Visual Basic

Library and object Files

From Table 7-3, you'll notice that VBAEXE6.LIB is linked in with our own OBJ file (created from our files and modules). The library contains just one component (library files contain object files), NATSUPP.OBJ. (NATSUPP might stand for "native support.") You can find this object by using DUMPBIN /ARCHIVEMEMBERS VBAEXE6.LIB. (DUMPBIN.EXE is the Microsoft Common Object File Format [COFF] Binary File Dumper.) NATSUPP.OBJ can be extracted for further examination using the Microsoft Library Manager, LIB.EXE:

  lib /
  extract:c:\vbadev\r6w32nd\presplit\vbarun\obj\natsupp.obj vbaexe6.lib

The reason for including the path to the OBJ file is that the library manager expects us to specify exactly the name of the module-including its path. (This is embedded into the library file when the object file is first put into it and is discovered using DUMPBIN /ARCHIVEMEMBERS.) In other words, the object file probably "lived" at this location on someone's machine in Redmond! Similarly, we can tell that the source code for this object file was named NATSUPP.ASM and was in the directory C:\VBADEV\RT\WIN32. It was assembled using Microsoft's Macro Assembler, Version 6.13. (6.11 is the latest version available to the public, I believe.) Interestingly, it doesn't contain any code-just data-although what looks like a jump table (a mechanism often used to facilitate calls to external routines) appears to be included. To call a routine, you look up its address in the table and then jump to it, as shown in Table 7-6.

Table 7-6 Contents of NATSUPP.OBJ

Name Size Content
.text 0 Readable code
.data 4 Initialized readable writable data
.debug$S 140 Initialized discardable readable data
.debug$T 4 Initialized discardable readable data
The sections are as follows:
  • .text is where all the general-purpose code created by the compiler is output. (It's 0 bytes big, which probably means no code!)
  • .data is where initialized data is stored.
  • .debug$S and .debug$T contain, respectively, CodeView Version 4 (CV4) symbolic information (a stream of CV4 symbol records) and CV4 type information (a stream of CV4 type records), as described in the CV4 specification.

As well as statically linking with this library file, other object files reference exported functions in yet another library file, MSVBVM60.DLL This is a rather large DLL installed by the Visual Basic 6 Setup program in the WINDOWS\SYSTEM directory. (The file describes itself as Visual Basic Virtual Machine and at the time of writing was at version 6.00.8176 if you look a the version string.) Using DUMPBIN /EXPORTS MSVBVM60.DLL on this DLL yields some interesting symbolic information. For example, we can see that it exports a number of routines, 635 in fact! Some interesting-looking things, possibly routines for invoking methods and procedures, are in here as well: MethCallEngine and ProcCallEngine. Additionally, there are what look like stubs, prefixed with rtc ("run-time call," perhaps?), one for apparently all the VBA routines: rtcIsArray, rtcIsDate, rtcIsEmpty, … rtcMIRR , … rtcMsgBox, … rtcQBColor, and so on. And as with most DLLs, some cryptic, yet interesting exports, such as Zombie_Release, are included.

In addition to this symbolic information, the DLL contains a whole bunch of resources, which we can extract and examine using tools such as Visual C++ 6. Of all the resources the DLL contains, the one that really begs examination is the type library resource. If we disassemble this using OLEVIEW.EXE, we can see its entire type library in source form.

The type library contains all sorts of stuff as well as the interface definitions of methods and properties, such as the hidden VarPtr, ObjPtr, and StrPtr routines.

It turns out that this MSVBVM60.DLL is probably the run-time support DLL for any Visual Basic 6 native and p-code executable; that is, it acts like MFC42.DLL does for an MFC application. (MFC stands for Microsoft Foundation Classes, Microsoft's C++/Windows class libraries.) We can confirm this by dumping a built native code executable. Sure enough, we find that the executable imports routines from the DLL. (By the way, the Package And Deployment Wizard also lists this component as the Visual Basic Runtime.)

By dumping other separate object files, we can gather information about what is defined and where it is exported. For example, we can use DUMPBIN /SYMBOLS MODULE1.OBJ to discover that a function named Beep will be compiled using Microsoft's C++ name decoration (name mangling) regime and thus end up being named ?Beep@Module1@@AAGXXZ. Presumably, this function is compiled as a kind of C++ anyway; that is, in C++ it is defined as (private: void __stdcall Module1::Beep(void)). Or better yet, we can use DUMPBIN /DISASM ????????.OBJ to disassemble a module.

The same routine-Beep-defined in a class, Class1 for example, looks like this:

  ?Beep@Class1@@AAGXXZ (private: void __stdcall Class1::Beep(void)).

Maybe now we can see why, since Visual Basic 4, we've had to name modules even though they're not multiply instantiable. Each seems to become a kind of C++ class. According to the name decorations used, Beep is a member of the C++ Classes Class1 and Module1.