Load-Time Dynamic Linking

Load-time dynamic linking is similar to static linking in that an application can call a routine that is not found in the application's source file. In load-time dynamic linking, however, an application is linked with a library file that contains a record that describes where the routine can be found instead of with a file that contains the code for the routine. The resulting application executable file contains this record and not a copy of the routine's code. The following figure illustrates the process of building a load-time dynamically linked application.

In the example in the following figure, the LIB file contains a record that describes where the code for a set of functions can be found. In this case, the code for the function Your_Routine can be found in the file, or module, Your_Routines.DLL under the entry point name Your_Routine. (The entry point name does not have to match the function name.) The function code also can be referenced by its ordinal value.

   My_Application.OBJ
  ЪДДДДДДДДДДДДДДДДДДДДДДДДї
  і EXTERNAL               і
  і     Your_Routine       і
  ГДДДДДДДДДДДДДДДДДДДДДДДДґ
  і .                      і                  My_Application.EXE
  і .                      і                 ЪДДДДДДДДДДДДДДДДДДї
  і CALL ???; Your_Routine ГДДї              і .                і
  і                        і  і  ЪДДДДДДї    і .                і
  АДДДДДДДДДДДДДДДДДДДДДДДДЩ  ГДі LINK ГДДДі CALL ???         і
                              і  АДДДДДДЩ    і .                ГДї
   Your_Library.LIB           і              і                  і  і
  ЪДДДДДДДДДДДДДДДДДДДДДДДДї  і              ГДДДДДДДДДДДДДДДДДДґ  і
  і                        і  і              і Reference to     і  і
  ГДДДДДДДДДДДДДДДДДДДДДДДДґ  і              і                  і  і
  і function name:         і  і              і Your_Routine.1   і  і
  і       Your_Routine     і  і              і Your_Routine.    ГДДЩ
  і                        і  і              і     Your_Routine і
  і module name:           і  і              і                  і
  і       Your_Routines    ГДДЩ              АДДДДДДДДДДДДДДДДДДЩ
  і                        і
  і entry point            і
  і       ordinal value: 1 і
  і       entry point name і
  і           Your_Routine і
  ГДДДДДДДДДДДДДДДДДДДДДДДДґ
  і                        і
  АДДДДДДДДДДДДДДДДДДДДДДДДЩ

Dynamic Linking

When the application is loaded, the system resolves the dynamic-link references, as shown in the following figure.

My_Application.EXE               Your_Routine.DLL
ЪДДДДДДДДДДДДДДДДДї              ЪДДДДДДДДДДДДДДДДДї
і  .              і              і   Entry Table   і
і  .              і              і       1    ДДДДДЕДДї
і  .              і              і.................і  і
і Call ???        іДї           і                 і  і
і  .              і  і           і                 і  і
і                 і  і   yyy:    і  Your_Routine:  іДЩ
і                 і  і           і                 і
і.................і  і           і                 і
іReference to     і  і           і                 і
і                 і  і           і                 і
іYour_Routine.1   ГДДЩ           АДДДДДДДДВДДДДДДДДЩ
іYour_Routine.    і                       і
і     Your Routineі                       і
АДДДДДДДВДДДДДДДДДЩ                       і
        і                                 і
        АДДДДДДДДДДДДДДДДДВДДДДДДДДДДДДДДДЩ
                          і
                       ЪДДБДДї
                       іLOAD і
                       АДДВДДЩ
                          і
        ЪДДДДДДДДДДДДДДДДДБДДДДДДДДДДДДДДДї
        і                                 і
My_Application code              Other code
ЪДДДДДДДДДДДДДДДДДї              ЪДДДДДДДДДДДДДДДДДї
і   Call yyy  ДДДДЕДДДДД yyy:   і  Your_Routine   і
АДДДДДДДДДДДДДДДДДЩ              АДДДДДДДДДДДДДДДДДЩ

Resolving Dynamic Link References

If a program contains dynamically linked references, the system must process the information in the references. If the DLL already is in memory, the system adds information to the executable code so that the code can use the DLL functions. If the required DLLs are not already in memory, OS/2 searches the path specified by the LIBPATH environment variable. If the system cannot find the DLLs, it stops loading the application and reports the error. If the system finds the DLLs, it loads them. When DLLs are loaded into memory, OS/2 notifies the application where the DLL functions can be found.

When a DLL is loaded into memory is determined by how the DLL was built. A DLL is built like an application, using a module-definition (.DEF) file. The CODE statement in a DEF file describes the attributes of application or DLL code. The load option for the CODE statement determines when application or DLL code is loaded:

PRELOAD

Code is loaded as soon as a process accesses the DLL. This leads to increased performance (in terms of accessing the DLL functions) while decreasing available memory. This option might be preferable if only one program is running.

Code is loaded when needed. This is the recommended choice and the default.

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