What do all those keywords mean when making a DLL?

Okay, with the help of some of my peers, I've solved the mystery surrounding the seemingly redundant SINGLE/MULTIPLE and SHARED/NONSHARED attributes.

First, some prep up work...

Every DLL needs and gets a data segment ("automatic data segment"). It is the place where the DLL stores all its STATIC data--basically the data declared in the DLL module but declared outside any function AND (I venture) variables declared STATIC within a DLL function as well. The STACK is NOT!! part of a DLL's automatic data segment (thoug it seems to be for an EXE). The stack that is used when 'running' a DLL function is that of the calling thread.

Now here's the clincher...

DLLs CAN ALSO HAVE ADDITIONAL DATA SEGMENTS!! (as can EXEs, I imagine)

Bingo! Now we understand why there are two different sets of attributes--specifically, NONE/SINGLE/MULTIPLE and SHARED/NONSHARED. Though they basically do the same thing, they APPLY to different things.

NONE/SINGLE/MULTIPLE

applies to the one and only default data segment. This is the data segment that is "automatically" created for your DLL (or EXE). I imagine that for 90% of the DLLs written, this is the only type of data segment that exists. Whether or not this data segment is shared or whether a private copy is created for each linking application is A DIRECT FUNCTION OF THIS ATTRIBUTE--'SINGLE'=it's shared, 'MULTIPLE'=everyone get's his own copy.

This SETS THE DEFAULT!! FOR ALL DATA SEGMENTS other than the "automatic" data segment. DEFAULT is the key word, because you can specify, through the SEGMENTS statement, the characteristics of these additional segments on an individual basis (which in all certainty, override this value.)

So...to answer my own question >could {someone} explain what the following statements would >produce:
>
> DATA MULTIPLE SHARED

Every application which links to the DLL will have it's own private copy of the automatic data segment. If the DLL has no additional data segments, the SHARED keyword is meaningless and can be omitted. If there are additional data segments, only one copy of each will exist and they will be shared by all applications (unless a SEGMENTS statement follows which overrides this default for a specific segment) > or
> DATA SINGLE NONSHARED

Every application which links to the DLL will share a single automatic data segment (hence, each app that affects the DLL STATIC data will affect it for all apps). If there are no additional data segments, the NONSHARED keyword is meaningless and can be omitted. If there are, a private copy will be created for each app (unless overriden by a SEGMENTS statement).

Note that in the absence of BOTH a SINGLE/MUTLIPLE and SHARED/NONSHARED keyword, the default is for ALL data segments (automatic and additional ones) to be shared (i.e., only one copy). In the absence of one OR the other, but not both, the property of ALL data segments are mandated by the single keyword. In other words, DATA SINGLE
and
DATA SHARED

do the same exact thing.

Credit: John Cortell