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DLL call in Delphi

Hi all,
I am trying to write some functions for a dll which means I need to declare the function, and call the function - sorry im not very good at these!
I have an example piece of code which was written in C++ and this is how it calls the function
RDR_SendCommandGetData(reader_handle, "select", "", buffer);

I have details on how what the functions requires here

RDR_SendCommandGetData // this is the function name
The RDR_SendCommandGetData function sends a command to a reader specified by its
handle and receives data.

Win char* RDR_SendCommandGetData(void* hReader, char* command, char* / this is some kind of declaration or description on the function
data, char* buffer);

Parameter Description // next are the defination of the variables required.
hReader:      Handle of reader to send a command.
command: 0 terminated string with the command e,g one of the commands I know is "select"
data: ASCII 0 terminated string containing the data
                  BIN length of data (1 byte) + data // note when using the select command there is no data
buffer: A buffer of the return value.
                        ASCII protocol: The buffer size has the range of 514 bytes.
                        BIN protocol: The buffer size has the range of 256 bytes.

I am getting confused with the char variable types and think they need to be pointers or something like that, I have put my code below but please appreciate it is wrong as I have been trying allsorts

function declared

function RDR_SendCommandGetData(readerhandle:integer;command:pointer;data:pointer;cardid:pointer):integer; stdcall; external 'Reader.dll' name 'RDR_SendCommandGetData'; // gets card ID

Im trying to call it as follows

readcard := RDR_SendCommandGetData(readerhandle, @command,@buffer,@cardid);
I have set command,buffer, and cardid as string type variables.

I really appreciate any help as my normal programmer is on leave and Im trying to sort this on my own (fat chance without you guys!)

thanks

Dave

2266180

Win char* RDR_SendCommandGetData(void* hReader, char* command, char* data, char* buffer);

translates to

function RDR_SendCommandGetData(hReader:pointer; command:pchar; data:pchar; buffer:pchar):pchar; stdcall; external 'dll_name_here.dll';

monitorwa

ASKER

Thanks ciuly - I dont suppose you could add how you call the function and declare the variables?

thanks

Dave

2266180

just like the parameters...

var
readcard:pchar;
readerhandle:pointer;
command, buffer, cardid:pchar;
begin
initialize readerhandle with some RDR_ call I suppose
command:='select';
buffer:='';// whatever
cardid:='';// whatever
readcard := RDR_SendCommandGetData(readerhandle, command,buffer,cardid);
close readerhandle if needed with some other RDR_ call
end;

monitorwa

ASKER

sorry for the slow response, I have been trying all sorts of things with no success. I have done exactly as you listed above and keep getting an access violation at address 000000

Any odeas?

thanks
Dave

monitorwa

ASKER

ok I think it may be from the way I had opened the port and reader, the instructions process is as follows

RDR_OpenComm
RDR_DetectReader
RDR_OpenReader
RDR_SendCommand
RDR_GetData
RDR_CloseReader
RDR_CloseComm

and the C++ definition is as follows

void* RDR_OpenComm(char* device, char autodetect, struct
presetSettings* settings);

char* RDR_DetectReader(void* hComm, char* buffer);

void* RDR_OpenReader(void* hComm, unsigned char id, short
knownReader);

char* RDR_SendCommandGetData(void* hReader, char* command, char*
data, char* buffer);

My attempts that dont work are as follows - first the defination
function RDR_OpenComm(com_port:pchar;autodetect:char;settings:tdcb):pointer; stdcall; external 'Reader.dll' name 'RDR_OpenComm'; // returns port handle
function RDR_DetectReader(porthandle:pointer;detect_buffer:pchar):pchar; stdcall; external 'Reader.dll' name 'RDR_DetectReader'; // returns reader handle

and called as follows

com_port := 'com4';
porthandle := RDR_OpenComm(com_port,'1',structure);
RDR_DetectReader(porthandle, detect_buffer);

vars are
structure: tdcb;
com_port : pchar;
readerhandle,porthandle, readeropen : pointer;
reply, command1,data,buffernew : string;
readcard :pchar;
command, buffer, cardid , detect_buffer , readerhand:pchar;

I have also attached the document that list all of these and more - Im really struggling here, it appears to get the com handle ok but cant get past the next stage

thanks

Dave

Reader-DLL-V3-3-0-User-Manual-03.zip

monitorwa

ASKER

I guess Ill just have to put this one down as to hard for everyone!
Thanks to Ciurly but his answers didnt actually work

Dave

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2266180

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monitorwa

ASKER

Hi Ciuly,
I really appreciate your assistance so far and fully understand what you are saying - yea who gets spare time, certainly not me either! What I will do is award you the points as I am sure we would get there int he end but I dont need to waste your time anymore as my developer has come back from holidays and I think he has it all working now. I was really just trying to get a better understanding of these things but its not important enough for you to spend a lot of time trying to more or less teach me, so thanks again and maybe you will help with other questions I have