VC++ DLL and Delphi PChars
Dick Rowe
and at work with PowerBuilder v6. everything works fine from PowerBuilder,
but I am having the following 'difficulties' getting some of it to work with
Delphi.
The Delphi "interface section" includes these imported functions from the
dll ...
function Count_Chars( Source : Pchar; LookFor : Pchar ) : integer; cdecl;
function Get_Token( Source : Pchar; Separator : Pchar ) : Pchar; cdecl;
function Get_String_Map( Map : Pchar; Data : Pchar; Key : Pchar; Token :
Pchar ) : Pchar; cdecl;
function Get_Next_Map( Data : Pchar; Key : Pchar; Token : Pchar ) : Pchar;
cdecl;
function IsArchiveBitSet( FileName : PChar ) : boolean; cdecl;
function ClearArchiveBit( FileName : PChar ) : integer; cdecl;
The functions above which return boolean or integer values to Delphi from
the dll work fine in both the Delphi IDE and from the created Delphi EXE.
The problem I am having is that the functions expecting a PChar (char *)
from the dll work fine when I run the Delphi program from the Delphi IDE,
but crash with a memory write violation when executing the Delphi EXE file
from disk.
What am I missing? I assume that I have to set a compiler or linker switch
in Delphi, but don't know what it might be.
Alex
I had the same problem. The only workaround I found was to get rid of
the PChar's and use pointer to array of chars, smth. like that:
szFileName : array[0..MAX_PATH] of char;
and use @szFileName. You can try the same approach.
Alex
ErgoLight Usability Software
Visit us at www.ergolight-sw.com
Peter Below (TeamB)
It has nothing to do with any switches, it has to do with the fact that a
PChar is a pointer type. Declaring a variable
Var
Buf: PChar;
Gives you 4 bytes of storage to hold a pointer, it does not allocate memory
for the character buffer itself. And you need to do that if you want to pass
Buf to a DLL function that is supposed to copy characters into the buffer. And
you better be able to tell the DLL how large the buffer is, or else you need a
way to figure out how large the buffer needs to be to fit the data.
Use StrAlloc to allocate memory for PChar variables, or declare the variables
as array [0..x] of char instead. This really gives you x+1 bytes of storage
and Delphi will automatically pass the address of the buffer when you try to
pass such a variable to a Pchar parameter.
Oh, you should not use Boolean as a return type in functions that are to be
used in non-delphi environments. Use LongBool instead, or BOOL.
> function Get_Token( Source : Pchar; Separator : Pchar ) : Pchar; cdecl;
> function Get_String_Map( Map : Pchar; Data : Pchar; Key : Pchar; Token :
> Pchar ) : Pchar; cdecl;
> function Get_Next_Map( Data : Pchar; Key : Pchar; Token : Pchar ) : Pchar;
> cdecl;
I always shudder when I see functions that return a PChar. This is generally a
receipe for desaster. You will note
that the API contains no functions of this type, for a good reason (who
owns the returned pointer and has to free it?). If a DLL routine needs
to return data as a zero-terminated string it has to receive a
preallocated buffer (specifically: a pointer to such a buffer) from the
caller, hopefully together with information on the buffers size. A
Delphi routine would use StrLCOpy or StrPLCopy to copy data into such a
buffer. If the data can vary widely in size there should be a way for
the application to ask the DLL what size the buffer needs to be (can be
a separate function, can be handled by returning the required size if
the buffer is too small).
Peter Below (TeamB) 10011...@compuserve.com)
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