Sharing Memory by Different Processes

Now, it's time to consider how different instances of the same application or different processes use the DLL. If you are acquainted with the principles of Windows operation, then such a formulation may stun you. You might say: Every process has its own address space, in which the DLL is loaded. Naturally, this isn't the most efficient approach; however, it is safe. The use of memory will be covered in detail in Chapter 19, but here it is necessary to point out that the application generally can initialize so-called shared memory. I'll return to this problem later and more than once. For the moment, however, consider this problem in relation to DLLs. Consider a practical situation: Suppose that the application loads a DLL and calls a procedure from there, which modifies the data located in that DLL. Now, suppose that the user starts the second instance of the same application. It will also load an instance of the same DLL. It might be desirable to inform the second instance of the application that the DLL data have been changed by the command from the first instance of the same application. Naturally, in this case the data, over which the DLL operates, must be shared. Technically, this is easy to implement. The LINK program has the following option: /section : name , attributes . This allows you to explicitly declare the properties of this section. In this interpretation, the section is simply a segment. Using TLINK32, the same result can be obtained using the DEF file.

The programs presented in Listing 16.6 to illustrate this idea are very simple. Actually, they only demonstrate the use of the shared memory. Before exiting the DLL procedure, the string stored in the shared memory section is modified. The application doesn't terminate its execution. When the next application instance is started, the string modified by the first application instance will be displayed.

Listing 16.6: The use of shared memory in dynamic link libraries

 ; The DLL4.ASM library .586P ; Flat memory model IFDEF MASM   .MODEL FLAT, stdcall ELSE   .MODEL FLAT ENDIF PUBLIC DLLP1 IFDEF MASM ; MASM ; Prototypes of external procedures        EXTERN      MessageBoxA@16:NEAR ; INCLUDELIB directives for the linker        includelib c:\masm32\lib\user32.lib        includelib c:\masm32\lib\kernel32.lib ELSE ; TASM        EXTERN      MessageBoxA:NEAR        MessageBoxA@16 = MessageBoxA        includelib c:\tasm32\lib\import32.lib ENDIF ;--------------------------------------------- ; Data segment _DATA SEGMENT        TEXT DB "String in a DLL", 0        MS   DB "Message", 0 _DATA ENDS ; Code segment _TEXT SEGMENT ; [EBP+10H]   ; Reserved parameter ; [EBP+0CH]   ; Cause of call ; [EBP+8]     ; DLL module identifier DLLENTRY:        MOV    EAX, 1        RET    12 ;------------------- ; Addresses of parameters DLLP1 PROC EXPORT        PUSH   EBP        MOV    EBP, ESP        PUSH   0        PUSH   OFFSET MS        PUSH   OFFSET TEXT        PUSH   0        CALL   MessageBoxA@16 ; Modify the string located in the shared memory        MOV    TEXT, '0'        MOV    TEXT+1, 'f'        POP    EBP        RET DLLP1 ENDP _TEXT ENDS END DLLENTRY ; The DLLEX4.ASM main module that calls ; a procedure from the DLL .586P ; Flat memory model .MODEL FLAT, stdcall ; Constants ; Prototypes of external procedures IFDEF MASM ; MASM        EXTERN      GetProcAddress@8: NEAR        EXTERN      LoadLibraryA@4:NEAR        EXTERN      FreeLibrary@4:NEAR        EXTERN      ExitProcess@4:NEAR        EXTERN      MessageBoxA@16:NEAR ; INCLUDELIB directives for the linker        includelib c:\masm32\lib\user32.lib        includelib c:\masm32\lib\kernel32.lib ELSE ; INCLUDELIB directives for the linker        includelib c:\tasm32\lib\import32.lib        EXTERN      GetProcAddress:NEAR        EXTERN      LoadLibraryA:NEAR        EXTERN      FreeLibrary:NEAR        EXTERN      ExitProcess:NEAR        EXTERN      MessageBoxA:NEAR        GetProcAddress@8 = GetProcAddress        LoadLibraryA@4 = LoadLibraryA        FreeLibrary@4 = FreeLibrary        ExitProcess@4 = ExitProcess        MessageBoxA@16 = MessageBoxA ENDIF ;---------------------------------------------- ; Data segment _DATA SEGMENT        TXT  DB 'DLL error', 0        MS   DB 'Message', 0        LIBR DB 'DLL4.DLL', 0        HLIB DD ? IFDEF MASM        NAMEPROC DB '_DLLP1@O', 0 ELSE        NAMEPROC DB 'DLLP1', 0 ENDIF _DATA ENDS ; Code segment _TEXT SEGMENT ; [EBP+10H]   ; Reserved parameter ; [EBP+0CH]   ; Cause of the call ; [EBP+8]     ; DLL module identifier START: ; Load the library        PUSH   OFFSET LIBR        CALL   LoadLibraryA@4        CMP    EAX, 0        JE     _ERR        MOV    HLIB, EAX ; Get the address        PUSH   OFFSET NAMEPROC        PUSH   HLIB        CALL   GetProcAddress@8        CMP    EAX, 0        JNE    YES_NAME ; Error message _ERR:        PUSH   0        PUSH   OFFSET MS        PUSH   OFFSET TXT        PUSH   0        CALL   MessageBoxA@16        JMP    _EXIT YES_NAME:        CALL   EAX        PUSH   0        PUSH   OFFSET MS        PUSH   OFFSET MS        PUSH   0        CALL.  MessageBoxA@16 ; Close the library ; The library is automatically closed ; when exiting the program        PUSH   OFFSET NAMEPROC        PUSH   HLIB        CALL   FreeLibrary@4 ; Exit _EXIT:        PUSH   0        CALL   ExitProcess@4 _TEXT ENDS END START 

 

To translate this program, issue the following commands using MASM32:

 ml /c /coff /DMASM dll4.asm      link /subsystem:windows /DLL /section:.data, SRW dll2.obj      ml /c /coff /DMASM dllex4.asm      link /subsystem:windows dllex4.obj 

Attributes of the SECTION option are S-SHARED, R-READ , and W-WRITE . Issue the following commands using TASM32:

 tasm32 /ml dll4.asm      tlink32 -aa -Tpd dll4.obj,,,,dll4.def      tasm32 /ml dllex4.asm      tlink32 -aa dllex4.obj 

The contents of the DEF file are as follows :

 SECTIONS .DATA SHARED      EXPORTS DLLP1