For C source code compiled without using the DLL or XPLINK compiler options, that code cannot reference (import) functions or variables that are exported by a DLL. NODLL is the default when compiling C source code, and the XPLINK compiler option is not used. C source code compiled with the DLL or XPLINK compiler options, and all C++ source code, can reference exported functions and variables. Source code that can reference exported functions and variables is called DLL application code. It need not itself be a DLL, in that it may not itself export any functions or variables.
When compiling DLL application source code, the compiler generates object code in such a way that references to external functions and variables can be resolved statically or dynamically (that is, resolved to a DLL). If you are uncertain whether non-XPLINK C source code references a DLL, you should specify the DLL or XPLINK compiler options. Compiling source code as DLL application code eliminates the potential compatibility problems that may occur when binding DLL application code with non-DLL application code. See Building complex DLLs for more information on compatibility issues.
The decision to use XPLINK needs to be made independently from the decision to build a DLL application. While XPLINK compiled code is always DLL application code, the XPLINK and non-XPLINK function call linkages are different. There is DLL compatibility for XPLINK and non-XPLINK at the DLL boundary, but XPLINK and non-XPLINK object modules cannot be mixed in the same DLL. Also, there is a performance penalty when transitioning between XPLINK and non-XPLINK DLLs (and vice versa). It is best to have a DLL application made up of all XPLINK or all non-XPLINK executable modules to the extent that is possible. For more information on XPLINK, see Using the XPLINK option.