>> EXTERN int my_util_var;    // always declared, but only defined if
compiling myutils.c

All of these ways of tricking the compiler into a definition instead of
a declaration are, I think, somewhat questionable.

Anything declared at file level has extern linkage, unless all
declarations of it within the file are given static linkage. Actually
specifying extern linkage does not change the standard semantics. The
presence of an initializer is what makes an extern declaration also a
definition, at least in C90 ("ANSI C"). What to do when there are
declarations but no definitions, or multiple definitions, is up to the
implementation and should be documented.

The K&R compiler used a "FORTRAN COMMON" model for storage with extern
linkage: the lack of initialization put the storage into a section that
was initialized to binary zeroes, multiple definitions were treated as
last definition wins. A lot of compilers and linkers implement this
model, but again, it is implementation dependent.

Incidentally, // for comments is a C99-ism.

Thanks.


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