XPLINK provides compatibility with non-XPLINK functions when calls are made across executable modules, using either the DLL or fetch() call mechanism. You should make a reference from XPLINK code into non-XPLINK code only if the reference is by an imported function or variable, or the function pointer is a parameter into the XPLINK code. This prevents incompatible references to a non-XPLINK function entry point.
A function pointer from a non-XPLINK application can be used as a callback by passing it as an argument into the XPLINK function, or as a member of a structure that is itself an argument to the XPLINK function.
Prior to z/OS V1R8, a function entry point from a non-XPLINK application only could be passed explicitly as an argument into a XPLINK function. This restriction did not apply if you used the compiler option XPLINK(CALLBACK) or the __callback qualifier where any such function pointers were used . Existing DLLs compiled using one of these options do not need to be recompiled. The use of these options can only be discontinued if the owner of the XPLink-compiled DLL is certain that any non-XPLink-compiled DLL callers have been recompiled with z/OS XL C/C++ V1R8 targetting z/OS Language Environment V1R8 or later, and those applications are targetted for and running on z/OS Language Environment V1R8 or later.