如何解决带有 Dart ffi 的 MySqlXC 连接器带来错误“NoSuchMethodError: The method 'FfiTrampoline' was called on null.”
我正在尝试使用 Dart 的 ffi 包和 MysqL 的本机 C-Connector 直接连接到带有 Dart 的 MysqL。
我的 Dart 代码是:
import 'dart:ffi';
import 'package:ffi/ffi.dart';
typedef CFun = Pointer Function(Pointer<Utf8>,Int32,Pointer<Utf8>,Pointer<Pointer>?);
typedef DartFun = Pointer Function(Pointer<Utf8>,int,Pointer<Pointer>?);
void main() {
final lib = DynamicLibrary.open('/usr/lib64/libMysqLcppconn8.so.2');
final fc = lib.lookup<NativeFunction<CFun>>('MysqLx_get_session');
print('We have C: $fc');
final DartFun fd = fc.asFunction();
print('We have Dart: $fd');
Pointer<Pointer>? error;
Pointer? session;
session = fd('localhost'.toNativeUtf8(),33060,'*user*'.toNativeUtf8(),'*password*'.toNativeUtf8(),''.toNativeUtf8(),error);
}
运行时我得到这个输出:
We have Pointer<NativeFunction<(Pointer<Utf8>,Pointer<Pointer<NativeType>>?) => Pointer<NativeType>>>: address=0x7f5b9c78f770
We have Closure: (Pointer<Utf8>,Pointer<Pointer<NativeType>>?) => Pointer<NativeType>
Unhandled exception:
NoSuchMethodError: The method 'FfiTrampoline' was called on null.
Receiver: null
Tried calling: FfiTrampoline()
#0 FfiTrampoline (dart:ffi)
#1 main (package:greudb/src/connection.dart:37:14)
#2 _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)
#3 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
#include <MysqL-cppconn/MysqLx/xapi.h>
#include <stdio.h>
int main()
{
MysqLx_session_t *session = 0;
MysqLx_error_t *error = 0;
session = MysqLx_get_session("localhost","*user*","*password*","",&error);
if(error != 0) {
printf("%s/n",MysqLx_error_message(error));
}
return 0;
}
我猜我的 Dart 函数签名不正确,但无论我尝试什么都无济于事。 MysqL 的 C 文档在这里:https://dev.mysql.com/doc/dev/connector-cpp/8.0/group__xapi__sess.html
我使用的是 Linux Fedora 33、MysqL 社区服务器 8、MysqL Connector C/C++ 8、Dart 202.8488。
解决方法
稍微环顾四周后,it appears 您可以使用以下方法为 error
创建适当的类型:
final error = calloc<Pointer>();
在幕后使用适当的 calloc.allocate<Pointer>(...)
调用 byteCount
。由于这是非托管内存,您需要记住通过调用来释放它:
calloc.free(error);
一旦你完成它。
幸运的是,mysqlx
API 似乎将所有结构视为不透明,因此您可以通过执行以下操作来确定实际错误是什么:
final message = errorMessage(error.value);
print(message.toDartString());
关于释放内存的话题,调用Pointer<Utf8>
创建的toNativeUtf8()
值也需要释放。
将所有这些放在一起:
import 'dart:ffi';
import 'package:ffi/ffi.dart';
typedef _GetSession = Pointer Function(Pointer<Utf8>,Int32,Pointer<Utf8>,Pointer<Pointer>);
typedef GetSession = Pointer Function(Pointer<Utf8>,int,Pointer<Pointer>);
typedef _CloseSession = Void Function(Pointer);
typedef CloseSession = void Function(Pointer);
typedef _ErrorMessage = Pointer<Utf8> Function(Pointer);
typedef ErrorMessage = Pointer<Utf8> Function(Pointer);
void main() {
final lib = DynamicLibrary.open('/usr/lib64/libmysqlcppconn8.so.2');
final GetSession getSession = lib.lookup<NativeFunction<_GetSession>>('mysqlx_get_session').asFunction();
final CloseSession closeSession = lib.lookup<NativeFunction<_CloseSession>>('mysqlx_session_close').asFunction();
final ErrorMessage errorMessage = lib.lookup<NativeFunction<_ErrorMessage>>('mysqlx_error_message').asFunction();
final host = 'localhost'.toNativeUtf8();
final user = 'user'.toNativeUtf8();
final password = 'password'.toNativeUtf8();
final database = ''.toNativeUtf8();
final error = calloc<Pointer>();
final session = getSession(host,33060,user,password,database,error);
calloc.free(host);
calloc.free(user);
calloc.free(password);
calloc.free(database);
if (error.value != nullptr) {
final message = errorMessage(error.value);
print(message.toDartString());
calloc.free(error);
return;
}
// Do something with session
closeSession(session);
}
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