on
the disk. There was no generally
supported way of specifying byte-exact
file sizes. The current size of a file
was maintained in the file’s file
control block (FCB) by the operating
system. Since many application
programs (such as text editors) prefer
to deal with files as sequences of
characters rather than as sequences of
records, by convention text files were
terminated with a control-Z character
(ASCII SUB, hexadecimal 1A).
Determining the end of a text file
therefore involved examining the last
record of the file to locate the
terminating control-Z. This also meant
that inserting a control-Z character
into the middle of a file usually had
the effect of truncating the text
contents of the file.