Loading a DLL explicitly

The use of DLLs can also be explicitly controlled by C/C++ application code at the source level. The application uses explicit source-level calls to one or more runtime services to connect the reference to the definition. The connections for the reference and the definition are made at run time.

The DLL application writer can explicitly call the following C runtime services:

dllload(), which loads the DLL and returns a handle to be used in future references to this DLL
dllqueryfn(), which obtains a pointer to a DLL function
dllqueryvar(), which obtains a pointer to a DLL variable
dllfree(), which frees a DLL loaded with dllload()

The following runtime services are also available as part of the Single UNIX Specification, Version 3:

dlopen(), which loads the DLL and returns a handle to be used in future references to this DLL.
dlclose(), which frees a DLL that was loaded with dlopen().
dlsym(), which obtains a pointer to an exported function or exported variable.
dlerror(), which returns information about the last DLL failure on this thread that occurred in one of the dlopen() family of functions.

While you can use both families of explicit DLL services in a single application, you cannot mix usage across those families. So a handle returned by dllload() can be used only with dllqueryfn(), dllqueryvar(), or dllfree(). And a handle returned by dlopen() can be used only with dlsym() and dlclose().

Since the dlopen() family of functions are part of the Single UNIX Specification, Version 3, they should be used in a new application if cross-platform portability is a concern.

For more information about the C runtime services, see z/OS XL C/C++ Runtime Library Reference.

To explicitly call a DLL in your application:

Determine the names of the exported functions and variables that you want to use. You can get this information from the DLL provider's documentation or by looking at the definition side-deck file that came with the DLL. A definition side-deck is a directive file that contains an IMPORT control statement for each function and variable exported by that DLL.
If you are using the dllload() family of functions, include the DLL header file <dll.h> in your application. If you are using the dlopen() family of functions, include the DLL header file <dlfcn.h> in your application.
Compile your source as usual.
Bind your object with the binder using the same AMODE value as the DLL.
Note: You do not need to bind with the definition side-deck if you are calling the DLL explicitly with the runtime services, since there are no references from the source code to function or variable names in the DLL for the binder to resolve. Therefore the DLL will not be loaded until you explicitly load it with the dllload() or dlopen() runtime service.

Explicit use of a DLL in a C application and Explicit use of a DLL in a application using the dlopen() family of functions are examples of applications that use explicit DLL calls.