Building a DLL application

The DLL application may consist of multiple source modules. Some of the source modules may contain references to imported functions, imported variables, or both.

To use a load-on-call DLL in your DLL application, perform the following steps:

  1. Write the DLL application code. Write it as you would if the functions were statically bound. Assembler code that will access imported functions and/or imported variables must use the Language Environment macros.
  2. Compile the DLL application code.
    • Compile the C source files with the following compiler options:
      • LP64
      • RENT
      • LONGNAME

      These options instruct the compiler to generate special code when calling functions and referencing external variables.

    • Compile your C source files with the following compiler options:
    • Compile your C++ source files with the LP64 compiler option.
    • Assembler DLL application source files must be assembled with the GOFF option.
  3. Bind the DLL application code.
    • The binder option CASE(MIXED) is required when binding DLL applications that use mixed-case exported names.
    • The binder options RENT, DYNAM(DLL), and COMPAT(PM4) or COMPAT(CURRENT) are required.
    Include the definition sidedeck from the DLL provider in the set of object modules to bind. The binder uses the definition sidedeck to resolve references to functions and variables defined in the DLL. If you are referencing multiple DLLs, you must include multiple definition side-decks.
    Note: Because definition side-decks in automatic library call (autocall) processing will not be resolved, you must use the INCLUDE statement.

After final autocall processing of DD SYSLIB is complete, all DLL-type references that are not statically resolved are compared to IMPORT control statements. Symbols on IMPORT control statements are treated as definitions, and cause a matching unresolved symbol to be considered dynamically rather than statically resolved. A dynamically resolved symbol causes an entry in the binder B_IMPEXP to be created. If the symbol is unresolved at the end of DLL processing, it is not accessible at run time.

Addresses of statically bound symbols are known at application load time, but addresses of dynamically bound symbols are not. Instead, the runtime library that loads the DLL that exports those symbols finds their addresses at application run time. The runtime library also fixes up the importer's linkage blocks (descriptors) in C_WSA64 during program execution.

The following code fragment illustrates how a C++ application can use the TRIANGLE DLL described previously (see Writing your C++ DLL code). Compile normally and bind with the definition sidedeck provided with the TRIANGLE DLL. 
   extern int getarea(); /* function prototype */
   main () {
         ...
         getarea();      /* imported function reference */
         ...
   }
The following code fragment illustrates how an assembler routine can use the ADLL6EV2 DLL described previously (see Writing your Language Environment-conforming AMODE 64 assembler DLL code). Assemble and bind with the definition sidedeck provided with the ADLL6EV2 DLL.
Figure 1. Assembler DLL application calling an assembler DLL
DLLAPPL  CELQPRLG PSECT=ADLA6IFP
*
R3       EQU   3             RETURN VALUE
R5       EQU   5             ENVIRONMENT
R6       EQU   6             ENTRY POINT ADDRESS
R7       EQU   7             RETURN POINT ADDRESS
R8       EQU   8             WORK REGISTER
R9       EQU   9             WORK REGISTER
*
         WTO   'ADLA6IV4: Calling imported function dllfunc64',ROUTCDE=11
*
         CELQCALL dllfunc64,WORKREG=10
*
         WTO   'ADLA6IV4: Getting address of imported var DllVar64',   X
               ROUTCDE=11
*
         CEEPLDA DllVar64,REG=9
*
* Set value of imported variable to 789
*
         LA    R8,789
         ST    R8,0(,R9)
*
         WTO   'ADLA6IV4: Done.',ROUTCDE=11
*
         SR    R3,R3
RETURN   DS   0H
         CELQEPLG
*
         CEEPDDA DllVar64,SCOPE=IMPORT
         LTORG
CEEDSAHP CEEDSA SECTYPE=XPLINK
         CEECAA
*
         END      DLLAPPL

See Figure 1 for a summary of the processing steps required for the application (and related DLLs).