Q35032: Don’t Place Communal Data in Libraries

Article: Q35032
Product(s): See article
Version(s): 3.00 4.00 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_LIB S_LINK lib link | mspl13_c
Last Modified: 8-SEP-1988

Do not declare unitialized (communal) data in modules that will
be placed in libraries. Doing so may result in the module not
being included at link time.

This information is documented in the last paragraph on Page 84 of the
"Microsoft C Language Reference" manual, which states the following:

"Unitialized variable declarations at the external level are
not recommended for any file that might be placed in a library."

Microsoft C supports four different storage classes for variables
declared at the external level, as follows:

1.  Static variables are local to the module and do not generate
    external references or public definitions, i.e., no other modules
    may reference these variables.

2.  Initialized variables, without static, are allocated storage
    and generate public definitions, i.e., other modules may share
    these variables.

3.  Uninitialized variables, with extern, are not allocated
    storage and they generate external references, i.e, this module
    isn't declaring the data, but it wants to share the data that other
    modules declare.

4.  Communal variables are global variables that are not initialized
    or given the extern modifier. A communal variable is one that is
    used by everybody but not necessarily declared public by anybody;
    the linker will allocate storage for it, if needed.

    Communal variables do not have external references or public
    definitions. When the program is linked, if a public definition
    with the same name is found, all communal declarations are treated
    as external references. If no definition is found, the linker will
    allocate storage and again the communal declarations are treated
    like references. When a module is added to a library, the librarian
    takes all public definitions and adds them to the dictionary to be
    searched when the linker resolves references to the library.

Note: Communal variables are NOT added to the dictionary. To do so
would cause conflicts with other communal declarations and a possible
public definition.

A good way to declare data in a multi-module program is to use a
single module that contains public definitions while every other
module uses the extern modifier. You do not need to actually
initialize all variables in this one module as long as one of them is
initialized. This process is sufficient to force the module to be
brought in while linking.

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