How can I receive “char *d” from DLL in VBA(Excel Visual Basic)?

Question

Now I'm trying to get a char string as the below c function.

This is a C function of DLL,

char _stdcall *fmt_hex(long long num, int nbits, char *d)
{
    sprintf(d, "0x%0*llX", (nbits - 1) / 4 + 1, num);
    return (d);
}

and the below is the Excel VBA function.

Option Explicit

Private Declare PtrSafe Function pll_dll Lib "F:\work\pll_dll\x64\Debug\pll_dll.dll" (ByVal num As LongLong, ByVal nbits As Integer, ByRef d As String)

Dim d As String

Sub useSquareInVBA()
    
    Cells(4, 4).Text = pll_dll(3, 4, d)

End Sub

but I've got a program halt when I run this code. Can you give me what am I supposed to do to resolve this problem?

Update2

When I ran with this code "C"

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double _stdcall pll_dll(double* datain0, double* datain1, double* dataout0, double* dataout1,  char * str)
{
    dataout0[0] = datain0[0] + 10;
    dataout1[0] = datain1[0] + 10;
    
    str ="c";
    
    return 0;
}

"VBA"

Option Explicit

Private Declare PtrSafe Function pll_dll Lib "F:\work\pll_dll\x64\Debug\pll_dll.dll" _
(ByRef x_in As Double, ByRef y_in As Double, ByRef x_out As Double, ByRef y_out As Double, ByRef str As String) As Double

Dim Error As Integer

Dim d1 As Double
Dim d2 As Double
Dim d3 As Double
Dim d4 As Double
Dim sometext As String


Function pll_dll_excel(data1 As Double, data2 As Double, data3 As Double, data4 As Double, text As String) As Double

pll_dll_excel = pll_dll(data1, data2, data3, data4, text)
    
End Function

Sub useSquareInVBA()
    MsgBox pll_dll_excel(3, 4, d1, d2, sometext)
    Cells(5, 5).Value = d1
    Cells(6, 6).Value = d2
    Cells(7, 7).Value = sometext
End Sub

No, program halt, but sometext is always empty "" value.

Can you help me please?

update3

I've checked with the article from here.

How to return a string of unknown size from DLL to Visual Basic

When I follow this article, I realized that is not "C". How do I make as a "C"?

Answer 1

I ran into a similar issue and this post is the most relevant search result.

Instead of giving a direct solution to the question (credit to GSerg for bringing out the correct approach), am posting a generic version of how I achieved receiving char array passed as reference:

C function (shared library):

int func(int a, double b, char *result) {
    ...
    result[0] = 'A';
    result[1] = 'B';
    result[2] = '\0';
    return EXIT_SUCCESS;
}

VBA:

Option Explicit

' Import DLL function
Private Declare PtrSafe Function func Lib "C:\path\to\c-shared-library.dll" _
    (ByVal a As Integer, ByVal b As Double, ByVal result As String) As Integer

' Functions for use by Excel - DLL functions cannot be directly used
Function getResult(ByVal a As Integer, ByVal b As Double) As String
    Dim status As Integer
    Dim result As String
    result = Space$(2)
    status = func(a, b, result)
    If status = 0 Then getResult = result Else Err.Raise 1000, "Error", "Failed to get result"
End Function

In my case the result are always going to be a two char array, but I think the above methodology can be accordingly modified.