Dart 如何使用C語言函式庫 (Windows)

　　雖然Dart 語言本身支援跨平台的編譯方式，但在實務開發時還是不免需要使用外部非Dart語言所提供的函式庫進行功能開發且由於C 語言是最為廣泛且通用的程式語言，因此Dart語言也有提供支援與C語言函式庫互通性的方式；本篇主要是以MSVC作為C的編譯器來實作說明如何引用C語言會遇到的作法。

1.　　動態載入C語言函式庫

#include <windows.h> 
 
// 1. Define function pointer for imported function prototypes.
typedef BOOL(__cdecl* CreateClassInstanceProc) (__inout HANDLE* ClassHandle);
// 2. Open and load DLL  
HINSTANCE hinstLib = LoadLibrary(TEXT("crosscode_wrapc_lib.dll"));
// 3. Find function addresses in loadded DLLs.
CreateClassInstanceProc CCIP =
　　(CreateClassInstanceProc) GetProcAddress(hinstLib, "CreateClassInstance");

import 'dart:ffi'
 
// 1. Define function pointer for imported function prototypes.
typedef CreateClassInstanceForNative = Int32 Function(Pointer classHandle);
typedef CreateClassInstanceForDarte = int Function(Pointer classHandle);
// 2. Open and load DLL  
final _wrapc_lib = 
　　DynamicLibrary.open("crosscode_wrapc_lib.dll");
// 3. Find function addresses in loadded DLLs.
final createClassInstanceProc = _wrapc_lib.lookupFunction<CreateClassInstanceForNative,　CreateClassInstanceForDarte>('CreateClassInstance');

2.　　定義資料結構

typedef bool (*CallbackFunc)(double* pDoubleArray);
typedef bool (*CopyStructCallbackFunc)(PVOID pStruct);
typedef struct _UnrestrictedAlignmentStruct {
　　int	intField;
　　TCHAR charField;
　　CallbackFunc funcptrField;
　　float floatField;
　　wchar_t stringField[15];
　　double dubaryField[10];
} UnrestrictedAlignmentStruct, * pUnrestrictedAlignmentStruct;
#pragma pack(push, 1)
typedef struct _RestrictedAlignmentStruct {
　　int	intField;
　　TCHAR charField;
　　CallbackFunc funcptrField;
　　float floatField;
　　wchar_t stringField[15];
　　double dubaryField[10];
} RestrictedAlignmentStruct, *pRestrictedAlignmentStruct;
#pragma pack(pop)

import 'dart:ffi';
typedef CallbackFuncForDart = int Function( Pointer<Double> pDoubleArray);
typedef CallbackFuncForFFI = Uint32 Function(Pointer<Double> pDoubleArray);
typedef CopyStructCallbackFuncForFFI = Bool Function(Pointer pStruct);
class UnrestrictedAlignmentStruct extends Struct {
　　@Int32()
　　external int intField;　　
　　@Uint16()
　　external int charField;  
　　external Pointer<NativeFunction<CallbackFuncForFFI>> funcptrField;　　
　　@Float()
　　external double floatField;　　
　　@Array(15)
　　external Array<Int16> stringField;　　
　　@Array(10)
　　external Array<Double> dubaryField; 
}
@Packed(1)
class RestrictedAlignmentStruct extends Struct {
　　@Int32()
　　external int intField;
　　@Int16()
　　external int charField;　　
　　external Pointer<NativeFunction<CallbackFuncForFFI>> funcptrField;　　
　　@Float()
　　external double floatField;
　　@Array(15)
　　external Array<Int16> stringField;
　　@Array(10)
　　external Array<Double> dubaryField; 
}

3.　　實作測試函式

　　3-1.　　記憶體配置的長度

// Create a data struct in C executables
UnrestrictedAlignmentStruct sUnrestrictedAlignmentStruct = { 0 };
// Define the function prototype of the C library
typedef int(__stdcall* GetSizeOfUnrestrictedAlignmentStructProc) 
　　　　　　　　(HANDLE pClassHandle, UnrestrictedAlignmentStruct sStrut);
// Find the the pointer of function in the C library
GetSizeOfUnrestrictedAlignmentStructProc getSizeOfUnrestrictedAlignmentStructProc =
　　　　　　　　(GetSizeOfUnrestrictedAlignmentStructProc)GetProcAddress(hinstLib, "GetSizeOfUnrestrictedAlignmentStruct");
                  
// Display the memory size of data structures in executeables and libraries.
wprintf(L"Used memory size of an unrestricted alignment struct in execute(%llu) and library(%d)\n", 
　　　　　　　　sizeof(sUnrestrictedAlignmentStruct),
　　　　　　　　getSizeOfUnrestrictedAlignmentStructProc(hinstLib, sUnrestrictedAlignmentStruct));

// Create a data struct in Dart executables
final sUnrestrictedAlignmentStruct = calloc<UnrestrictedAlignmentStruct>()
// Define the function prototype of the C library
typedef GetSizeOfUnrestrictedAlignmentStructProcForNative = 
　　　　int Function(Pointer classHandle,RestrictedAlignmentStruct confirmStrut);
typedef GetSizeOfUnrestrictedAlignmentStructProcForDart = 
　　　　Int32 Function(Pointer classHandle,RestrictedAlignmentStruct confirmStrut);
// Find the the pointer of function in the C library
late final getSizeOfUnrestrictedAlignmentStructProc = _wrapc_lib.lookupFunction<
　　　　GetSizeOfUnrestrictedAlignmentStructProcForDart,
　　　　GetSizeOfUnrestrictedAlignmentStructProcForNative
　　>('GetSizeOfUnrestrictedAlignmentStructProc');
// Display the memory size of data structures in executeables and libraries.
int retSize = getSizeOfUnrestrictedAlignmentStructProc(classHandle.value, sUnrestrictedAlignmentStruct.ref);
print("Used memory size of an unrestricted alignment struct in execute(${sizeOf<UnrestrictedAlignmentStruct>()}) and library( $retSize)");

　　3-2.　　字元轉換的正確性　　　　

// Build and initial data structure in the C executable.
static const TCHAR* pCmpStr = L"Test String";
UnrestrictedAlignmentStruct sUnrestrictedAlignmentStruct = { 0 };
wcsncpy_s(sUnrestrictedAlignmentStruct.stringField, wcslen(pCmpStr) + 1, pCmpStr, sizeof(sUnrestrictedAlignmentStruct.stringField));
// Define the function prototype of the C library.
typedef bool (__stdcall* CompareStringOfUnrestrictedAlignmentStructProc)
　　　　　　(HANDLE pClassHandle, pUnrestrictedAlignmentStruct pStrut);
// Find the the pointer of function in the C library.
CompareStringOfUnrestrictedAlignmentStructProc compareUnrestrictedAlignmentStructProc =
　　　　　　(CompareStringOfUnrestrictedAlignmentStructProc)GetProcAddress(hinstLib, "CompareStringOfUnrestrictedAlignmentStruct");
        
// Compare the string of data structure in the C library.
wprintf(L"Compare string in C library : %s\n",
　　　　　　compareUnrestrictedAlignmentStructProc(hinstLib, &sUnrestrictedAlignmentStruct) == 1 ? L"True " : L"False");

// Convert string to int array and copy to the stringField of structure.
extension StringArrayFill<T> on Array<Int16> {
　　void fillFromList(List<T> list) {
　　　　for (var i = 0; i < list.length; i++) {
　　　　　　this[i] = list[i] as int;
}}}
// Build and initial data structure in the Dart executable.
final String pCmpStr = "Test String";
final sUnrestrictedAlignmentStruct = calloc<UnrestrictedAlignmentStruct>()
　　　　..ref.stringField.fillFromList(pCmpStr.codeUnits);
// Define the function prototype of the C library.
typedef CompareStringOfUnrestrictedAlignmentStructProcForNative = int Function(Pointer classHandle,Pointer<UnrestrictedAlignmentStruct> pStruct);
typedef CompareStringOfUnrestrictedAlignmentStructProcForDart = Int32 Function(Pointer classHandle,Pointer<UnrestrictedAlignmentStruct> pStruct);
// Find the the pointer of function in the C library.
late final compareStringOfUnrestrictedAlignmentStructProc = _wrapc_lib.lookupFunction<
　　　　CompareStringOfUnrestrictedAlignmentStructProcForDart,
　　　　CompareStringOfUnrestrictedAlignmentStructProcForNative
　　>('CompareStringOfUnrestrictedAlignmentStruct');
// Compare the string of data structure in the C library
String isSame = compareStringOfUnrestrictedAlignmentStructProc(classHandle.value, sUnrestrictedAlignmentStruct) == 1 ? "True" : "False";  
print("Compare string in C library : $isSame&quot;);   

　　3-3.　　回調函數的執行

// Define the prototype of the callback function
typedef int (*CallbackFunc)(double* pDoubleArray);
typedef bool (*CopyStructCallbackFunc)(PVOID pStruct);
// Define the callback function of data structure.
int comparArray(double *cmpArray) {
　　for (int i = 0; i < 10; i++) {
　　　　if (cmpArray[i] != notifyDouble[i]) 
　　　　　　return 0;
　　}
　　return 1;
}
// Define double array 
static double notifyDouble[] = {
　　0, 0.1, 2.2, 3.33, 44.44, 55.555, 666.666, 777.7777,
　　8888.8888, 99999.9999 };
// Define the function prototype of the C library.
typedef bool(__stdcall* NotifyCallbackOfUnrestrictedAlignmentStructProc)
　　　　　　(HANDLE pClassHandle, pUnrestrictedAlignmentStruct pStruct);
// Build and initial data structure in the C executable.
UnrestrictedAlignmentStruct sUnrestrictedAlignmentStruct = { 0 };
sUnrestrictedAlignmentStruct.funcptrField = comparArray;
for (int i = 0; i < 10; i++) {
　　sUnrestrictedAlignmentStruct.dubaryField[i] = notifyDouble[i];
}
// Find the the pointer of function in the C library.
NotifyCallbackOfUnrestrictedAlignmentStructProc notifyCallbackOfUnrestrictedAlignmentStructProc =
　　　　　　(NotifyCallbackOfUnrestrictedAlignmentStructProc)GetProcAddress(hinstLib, "NotifyCallbackOfUnrestrictedAlignmentStruct");
  
// Execute callback function for unrestricted alignment structure in C library.
wprintf(L"Execute callback function in C library : %d\n", 
　　　　　　notifyCallbackOfUnrestrictedAlignmentStructProc(hinstLib, &sUnrestrictedAlignmentStruct));

// Define the prototype of the callback function
typedef CallbackFuncForDart = int Function( Pointer<Double> pDoubleArray);
typedef CallbackFuncForFFI = Uint32 Function(Pointer<Double> pDoubleArray);
typedef CopyStructCallbackFuncForFFI = Bool Function(Pointer pStruct);
// Define the callback function of data structure.
int checkArray(List<double> cmpArray){
　　for (int i =0; i < cmpArray.length;i++){
　　　　if (cmpArray.elementAt(i) != notifyDouble.elementAt(i)) {  
　　　　　　return 0;
　　}}
　　return 1;
}
// Define double array 
final List<double> notifyDouble = [ 0, 0.1, 2.2, 3.33, 44.44, 55.555, 666.666, 777.7777,
　　　　8888.8888, 99999.9999];
// Define the function prototype of the C library.
typedef NotifyCallbackOfUnrestrictedAlignmentStructProcForNative = Int32 Function(Pointer pClassHandle, Pointer<UnrestrictedAlignmentStruct> pStruct);
typedef NotifyCallbackOfUnrestrictedAlignmentStructProcForDart = int Function(Pointer pClassHandle, Pointer<UnrestrictedAlignmentStruct> pStruct);
// Build and initial data structure in the Dart executable.
final sUnrestrictedAlignmentStruct = calloc<UnrestrictedAlignmentStruct>()
　　　　..ref.funcptrField = Pointer.fromFunction<CallbackFuncForFFI>(comparArray, 0)      
// Find the the pointer of function in the C library.
late final notifyCallbackOfUnrestrictedAlignmentStructProc = wrapclib.lookupFunction<
　　　　NotifyCallbackOfUnrestrictedAlignmentStructProcForNative,
　　　　NotifyCallbackOfUnrestrictedAlignmentStructProcForDart
　　>('NotifyCallbackOfUnrestrictedAlignmentStruct');
// Execute callback function for unrestricted alignment structure in C library.
isSame = notifyCallbackOfUnrestrictedAlignmentStructProc(classHandle.value, sUnrestrictedAlignmentStruct) == 1 ? "True":"False";
print("Execute callback function in C library : $isSame");

　　3-4.　　驗證資料結構的資料

// Define a callback function to display replicated data. 
bool getUnrestrictedAlignmentStructProc(PVOID pStruct) {    
　　pUnrestrictedAlignmentStruct pUAS = (pUnrestrictedAlignmentStruct) pStruct;
　　wprintf(L"stringField : %s    floatField : %f     intField : %d     charField : %c \n",
        
　　pUAS->stringField, pUAS->floatField, pUAS->intField, pUAS->charField);
　　wprintf(L"Verify the data of dubaryField : %s\n", comparArray(pUAS->dubaryField) == 1 ? L"True" : L"False");
　　bool bRetVal = pUAS->funcptrField(notifyDouble) == 1;
　　wprintf(L"Execute the function porinter of pointerField : %s", bRetVal ? L"True" : L"False");
　　return bRetVal;
}
// Define the function prototype of the C library.
typedef bool(__stdcall* CopyDataOfUnrestrictedAlignmentStructWithCallbackProc) 
　　　　　　(HANDLE pClassHandle, UnrestrictedAlignmentStruct sStruct, CopyStructCallbackFunc pCopyCallbackFunc);
// Build and initial data structure in the C executable.
UnrestrictedAlignmentStruct sUnrestrictedAlignmentStruct = { 0 };
sUnrestrictedAlignmentStruct.charField = 'A';
sUnrestrictedAlignmentStruct.floatField = 456.123f;
sUnrestrictedAlignmentStruct.intField = 123;
sUnrestrictedAlignmentStruct.funcptrField = comparArray;
wcsncpy_s(sUnrestrictedAlignmentStruct.stringField, wcslen(pCmpStr) + 1, pCmpStr, sizeof(sUnrestrictedAlignmentStruct.stringField));
for (int i = 0; i < 10; i++) {
　　sUnrestrictedAlignmentStruct.dubaryField[i] = notifyDouble[i];
}
// Find the the pointer of function in the C library.
CopyDataOfUnrestrictedAlignmentStructWithCallbackProc copyDataOfUnrestrictedAlignmentStructWithCallbackProc =
　　　　　　(CopyDataOfUnrestrictedAlignmentStructWithCallbackProc) GetProcAddress(hinstLib, "CopyDataOfUnrestrictedAlignmentStructWithCallback");
       
// Validate each field of the replicated data structure in C library.
wprintf(L"\nCopy structure data in C library : %d\n", 
　　　　　　copyDataOfUnrestrictedAlignmentStructWithCallbackProc(hinstLib, sUnrestrictedAlignmentStruct, getUnrestrictedAlignmentStructProc));

// Define a callback function to display replicated data. 
int getUnrestrictedAlignmentStructProc(Pointer pStruct){
　　int iRetVal = 0;
　　var pUAS = pStruct.cast<UnrestrictedAlignmentStruct>();
　　List<int> strList = [];
　　List<double> doubleList =[] ;

　　pUAS.ref.dubaryField.fillFromArray(doubleList,10);
　　pUAS.ref.stringField.fillFromArray(strList, 15);
　　print ('stringField : ${String.fromCharCodes (strList)}   floatField : ${pUAS.ref.floatField}   intField : ${pUAS.ref.intField}   charField : ${String.fromCharCode(pUAS.ref.charField)}');

　　var pPassParm = calloc.allocate<Double>(sizeOf<Double>() * 10);
　　for (int i = 0;i<10;i++) {
　　　　pPassParm[i] = doubleList[i];
　　}

　　String isSame = comparArray(pPassParm) == 1 ? "True" : "False";
　　print("Verify the data of dubaryField : $isSame");

　　final callbackFuncProc = pUAS.ref.funcptrField
　　　　　　.cast<NativeFunction<CallbackFuncForFFI>>().asFunction<CallbackFuncForDart>();

　　iRetVal = callbackFuncProc(pPassParm);
　　isSame = iRetVal == 1 ? "True" : "False";
　　print ("Execute the function porinter of pointerField : $isSame");

　　calloc.free(pPassParm);
　　return iRetVal;
}
// Define the function prototype of the C library.
typedef CopyDataOfRestrictedAlignmentStructWithCallbackProcForNative = Int32 Function(Pointer classHandle, RestrictedAlignmentStruct sStruct, Pointer<NativeFunction<CopyStructCallbackFuncForFFI>> pCopyCallbackFunc);
typedef CopyDataOfRestrictedAlignmentStructWithCallbackProcForDart = int Function(Pointer classHandle, RestrictedAlignmentStruct sStruct, Pointer<NativeFunction<CopyStructCallbackFuncForFFI>> pCopyCallbackFunc);
 
// Build and initial data structure in the Dart executable.
final sUnrestrictedAlignmentStruct = calloc<UnrestrictedAlignmentStruct>()
　　　　　　..ref.charField = 'A'.codeUnitAt(0)     
　　　　　　..ref.floatField = 456.123
　　　　　　..ref.intField = 123
　　　　　　..ref.funcptrField = Pointer.fromFunction<CallbackFuncForFFI>(comparArray, 0)       
　　　　　　..ref.stringField.fillFromList(pCmpStr.codeUnits)
　　　　　　..ref.dubaryField.fillFromList(notifyDouble);   
// Find the the pointer of function in the C library.
late final copyDataOfUnrestrictedAlignmentStructWithCallbackProc = wrapclib.lookupFunction<
　　　　CopyDataOfUnrestrictedAlignmentStructWithCallbackProcForNative,
　　　　CopyDataOfUnrestrictedAlignmentStructWithCallbackProcForDart
　　>('CopyDataOfUnrestrictedAlignmentStructWithCallback');
// Validate each field of the replicated data structure in C library.
isSame =  copyDataOfUnrestrictedAlignmentStructWithCallbackProc(
　　　　　　　　classHandle.value, 
　　　　　　　　sUnrestrictedAlignmentStruct.ref, 
　　　　　　　　Pointer.fromFunction<CopyStructCallbackFuncForFFI>(getUnrestrictedAlignmentStructProc, false)
　　　　　　) == 1 ? "True" : "False";     
print("Copy structure data in C library : $isSame");

　　3-5.　　回傳資料結構的完整性

// Default alignment according to computer architecture.
typedef struct _UnrestrictedAlignmentStruct {
　　　　int intField;
　　　　TCHAR charField;
　　　　CallbackFunc funcptrField;
　　　　float floatField;
　　　　wchar_t stringField[15];
　　　　double dubaryField[10];
} UnrestrictedAlignmentStruct, * pUnrestrictedAlignmentStruct;
// Default alignment according to pack function.
#pragma pack(push, 1)
typedef struct _RestrictedAlignmentStruct {
　　　　int intField;
　　　　TCHAR charField;
　　　　CallbackFunc funcptrField;
　　　　float floatField;
　　　　wchar_t stringField[15];
　　　　double dubaryField[10];
} RestrictedAlignmentStruct, *pRestrictedAlignmentStruct;
#pragma pack(pop)

// Default alignment according to computer architecture.
class UnrestrictedAlignmentStruct  extends Struct {
　　@Int32()
　　external int intField;
　　@Uint16()
　　external int charField;  
　　external Pointer<NativeFunction<CallbackFuncForFFI>> funcptrField;
　　@Float()
　　external double floatField;
　　@Array(15)
　　external Array<Int16> stringField;
　　@Array(10)
　　external Array<Double> dubaryField; 
}
// Default alignment according to pack function.
@Packed(1)
class RestrictedAlignmentStruct  extends Struct {
　　@Int32()
　　external int intField;
　　@Int16()
　　external int charField;  
　　external Pointer<NativeFunction<CallbackFuncForFFI>> funcptrField;
　　@Float()
　　external double floatField;
　　@Array(15)
　　external Array<Int16> stringField;
　　@Array(10)
　　external Array<Double> dubaryField; 
}

4.　　自動轉換C語言標頭檔套件

　　4-1.　　設定流程

// pubspec.yaml
dev_dependencies:
　　ffigen: ^4.0.0

name: AutoConvertByffigen
description: A starting point for Dart libraries or applications.
output: 'lib/crosscode_by_auto.dart'
headers:
　　entry-points:   
　　　　- 'third_party/crosscode_wrap_cfunc.h'   
   
　　# 只轉換下列的header文件而其內所引用的相關header文件將不會被轉換
　　include-directives:  
　　　　- '**crosscode_defstruct.h'
　　　　- '**crosscode_wrap_cfunc.h'
structs:
　　include:
　　　　- '.*' 
　　exclude:
　　　　- '_ExcludeUnrestrictedAlignmentStruct'  
　　# 移除起始"_"符號並加入FFI字串
　　rename:
　　　　'_(.*)': 'FFI$1'
# 此選項中只有被引用的typedef才會進行轉換
typedefs:
　　# 若要直接引用其原始定義而不要建立其對應的定義
　　exclude:
　　　　- 'CallbackFunc2'
　　　　- 'UChar'
　　# 將轉換後的定義變數名稱加入ForFFI字串
　　rename: 
　　　　'(.*)': '$1'
functions: 
　　include:
　　　　- '.*' 
　　# 指定排除轉換的函式
　　exclude:
　　　　- 'ExcludeFunction' 
# 指定型別對應至NativeType型別  
typedef-map:
　　'TCHAR': 'Int16' 
comments:
　　style: any
　　length: full
preamble:
　　/* ************** Cross Code Co.Ltd ********************
　　how to auto convert from C to dart by ffigen packages       
　　***************************************************** */

name: crosscode_wrapc_exe
description: A sample command-line application.
version: 1.0.0
homepage: https://vocus.cc/user/61ca9f1bfd89780001efa2a6
environment:
　　sdk: '>=2.15.1 <3.0.0'
dependencies:
　　ffi: ^1.1.0
dev_dependencies:
　　lints: ^1.0.0
　　test: ^1.16.0
　　ffigen: ^4.0.0
ffigen:
　　name: AutoConvertByffigen
　　description: Convert Header file of C to dart file of Dart by ffigen package
　　output: 'lib/crosscode_by_auto.dart'
　　headers:
　　　　entry-points:   
　　　　　　- 'third_party/crosscode_wrap_cfunc.h'   
　　　　# 只轉換下列的header文件而其內所引用的相關header文件將不會被轉換
　　　　include-directives:  
　　　　　　- '**crosscode_defstruct.h'
　　　　　　- '**crosscode_wrap_cfunc.h'
　　structs:
　　　　include:
　　　　　　- '.*' 
　　　　exclude:
　　　　　　- '_ExcludeUnrestrictedAlignmentStruct' 
　　　　# 移除起始"_"符號並加入FFI字串
　　　　rename:
　　　　　　'_(.*)': 'FFI$1'
　　# 此選項中只有被引用的typedef才會進行轉換
　　typedefs:
　　　　# 若要直接引用其原始定義而不要建立其對應的定義
　　　　exclude:
　　　　　　- 'CallbackFunc2'
　　　　　　- 'UChar'
　　　　# 將轉換後的定義變數名稱加入ForFFI字串
　　　　rename: 
　　　　　　'(.*)': '$1'
　　functions: 
　　　　include:
　　　　　　- '.*' 
　　　　# 指定排除轉換的函式
　　　　exclude:
　　　　　　- 'ExcludeFunction' 
　　# 指定C型別對應至NativeType型別
　　typedef-map:
　　　　'TCHAR': 'Int16' 
　　comments:
　　　　style: any
　　　　length: full
　　preamble:
　　/* ************** Cross Code Co.Ltd ********************
　　　　how to auto convert from C to dart by ffigen packages       
　　***************************************************** */

GitHub : https://github.com/crosscode-software/dart_wrap_c