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The 32-bit edition of Visual C++ uses _declspec(dllimport) and
_declspec(dllexport) to replace the __export keyword previously used in
16-bit versions of Visual C++.
You do not need to use _declspec(dllimport) for your code to compile
correctly, but doing so allows the compiler to generate better code. The
compiler is able to generate better code because it knows for sure whether
a function exists in a DLL or not, so the compiler can produce codes that
skip a level of indirection that would normally be present in a function
call that crossed a DLL boundary.
With the proper .DEF file EXPORTS section, _declspec(dllexport) is not
required. _declspec(dllexport) was added to provide an easy way to export
functions from an .EXE or .DLL without using a .DEF file.
The remainder of this article provides a fairly low-level, thorough
discussion of these issues.
The Win32 Portable Executable format is designed to minimize the number of
pages that must be touched to fix imports. To do this, it places all the
import addresses for any program in one place called the Import Address
Table. This allows the loader to modify only one or two pages when
accessing these imports.
Using _declspec(dllimport) for Function Calls
In the following code example, assume func1 is a function that resides in a
DLL separate from the .EXE file that contains the main() function.
Without _declspec(dllimport), given this code:
void main(void) {
func1();
}
the compiler generates code that looks like this:
call func1
and the linker translates the call into something like this:
call 0x4000000 ; The address of 'func1'.
If 'func1' exists in another DLL, the linker can't resolve this directly
because it has no way of knowing what the address of 'func1' is. In 16-bit
environments, the linker adds this code address to a list in the .EXE
that the loader would patch at run-time with the correct address. In 32-bit
environments, the linker generates a thunk for which it does know the
address. The thunk looks like this:
0x40000000: jmp DWORD PTR __imp_func1
Here __imp_func1 is the address for func1's slot in the import address
table of the .EXE file. All the addresses are thus known to the linker. The
loader only has to update the .EXE file's import address table at load time
for everything to work correctly.
Therefore, using _declspec(dllimport) is better because it is better if the
linker does not generate a thunk if it does not have to. Thunks make the
code larger (on RISC systems, it can be several instructions) and can
degrade your cache performance. If you tell the compiler the function is in
a DLL, it can generate an indirect call for you.
So now this code:
__declspec(dllimport) void func1(void);
void main(void) {
func1();
}
generates this instruction:
call DWORD PTR __imp_func1
There is no thunk and no jmp instruction, so the code is smaller and
faster.
On the other hand, for function calls inside a DLL, you don't want to have
to use an indirect call. You already know a function's address. Time and
space are required to load and store the address of the function before an
indirect call, so a direct call is always faster and smaller. You only want
to use __declspec(dllimport) when calling DLL functions from the outside
the DLL itself. Don't use __declspec(dllimport) on functions inside a DLL
when building that DLL.
Using _declspec(dllexport)
Microsoft introduced __export in the 16-bit compiler version to allow the
compiler to generate the export names automatically and place them in a
.LIB file. This .LIB file could then be used just like a static .LIB to
link with a DLL.
Microsoft added __declspec(dllexport) to continue this convenience. Its
purpose is to add the export directive to the object file so you don't need
a .DEF file.
This convenience is most apparent when trying to export decorated C++
function names. There is no standard specification for name decoration, so
the name of an exported function may change between compiler versions. If
you use _declspec(dllexport), recompiling the DLL and dependent .EXE files
is necessary only to account for any naming convention changes.
Many export directives such as ordinals, NONAME, or PRIVATE, can be made
only in a .DEF file, and there is no way to specify these attributes
without a .DEF file. However, using _declspec(dllexport) in addition to
using a .DEF file does not cause build errors.
As a reference, search through the Win32 WINBASE.H header file. It contains
examples of preferred __declspec(dllexport) and __declspec(dllimport)
usage.
Using _declspec(dllexport) and _declspec(dllimport) on Data
In the case of data, using _declspec(dllimport) is a convenience item that
removes a layer of indirection. When you import data from a DLL, you still
have to go through the import address table. In the Win32 days before
_declspec(dllimport), this meant you had to remember to do an extra level
of indirection when accessing data exported from the DLL:
// project.h
#ifdef _DLL // If accessing the data from inside the DLL
ULONG ulDataInDll;
else // If accessing the data from outside the DLL
ULONG *ulDataInDll;
#endif
You would then export the data in your .DEF file:
// project.def
LIBRARY project
EXPORTS
ulDataInDll CONSTANT
and access it outside the DLL:
if (*ulDataInDll == 0L) {
// Do stuff here
}
When you mark the data as __declspec(dllimport), the compiler automatically
generates the indirection code for you. You no longer have to worry about
the steps above. As stated previously, do not use _declspec(dllimport)
declaration on the data when building the DLL. Functions within the DLL
will not use the Import Address Table to access the data object. Therefore,
you will not have the extra level of indirection present.
To export the data automatically from the DLL, use this declaration:
__declspec(dllexport) ULONG ulDataInDLL;
Using a .DEF File
If you choose to use __declspec(dllimport) along with a .DEF file, you
should change the .DEF file to use DATA in place of CONSTANT to reduce the
likelihood that incorrect coding will cause a problem:
// project.def
LIBRARY project
EXPORTS
ulDataInDll DATA
The following chart shows why:
Keyword Emits in the import lib Exports
CONSTANT __imp_ulDataInDll ulDataInDll
__ulDataInDll
DATA __imp_ulDataInDll ulDataInDll
Using _declspec(dllimport)and CONSTANT lists both the __imp_ version and
the undecorated name in the .LIB DLL import library that is created to
allow explicit linking. Using _declspec(dllimport) and DATA lists just the
__imp_ version of the name.
If you use CONSTANT, either of the following code constructs could be used
to access the ulDataInDll:
__declspec(dllimport) ULONG ulDataInDll; /*prototype*/
if (ulDataInDll == 0L) /*sample code fragment*/
ULONG *ulDataInDll; /*prototype*/
if (*ulDataInDll == 0L) /*sample code fragment*/
However, if you use DATA in your .DEF file, only code compiled with the
following definition can access the variable ulDataInDll:
__declspec(dllimport) ULONG ulDataInDll;
if (ulDataInDll == 0L) /*sample code fragment*/
Using CONSTANT is more risky because if you forget to use the extra level
of indirection, you could potentially access the Import Address Table's
pointer to the variable -- not the variable itself. This type of problem
can often manifest as an Access Violation because the Import Address Table
is currently made read-only by the Microsoft compiler and linkers.
The Current Visual C++ linker issues a warning if it sees CONSTANT in
the .DEF file to account for this case. The only real reason to use
CONSTANT is if you can't recompile some object file where the header file
didn't list dllimport on the prototype.