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<hr><p align="left"><small>发信人: strayli (stray), 信区: Delphi <br>
标 题: Importing or "Wrapping" DLL Function Calls <br>
发信站: BBS 水木清华站 (Tue Sep 22 18:22:58 1998) <br>
<br>
Importing or "wrapping" dll function calls <br>
<br>
Two methods exist for importing or loading functions <br>
from a Dynamic Link Library(DLL). The first method <br>
(which is discussed extensively in this document) is <br>
called "implicit" loading. Implicit loading involves <br>
loading the DLL statically at startup and accessing <br>
the functions through an Object PASCAL interface. <br>
This method should be used when an application is totally <br>
dependent upon the loading of a DLL for proper functioning. <br>
The other method available is referred to as "explicit" <br>
loading because the DLL is loaded dynamically on demand. <br>
This method requires a little more coding and should be <br>
used if the application needs to run even if the DLL fails <br>
to load properly. <br>
<br>
What is a "wrapped" function call? <br>
<br>
A wrapped function or set of functions consists <br>
of an entry into the interface section and an <br>
entry into the implementation section (along <br>
with associated constants or types) which corresponds <br>
to a function or set of functions to be imported from <br>
a DLL. A wrapper is simply a declaration in a PASCAL <br>
unit that provides an entry Point into a DLL. With Delphi, <br>
this wrapper is represented by a unit file containing <br>
object pascal code. The Delphi development team has <br>
already conveniently wrapped many standard Windows <br>
controls and functions for you. On occasion it may become <br>
necessary to create wrappers for dll function calls <br>
that are not already wrapped in Delphi. <br>
<br>
The first and often the most difficult step in this <br>
process is locating information about the function(s). <br>
One of the best sources for locating documentation of <br>
the function(s) in question is the World Wide Web. <br>
An extensive search beginning with the MSDN and <br>
extending to the numerous available search engines will <br>
often reveal some pertinent information.Search C++ header <br>
files in a product like Borland C++ or MS Visual C++ to <br>
get the structure of the function calls. Type conversion <br>
get the structure of the function calls. Type conversion <br>
and calling conventions are generally able to be resolved <br>
when converting between C++ and PASCAL. A good resource <br>
for compatibility between Delphi and C++ can be found <br>
on the Borland web site at: <br>
"http://www.borland.com/delphi/papers/brick.html". <br>
<br>
After locating an example or documentation of the DLL in <br>
question the next step is to create a new unit file. The <br>
interface of the unit will contain the types and constants <br>
that are specific to the function calls of the DLL along <br>
with the function headers. These function headers are the <br>
Object PASCAL interface that is being provided for other <br>
Delphi applications to call the DLL function. Once <br>
the interface section of the unit is complete the next <br>
section is the implementation. The implementation section <br>
of the unit contains declarations of the imported external <br>
functions. These headers are not identical to those found in <br>
the interface section of the unit (these contain the actual <br>
function identifiers plus other important implementation <br>
information). For a more thorough treatment of this subject <br>
see the help topic "DLLs:accessing procedures and functions" <br>
in the Delphi 3 help file. <br>
<br>
For example, say there exists a function called BOB in a DLL <br>
called 'BLODGE.DLL'. (detailed below are the steps required to <br>
implicitly load the DLL) <br>
<br>
1) WWW Research on the function BOB reveals that it returns a <br>
boolean and takes a word and a boolean as it's arguments. <br>
<br>
2) Create a new unit file named 'UseBob.pas' via Delphi <br>
(File|New and choose unit) <br>
<br>
3) The following line of code would go in the interface section <br>
of the new unit: <br>
<br>
function BOB(Fire: Word; Dances: Boolean): Boolean; stdcall; <br>
<br>
4) The following line of code would go in the implementation <br>
section of the new unit: <br>
<br>
function BOB; external 'BLODGE'; <br>
<br>
5) Save the unit, name it 'UseBob.pas'. <br>
5) Save the unit, name it 'UseBob.pas'. <br>
<br>
6) It is also necessary to make sure that UseBob.pas <br>
is in the same directory of the current project or that it <br>
resides in a directory that is in the Delphi search path. <br>
<br>
7) Add the 'UseBob' unit to a new project's uses clause. The <br>
function BOB can be called from this new project just like any <br>
other standard function. <br>
<br>
8) At runtime BLODGE.DLL must be in the path of the current <br>
process's environment. <br>
<br>
The steps required to explicitly load the 'BLODGE.DLL' are <br>
slightly different and involve a bit of coding. As before a <br>
knowledge of the function/procedure arguments is necessary <br>
(and a result type if it's a function). <br>
<br>
What follows is a unit that implements the BOB function call <br>
on a button click event: <br>
<br>
unit UDLLTest; <br>
<br>
<br>
interface <br>
<br>
uses <br>
Windows, Messages, SysUtils, Classes, Graphics, Controls, <br>
Forms, Dialogs, StdCtrls; <br>
<br>
type <br>
TForm1 = class(TForm) <br>
Button1: TButton; <br>
procedure Button1Click(Sender: TObject); <br>
private <br>
{ Private declarations } <br>
public <br>
{ Public declarations } <br>
end; <br>
<br>
{ Here's a type which points to our bob function } <br>
TBOB = function(Fire: Word; Dances: Boolean): Boolean; stdcall; <br>
<br>
var <br>
Form1: TForm1; <br>
<br>
<br>
implementation <br>
<br>
{$R *.DFM} <br>
<br>
procedure TForm1.Button1Click(Sender: TObject); <br>
var <br>
BOB: TBOB; <br>
hDLLInst: THandle; <br>
IsAlive,IsDancing: Boolean; <br>
Years: Word; <br>
<br>
begin <br>
{ Load a handle to our BLODGE.DLL } <br>
hDLLInst:= LoadLibrary('BLODGE.DLL'); <br>
{ If the load was unsuccessful raise a custom exception } <br>
if (hDLLInst <= 0) then <br>
raise exception.create('[LoadLibrary Fail]'); <br>
{ Try to load the address of the BOB function } <br>
try <br>
@BOB:= GetProcAddress(hDLLInst,'BOB'); <br>
if not assigned(BOB) then <br>
raise exception.Create('[GetProcAddress Fail]'); <br>
Years:= 25; <br>
IsDancing:= True; <br>
{ Now we can execute the BOB function } <br>
IsAlive:= BOB(Years,IsDancing); <br>
finally <br>
{ Free the handle to the DLL } <br>
FreeLibrary(hDLLInst); <br>
end; <br>
end; <br>
end. <br>
<br>
-- <br>
※ 来源:·BBS 水木清华站 bbs.net.tsinghua.edu.cn·[FROM: 210.45.208.4] <br>
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