TI3299 - Importing or "wrapping" DLL function calls
- Product: Delphi
- Version: All
- Platform: Windows/Win32
For more complete information on this subject, see:
Importing or "wrapping" dll function calls
First of all, what is a "wrapped" function call?
A wrapped function or set of functions consists
of an entry into the interface section and an
entry into the implementation section (along
with associated constants or types) which corresponds
to a function or set of functions to be imported from
a Dynamic Link Library(DLL). A wrapper is simply a
declaration in a PASCAL unit that provides an entry
Point into a DLL. With Delphi, this wrapper is
represented by a unit file containing object pascal
code. The Delphi development team has already
conveniently wrapped many standard Windows controls
and functions for you. On occasion it may become
necessary to create wrappers for dll function calls
that are not already wrapped in Delphi.
The first and often the most difficult step in this
process is locating information about the function(s).
One of the best sources for locating documentation of
the function(s) in question is the World Wide Web.
An extensive search beginning with the MSDN and
extending to the numerous available search engines will
often reveal some pertinent information.Search C++ header
files in a product like Borland C++ or MS Visual C++ to
get the structure of the function calls. Type conversion
and calling conventions are generally able to be resolved
when converting between C++ and PASCAL. A good resource
for compatibility between Delphi and C++ can be found
on the Borland web site at
"http://www.borland.com/delphi/papers/brick.html".
After locating an example or documentation of the DLL in
question the next step is to create a new unit file. The
interface of the unit will contain the types and constants
that are specific to the function calls of the DLL along
with the function headers. These function headers are the
Object PASCAL interface that is being provided for other
Delphi applications to call the DLL function. Once
the interface section of the unit is complete the next
section is the implementation. The implementation section
of the unit contains declarations of the imported external
functions. These headers are not identical to those found in
the interface section of the unit (these contain the actual
function identifiers plus other important implementation
information). For a more thorough treatment of this subject
see the help topic "DLLs:accessing procedures and functions"
in the Delphi 3 help file.
For example, say there exists a function called BOB in a DLL
called 'BLODGE.DLL'.
1) WWW Research on the function BOB reveals that it returns a
boolean and takes a word and a boolean as it's arguments.
2) Create a new unit file named 'UseBob.pas' via Delphi
(File|New and choose unit)
3) The following line of code would go in the interface section
of the new unit:
function BOB(Fire: Word, Dances: Boolean): Boolean; stdcall
4) The following line of code would go in the implementation section
of the new unit:
function BOB; external 'BLODGE';
5) Save the unit, name it 'UseBob.pas'.
6) It is also necessary to make sure that UseBob.pas
is in the same directory of the current project or that it
resides in a directory that is in the Delphi search path.
7) Add the 'UseBob' unit to a new project's uses clause. The
function BOB can be called from this new project just like any
other standard function.
8) At runtime BLODGE.DLL must be in the path of the current
process's environment.
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