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op_plugin.obj : error LNK2001: unresolved external symbol "void __cdecl quicksort(char *,unsigned int,unsigned int,int (__cdecl*)(char const *,char const *,unsigned int),unsigned int)" (?quicksort@@YAXPADIIP6AHPBD1I@ZI@Z)
quicksort is defined in qsort.cpp
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fullscreen.obj : error LNK2001: unresolved external symbol "char * __cdecl SkipWhiteSpace(char *)" (?SkipWhiteSpace@@YAPADPAD@Z)
SkipWhiteSpace is defined in Util.cpp
Sorry, those are the only ones I understand. For undeclared constants, you can try searching MSDN to see if there are alternative names supported in VC6, otherwise it's usually fine to just delete them.