Delphi – Sending Delphi string as a parameter to DLL

delphidllparametersstring

I want to call a DLL function in Delphi 2010. This function takes a string and writes it to a printer with an USB interface. I do not know in which language is the DLL developed. According to the documentation, the syntax of the function is:

int WriteUSB(PBYTE pBuffer, DWORD nNumberOfBytesToWrite);

How can I declare and use my function in Delphi?

I declare the function like this:

var
function WriteUSB(myP:pByte;n:DWORD): integer ; external 'my.dll';

Should I use stdcall or cdecl in the declaration?

I call the DLL function like this:

procedure myProc;
var 
   str : string:
begin
     str := 'AAAAAAAAAAAAAAAAAAAAA';
     WriteUSB(str,DWORD(length(tmp)));
end;

But this code give me exception all the time. I know that the problem is that String is Unicode and each character > 1 byte. I tried to convert to different string types ( AnsiChar and ShortString) but I failed.

What is the correct way to do this?

Best Answer

A couple things. First off, if this is a C interface, which it looks like it is, then you need to declare the import like this:

function WriteUSB(myP:pAnsiChar; n:DWORD): integer; cdecl; external 'my.dll';

Then to call the function, you need to use an Ansi string, and convert it to a PAnsiChar, like so:

procedure myProc;
var 
   str : AnsiString;
begin
     str := 'AAAAAAAAAAAAAAAAAAAAA';
     WriteUSB(PAnsiChar(str), length(str));
end;

(The cast to DWORD is unnecessary.) If you do it like this, it should work without giving you any trouble.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C++ – How to iterate over the words of a string

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