Re: [Code] mail system

From: Jeremy Elson (
Date: 01/22/97

> This is my first post, I am all excited!
> I use stock code with oasisolc and the enhanced editor
> when i send mail the system stops the mud process and returns a message
> saying that ASSERTION FAILED: mail.c line 208
> this is the line and the preceeding one:
> 207:  assert(sizeof(header_block_type) == sizeof(data_block_type));
> 208:  assert(sizeof(header_block_type) == BLOCK_SIZE);
> i simply commented 208 and the problem is fixed!! (mail can be sent)
> Now my question is:
> Anyone has a clue about the consequences? I'd like to know what assert
> does but you don't have to answer that.
> Thanks
> Mehdi Keddache from NYC

Assertions are a good way to make sure that code you release to the world
doesn't break.  For example, if I write an entire program assuming that
n = 4, and release the code, someday someone somewhere can change n to 5
causing my code to fail.  assert(n == 4) will make the program stop if n
is not 4.

The mail system is pretty hard to understand.  It's from the summer of
1992 -- the first major piece of MUD code I ever wrote.  It was part of
the original CircleMUD 1.0 (i.e. when Circle was actually a MUD, not a
code base).

The mail system has its own mini "file system" so that all the mail sent
of varying lengths can be stored somewhat efficiently in a single file.
Remember, this code was written in '92 when 1.5 megs of disk space was all
I could run a MUD in, so keeping disk usage down was a very high priority.
These days, disks are cheap and brain cycles are expensive.. so if I was
rewriting mail today, I would probably just keep the mail in plain text
format, probably one file per receipient, the same way that UNIX stores
mail in mail spools. 

I'm not going to get into a detailed description of the way the mail
"filesystem" works, but suffice it to say that the mail file is broken
down into 100-byte chunks, and each chunk can either be a "data block" or
"header block".  A piece of mail consists of exactly 1 header block
followed by 0 or more data blocks.  The blocks can appear in any order in
the file; each block has a link to the next one.  This works just like the
DOS FAT scheme except that the next-block link is stored along with the
data itself instead of in a seperate FAT as it is in DOS.  To read a piece
of mail one must first find its header block, then follow what can be
thought of as a linked list of data blocks, with each block containing a
pointer to the next.  (The "pointer" is actually a file offset.)

If you're still with me, it should be clear at this point that this scheme
depends heavily on being able to do random access in the file to any
block, and for this each block must be exactly the same length.  The
assertions in mail.c make sure that the definitions of the header and data
blocks were not screwed with, because for header and data blocks to be
different lengths would be fatal.  You should never see this assertion
fail in your code unless you've screwed with mail.c or mail.h.


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