DLL Programmer's Guide for TNS/R Systems

DLL Programmer’s Guide for TNS/R Systems—522203-002
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3 Dynamic Use of DLLs
An important attribute of the DLL facility is that a running PIC program or DLL can load
and open a previously not-loaded DLL and gain access to the symbols it offers. DLLs
invoked this way are called dynamically loaded DLLs.
One advantage of dynamically loading a DLL is that its name need not be known when
the program is constructed; instead, you can add this new DLL to an existing
application without even restarting the application. Also, you do not need to load
infrequently used DLLs when the application is loaded. Instead, you can load and use
these DLLs when needed and unload them when they are no longer required. They
can be reloaded whenever necessary.
Dynamic libraries make it possible to update facilities in a running application. If
specific business functions are implemented in a dynamically loaded DLL, a program
can unload that DLL and load an updated version that supports the same interfaces
with revised algorithms, tables, etc.
This section discusses how to dynamically load and unload a DLL from your running
process and how to link your loadfile with a dynamically loaded DLL.
Dynamic Loading Functions
Dynamic loading and linking use five associated C-language functions, dlopen(),
dlsym(), dlclose(), dlerror() and dlresultcode(). These functions compose
the dynamic library function calls described in this section, and they are declared in a
header file called dlfcn.h and defined in the loader’s library. Therefore, any C or
C++ source file that uses these functions must contain the following:
#include <dlfcn.h>
The same functions have pTAL external declarations in a file named HLDFCN. Both
header files also define parameter types and constants for using with these functions.
These functions are implemented in the public library ZRLDSRL. This library is not
named in LIBCOBEY or libc.obey, and is not supplied automatically by the compiler
driver when it runs the linker for you. Therefore, you must name this library explicitly
when building a loadfile that calls any of these functions. If you run ld manually, you
can add a -lib zrldsrl option in the command stream. If you let the compiler run the
linker, you can provide the option through the compiler command line; see Running the
Linker Through the Compiler on page 5-8.
The following summarizes the dynamic library function calls.
dlopen() loads and opens a specified DLL and the libraries in its loadList if they are
not loaded, and this function returns a handle for the named DLL.
dlsym() returns an address of a named symbol exported by a loadfile associated with
a dlopen handle. The calling process can assign that address to an appropriate
data or function pointer, which becomes a reference to that symbol.