The use of something like:
int x;
within a header file is, according to the C Standard, a tentative definition.
Any tentative definition for which an actual definition does not exist within a translation unit becomes a definition at the end of the translation unit when it is compiled. This means that the above use within a header file does result in multiple definitions of x if the header file is included in multiple translation units.
It is therefore perfectly reasonable for IAR to issue a diagnostic, but the standard does not require this behaviour (it is a link-time issue). GCC will do the same with the default -fno-common, but not with -fcommon.
As @Andrew Neil says above, it really is much better to use extern.
