Interfacing C functions
C programming language is good for writing optimized low layer and system side code. However, C programming language may require complex code for writing GUI front-end codes.
VB, VBA, VBScript, JavaScript, are programming languages and scripting languages to write frontend applications and also automate tasks in Office and Windows operating systems. GUI and frontend are easy to implement in VB and good they are excellent for rapid development. So the basic idea is to call our C code from the frontend applications and thus to utilize the best of both worlds. We will discuss how to bridge between a VB, VBA, VB Script application, and C code executable in a dynamic link library.
VB calls DLL
Visual Basic compiler /linker generates the same type of output binary as of C compiler. Both compilers push arguments from right to left and call the function. Low level machine instructions are the same. So applications written in Visual Basic can access C functions. Functions have to reside inside a dynamic link library(dll). Program loader loads dynamic link library either from current path or from absolute location or searchs all locations given in PATH variable.
VB vs C function prototypes
Apart from dll function address, Visual Basic also need function prototype so that it can push input variables in stack before calling the function. Calling C Functions should be of type stdcall. Stdcall cleans its own stack or callee cleans the stack. So all cleanups are done in Dll side. All Windows DLL works like this. We have below example showing how VB can call a function using call by value and call by reference.
C function prototypes
int extern "C" __stdcall __declspec(dllexport)
VBCallCByVal
(
char c,
short s,
long l,
float f,
double d,
LPCSTR str
);
int extern "C" __stdcall __declspec(dllexport)
VBCallCByRef
(
char *c,
short *s,
long *l,
float *f,
double *d,
LPSTR *str
)
VB function prototypes
Public Declare Function VBCallCByVal Lib "VBDLL.dll"
(
ByVal c As Byte, _
ByVal s As Integer, _
ByVal l As Long, _
ByVal f As Single, _
ByVal d As Double, _
ByVal str As String
) As Integer
Public Declare Function VBCallCByRef Lib "VBDLL.dll"
(
ByRef c As Byte, _
ByRef s As Integer, _
ByRef l As Long, _
ByRef f As Single, _
ByRef d As Double, _
ByRef str As String
) As Integer
C DLL project
We are creating a Win32 Dynamic Link Library VC++ project. We are selecting this project as Simple Dynamic Link Library Project.
VC++ editor will create the necessary project files including header and C/C++ implementation file.
DllMain() function will be implemented by default by the editor. We need to add VBCallCByVal, VBCallCByRef prototypes and the body of the functions.
/* C Windows DLL (VBDLL.dll) Source code */
#include <windows.h>
#include <stdio.h>
BOOL APIENTRY DllMain
(
HANDLE hModule,
DWORD ul_reason_for_call,
LPVOID lpReserved
)
{
switch (ul_reason_for_call)
{
case DLL_PROCESS_ATTACH:
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
}
return TRUE;
}
int extern "C" __stdcall __declspec(dllexport)
VBCallCByVal
(
char c,
short s,
long l,
float f,
double d,
LPCSTR str
)
{
char str_m [100];
sprintf ( str_m,
"Char %c, Short 0x%X, Long %ld,
Float %f, Double %lf, String \"%s\"",
c, (int)s, l, f, d, str);
MessageBoxA(NULL, (const char *)str_m, "Message from C", MB_OK);
return 0;
}
int extern "C" __stdcall __declspec(dllexport)
VBCallCByRef
(
char *c,
short *s,
long *l,
float *f,
double *d,
LPSTR *str
)
{
char str_m[100];
sprintf ( str_m,
"Char %c, Short 0x%X, Long %ld,
Float %f, Double %lf, String \"%s\"",
*c, (int)*s, *l, *f, *d, *str);
MessageBoxA(NULL, (const char *)str_m, "Values from VB", MB_OK);
*c = *c + 1;
*s = *s + 1;
*l = *l + 1;
*f = *f + 1;
*d = *d + 1;
strcpy(*str,"String from C");
return 0;
}
VB Project
We are creating a simple VB application project to call the C function.
We have added a button in the main form and added the calling code inside the click event of the button.
This is however not limited to VB. VBA, VB script and Javascript can call the function in the same way as VB does.
Public Declare Function VBCallCByVal Lib "VBDLL.dll"
(
ByVal c As Byte, _
ByVal s As Integer, _
ByVal l As Long, _
ByVal f As Single, _
ByVal d As Double, _
ByVal str As String
) As Integer
Dim c As Byte
Dim i As Integer
Dim l As Long
Dim f As Single
Dim d As Double
Dim s As String
Private Sub Command1_Click()
c = CByte(65)
i = CInt(12)
l = CLng(200)
f = CSng(1.2)
d = CDbl(4.8)
s = CStr("VB String")
Label1.Caption = "Char " + CStr(Chr(c)) + _
", Short " + CStr(i) + _
", Long " + CStr(l) + vbCrLf
Label1.Caption = Label1.Caption + _
"Float " + CStr(f) + _
", Double " + CStr(d) + _
", String " + s
r = VBCallCByVal(c, i, l, f, f, s)
Label2.Caption = "Char " + CStr(Chr(c)) + _
", Short " + CStr(i) + _
", Long " + CStr(l) + vbCrLf
Label2.Caption = Label2.Caption + _
"Float " + CStr(f) + _
", Double " + CStr(d) + _
", String " + s
End Sub
Public Declare Function VBCallCByRef Lib "VBDLL.dll"
(
ByRef c As Byte, _
ByRef s As Integer, _
ByRef l As Long, _
ByRef f As Single, _
ByRef d As Double, _
ByRef str As String
) As Integer
Dim c As Byte
Dim i As Integer
Dim l As Long
Dim f As Single
Dim d As Double
Dim s As String
Private Sub Command2_Click()
c = CByte(65)
i = CInt(12)
l = CLng(200)
f = CSng(1.2)
d = CDbl(4.8)
s = CStr("String from VB")
Label1.Caption = "Char " + CStr(Chr(c)) + _
", Short " + CStr(i) + _
", Long " + CStr(l) + vbCrLf
Label1.Caption = Label1.Caption + "Float " + CStr(f) + _
", Double " + CStr(d) + _
", String " + s
r = VBCallCByRef(c, i, l, f, d, s)
Label2.Caption = "Char " + CStr(Chr(c)) + _
", Short " + CStr(i) + _
", Long " + CStr(l) + vbCrLf
Label2.Caption = Label2.Caption + _
"Float " + CStr(f) + _
", Double " + CStr(d) + _
", String " + s
End Sub
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