Because z/OS® XL C/C++ supports a DLL function pointer call in non-DLL code, you are able to create a DLL to support both DLL and non-DLL applications. The z/OS XL C/C++ compiler inserts glue code at the beginning of a function descriptor to allow branching to a function descriptor. Glue code is special code that enables function pointer calls from non-DLL code to DLL code, including XPLINK code.
A function pointer in non-DLL code points to the function entry and a function pointer call branches to the function address. However, a DLL function pointer points to a function descriptor. A call made through this pointer in non-DLL code results in branching to the descriptor.
z/OS XL C/C++ executes a DLL function pointer call in non-DLL code by branching to the descriptor and executing the glue code that invokes the actual function.
The following examples and Figure 5 show a DLL function pointer call in non-DLL code, where a simplified sort() routine is used. Note that the sort() routine compiled as non-DLL code can be called from both a DLL application and a non-DLL application.