luajit利用ffi结合C语言实现面向对象的封装库

luajit中。利用ffi能够嵌入C。眼下luajit的最新版是2.0.4，在这之前的版本号我还不清楚这个扩展库详细怎么样，只是在2.04中，真的非常爽。



既然是嵌入C代码。那么要说让lua支持面向对象。不如说是让C语言模拟面向对象编程。然后让luajit的ffi嵌入。

要文字彻底来描写叙述清楚这个问题。我的表达能力非常有限，说不清楚，所以直接用代码来说吧。

//C++
class foo_type{
public:
    void foo1()
    {
        printf("%d", a + b);
    }

    void foo2(int n)
    {
        printf("%d", a + b + n);
    }
    int a;
    int b;
};

foo_type obj;
obj.foo1();
obj.foo2(100);

//在C语言要做到相同的事。最简单的做法例如以下。

typedef struct{
    int a;
    int b;
}foo_type;
void foo1(foo_type *obj)
{
    printf("%d", obj->a + obj->y);
}

void foo2(foo_type *obj, int n)
{
    printf("%d", obj->a + obj->y + n);
}

foo_type obj;
foo1(&obj);
foo2(&obj, 100);

/*****************************************
//C++从汇编语言的角度看
obj.foo1();
lea ecx, obj
call foo1
-----------------------

obj.foo2(100);
push 100
lea ecx, obj
call foo1
-----------------------

//C语言从汇编语言的角度看
foo1(&obj);
lea eax, obj
push eax
call foo1
-----------------------

foo2(&obj, 100);
push 100
lea eax, obj
push eax
call foo2

那么就能够看到，C和C++在实现这样的功能时的差别之处主要在于thiscall使用了ecx寄存器向下传递对象指针
而在C语言。仅仅能把结构指针push下去，这就有点像微软的COM包装类的stdcall的类成员了。

*****************************************/

//从汇编看到问题所在之后，要解决问题，也就在能够考虑写一个shellcode，这个shellcode实现将向下传递上层传入的參数。而且直接将结构指针以push參数的方式最后入栈。然后call就可以。
typedef struct{
    int a;
    int b;
}foo_type;
void foo1(foo_type *obj)
{
    printf("%d", obj->a + obj->y);
}

void foo2(foo_type *obj, int n)
{
    printf("%d", obj->a + obj->y + n);
}

typedef struct{
    void (*foo1)();
    void (*foo2)(int n);
}foo_class;

foo_type ft;
foo_class obj;
//与foo1相应的shellcode模板
BYTE scT1[] = {
    0x68, 0, 0, 0, 0, //push CONST
    0xE8, 0, 0, 0, 0, //call CONST
    0x83, 0xC4, 0x04, //add esp, 4
    0xC3              //ret
};

BYTE *psc = (BYTE*)VirtualAlloc(NULL, sizeof(scT1), MEM_COMMIT, PAGE_EXECUTE_READWRITE);
memcpy(psc, scT1, sizeof(scT1));
*(ULONG*)(psc + 1) = (ULONG)&ft;
*(ULONG*)(psc + 6) = (ULONG)&foo1 - (ULONG)(psc + 5) - 5;
*(ULONG*)&(obj.foo1) = psc;

//到这里之后，就已经能够这样用了
obj.foo1();

//与foo2相应的shellcode模板
BYTE scT2[] = {
        0xFF, 0x74, 0x24, 0x04,   //push dword ptr [esp + 4]
        0x68, 0, 0, 0, 0,         //call CONST
        0xE8, 0, 0, 0, 0,         //call CONST
        0x83, 0xC4, 0x08,         //add esp, 8
        0xC3                      //ret
};
BYTE *psc = (BYTE*)VirtualAlloc(NULL, sizeof(scT2), MEM_COMMIT, PAGE_EXECUTE_READWRITE);
memcpy(psc, scT2, sizeof(scT2));
*(ULONG*)(psc + 5) = (ULONG)&ft;
*(ULONG*)(psc + 10) = (ULONG)&foo2 - (ULONG)(psc + 5) - 5;
*(ULONG*)&(obj.foo2) = psc;

obj.foo2(100);

//这样就模拟了面向对象形式的调用，之后就须要考虑的就是一些怎样整合管理资源之类的问题，还须要将代码尽可能的利用一些宏来简化，这些东西就不在这里说了，这个思路深入下去。相信大家都会知道该怎么做了。

以下是一套我封装的luajit使用ODBC訪问数据库的代码，没有凝视。将就着看吧。

//luaodbc.h**************************************************
#pragma once

extern "C"{
    typedef struct{
        void (*close)();
        bool (*set)(int i, int type, char *data, int size);
        bool (*get)(int i, int type);
        void *(*data)();
        bool (*next)();
        char *(*getstr)(int i);
        int (*getint)(int i);
        char *(*getstr_fn)(const char *name);
        char *(*getint_fn)(const char *name);
        bool (*setstr)(int i, const char *str);
        bool (*setint)(int i, int num);
        bool (*setstr_fn)(const char *name, const char *str);
        bool (*setint_fn)(const char *name, int num);
    }luastmt_class;

    typedef struct{
        bool (*open)(const char*dsn);
        void (*close)();
        luastmt_class *(*exec)(const char *sql);
        void (*tran_begin)();
        void (*tran_end)(int ct);
    }luadbc_class;

    __declspec(dllexport) luadbc_class *luadbc_init();
    __declspec(dllexport) void luadbc_exit(luadbc_class *obj);
};

//luaodbc.cpp**************************************************
#include "stdafx.h"
#include "luaodbc.h"
#include <sql.h>
#include <sqlext.h>
#include <list>
#include <algorithm>
#include <string>

extern "C"{
#define LOS_CLOSED 0
#define LOS_INITILZED 1
#define LOS_CONNECTIONED 2
#define LOS_TRAN 4
#define LOS_STMTOPENED 8

    typedef struct _ciType{
        char name[68];
        int type;
        int type_len;
        char *data;
        int data_size;
        int nds;
    }ciType;

    typedef struct _stmtType{
        SQLHANDLE hstmt;
        ciType *cols;
        int col_count;
        int cp_size;
        unsigned int status;
        void *parent;
        luastmt_class thisobj;
        unsigned char *class_memory;
        int gci;
        _stmtType *next;
    }stmtType;

    typedef struct{
        int (*open)(const char*dsn);
        void (*close)();
        bool (*exec)(const char *sql);
        void (*tran_begin)();
        void (*tran_end)(int ct);
        void *dbc_obj;
    }luadbc_class_ex;

    typedef struct _odbcType{
        SQLHANDLE henv, hdbc;
        _stmtType *stmt_list;
        unsigned int status;
        luadbc_class_ex thisobj;
        unsigned char *class_memory;
    }odbcType;

    BYTE scT1[] = {
        0x68, 0, 0, 0, 0,
        0xE8, 0, 0, 0, 0,
        0x83, 0xC4, 0x04,
        0xC3
    };

    BYTE scT2[] = {
        0xFF, 0x74, 0x24, 0x04,
        0x68, 0, 0, 0, 0,
        0xE8, 0, 0, 0, 0,
        0x83, 0xC4, 0x08,
        0xC3
    };

    BYTE scT3[] = {
        0xFF, 0x74, 0x24, 0x08,
        0xFF, 0x74, 0x24, 0x08,
        0x68, 0, 0, 0, 0,
        0xE8, 0, 0, 0, 0,
        0x83, 0xC4, 0x0C,
        0xC3
    };

    BYTE scT4[] = {
        0xFF, 0x74, 0x24, 0x0C,
        0xFF, 0x74, 0x24, 0x0C,
        0xFF, 0x74, 0x24, 0x0C,
        0x68, 0, 0, 0, 0,
        0xE8, 0, 0, 0, 0,
        0x83, 0xC4, 0x10,
        0xC3
    };

    BYTE scT5[] = {
        0xFF, 0x74, 0x24, 0x10,
        0xFF, 0x74, 0x24, 0x10,
        0xFF, 0x74, 0x24, 0x10,
        0xFF, 0x74, 0x24, 0x10,
        0x68, 0, 0, 0, 0,
        0xE8, 0, 0, 0, 0,
        0x83, 0xC4, 0x14,
        0xC3
    };

#define SCTP1 1
#define SCTP2 SCTP1 + 4
#define SCTP3 SCTP2 + 4
#define SCTP4 SCTP3 + 4
#define SCTP5 SCTP4 + 4

    std::list<int> g_safe_luaodbc_list;
    CRITICAL_SECTION g_safe_luaodbc_cs;
    class luaodbc_startup{
    public:
        luaodbc_startup()
        {
            InitializeCriticalSection(&g_safe_luaodbc_cs);
            static_link = 1;
        }
        int static_link;
    };

    static luaodbc_startup g_luaodbc_link;
    static int ___g_static_link = g_luaodbc_link.static_link;

    void luastmt_close(stmtType *v)
    {
        if(v->status & LOS_STMTOPENED)
        {
            SQLFreeHandle(SQL_HANDLE_STMT, v->hstmt);
            v->hstmt = NULL;
            v->status ^= LOS_STMTOPENED;
        }
    }

    void luaodbc_coldata_realloc(stmtType *v, int i, int size)
    {
        delete v->cols[i].data;
        v->cols[i].data = new char[size];
    }

    BOOL luastmt_set(stmtType *v, int i, int type, char *data, int size)
    {
        if(!data || i <= 0)
            return FALSE;
        if(!(v->status & LOS_STMTOPENED))
            return FALSE;
        SQLINTEGER ncb = size;
        //printf("data:%s, i:%d, len:%d, type:%d\n", data,i, size, type);
        SQLFreeStmt(v->hstmt, SQL_UNBIND);
        SQLRETURN rc = SQLBindCol(v->hstmt, (SQLUSMALLINT)i, (SQLSMALLINT)type, (SQLPOINTER)data, (SQLINTEGER)size, &ncb);
        if(rc == SQL_SUCCESS || rc == SQL_SUCCESS_WITH_INFO)
            SQLSetPos(v->hstmt, 1, SQL_UPDATE, SQL_LOCK_NO_CHANGE);
        //printf("update:%d, %d\n", rc, ncb);
        SQLFreeStmt(v->hstmt, SQL_UNBIND);
        return TRUE;
    }

    BOOL luastmt_get(stmtType *v, int i, int type)
    {
        if(i <= 0)
            return FALSE;
        if(!(v->status & LOS_STMTOPENED))
            return FALSE;
        SQLRETURN rc;
        for(;;)
        {
            if(v->gci == i)
                SQLSetPos(v->hstmt, 1, SQL_POSITION, SQL_LOCK_NO_CHANGE);
            else
                v->gci = i;
            rc = SQLGetData(v->hstmt, i, type,
                v->cols[i - 1].data, v->cols[i - 1].data_size, (SQLINTEGER*)&(v->cols[i - 1].nds));
            if(rc == SQL_SUCCESS || rc == SQL_SUCCESS_WITH_INFO)
                return TRUE;
            if(v->cols[i - 1].data_size < v->cols[i - 1].nds)
                luaodbc_coldata_realloc(v, i - 1, v->cols[i - 1].nds + 1);
            else
                return FALSE;
        }
        return false;
    }

    void *luastmt_dataptr(stmtType *v, int i)
    {
        if(i <= 0)
            return false;
        return v->cols[i - 1].data;
    }

    BOOL luastmt_next(stmtType *v)
    {
        if(!(v->status & LOS_STMTOPENED))
            return false;
        v->gci = 0;
        SQLRETURN rc = SQLFetch(v->hstmt);
        return (rc != SQL_NO_DATA_FOUND);
    }

    const char *luastmt_getstr(stmtType *v, int i)
    {
        if(!luastmt_get(v, i, SQL_C_CHAR))
            return "";
        return v->cols[i - 1].data;
    }

    int luastmt_getint(stmtType *v, int i)
    {
        if(!luastmt_get(v, i, SQL_C_SLONG))
            return -1;
        return *(int*)(v->cols[i - 1].data);
    }

    int _colindexfromename(stmtType *v, const char *name)
    {
        std::string str = name;
        std::transform(str.begin(), str.end(), str.begin(), tolower);
        for(short i = 0; i < v->col_count; i++)
        {
            if(str == (v->cols)[i].name)
                return i + 1;
        }
        return -1;
    }

    const char *luastmt_getstr_fn(stmtType *v, const char *name)
    {
        if(!name)
            return "";
        return luastmt_getstr(v, _colindexfromename(v, name));
    }

    int luastmt_getint_fn(stmtType *v, const char *name)
    {
        if(!name)
            return -1;
        return luastmt_getint(v, _colindexfromename(v, name));
    }

    BOOL luastmt_setstr(stmtType *v, int i, const char *str)
    {
        if(i <= 0 || !str)
            return FALSE;
        return luastmt_set(v, i, SQL_C_CHAR, (char*)str, strlen(str) + 1);
    }

    BOOL luastmt_setint(stmtType *v, int i, int num)
    {
        if(!v || i <= 0)
            return FALSE;
        return luastmt_set(v, i, SQL_C_SLONG, (char*)&num, sizeof(int));
    }

    BOOL luastmt_setstr_fn(stmtType *v, const char *name, const char *str)
    {
        if(!v || !name)
            return false;
        return luastmt_setstr(v,  _colindexfromename(v, name), str);
    }

    BOOL luastmt_setint_fn(stmtType *v, const char *name, int num)
    {
        if(!name)
            return false;
        return luastmt_setint(v, _colindexfromename(v, name), num);
    }

    void luastmt_class_init(stmtType *v)
    {
        BYTE *psc = v->class_memory;
#define make_luastmt_shellcode(_member_, _sc_t_, _pup_, _proc_) *(DWORD*)&(v->thisobj._member_) = (DWORD)psc; \
    memcpy_s(psc, sizeof(_sc_t_), _sc_t_, sizeof(_sc_t_)); \
    *(DWORD*)(psc + _pup_) = (DWORD)v; \
    *(DWORD*)(psc + _pup_ + 5) = (DWORD)_proc_ - (DWORD)(psc + _pup_ + 5 - 1) - 5; \
    psc += sizeof(_sc_t_)
        make_luastmt_shellcode(close, scT1, SCTP1, luastmt_close);
        make_luastmt_shellcode(set, scT5, SCTP5, luastmt_set);
        make_luastmt_shellcode(get, scT3, SCTP3, luastmt_get);
        make_luastmt_shellcode(data, scT1, SCTP1, luastmt_dataptr);
        make_luastmt_shellcode(next, scT1, SCTP1, luastmt_next);
        make_luastmt_shellcode(getstr, scT2, SCTP2, luastmt_getstr);
        make_luastmt_shellcode(getint, scT2, SCTP2, luastmt_getint);
        make_luastmt_shellcode(getstr_fn, scT2, SCTP2, luastmt_getstr_fn);
        make_luastmt_shellcode(getint_fn, scT2, SCTP2, luastmt_getint_fn);
        make_luastmt_shellcode(setstr, scT3, SCTP3, luastmt_setstr);
        make_luastmt_shellcode(setint, scT3, SCTP3, luastmt_setint);
        make_luastmt_shellcode(setstr_fn, scT3, SCTP3, luastmt_setstr_fn);
        make_luastmt_shellcode(setint_fn, scT3, SCTP3, luastmt_setint_fn);
    }

    void luastmt_colinfo_release(stmtType *v)
    {
        for(int i = 0; i < v->cp_size; i++)
        {
            if(v->cols[i].data)
            {
                delete v->cols[i].data;
                v->cols[i].data = NULL;
            }
        }
        delete v->cols;
    }

    void luastmt_colinfo_init(stmtType *v)
    {
        short count;
        SQLNumResultCols(v->hstmt, &count);
        if(count > v->cp_size)
        {
            luastmt_colinfo_release(v);
            v->cp_size = count;
            v->cols = new ciType[count];
            memset(v->cols, 0, sizeof(ciType) * count);
        }
        v->col_count = count;
        char szColName[68];
        SQLSMALLINT cbColName, sqlColType, ibScale, fNullable;
        SQLUINTEGER cbColDef;
        for(short i = 0; i < count; i++)
        {
            SQLDescribeColA(v->hstmt, i + 1, (SQLCHAR*)szColName, 68, &cbColName, &sqlColType, &cbColDef,  &ibScale, &fNullable);
            for(short j = 0; j < cbColName; j++)szColName[j] = tolower(szColName[j]);
            memcpy_s(v->cols[i].name, 68, szColName, cbColName + 1);
            if(v->cols[i].data_size < (int)cbColDef + 1)
            {
                if(v->cols[i].data)
                    delete v->cols[i].data;
                v->cols[i].data_size = cbColDef + 1;
                v->cols[i].data = new char[cbColDef + 1];
            }
            v->cols[i].type = sqlColType;
            v->cols[i].type_len = cbColDef;
        }
    }

    void luastmt_release(stmtType *v)
    {
        luastmt_colinfo_release(v);
        VirtualFree(v->class_memory, 0, MEM_RELEASE);
        delete v;
    }

    stmtType *luaodbc_newstmt(_odbcType *parent)
    {
        stmtType *v = new stmtType;
        memset(v, 0, sizeof(stmtType));
        v->cp_size = 64;
        v->cols = new ciType[64];
        memset(v->cols, 0, sizeof(ciType) * 64);
        v->status = LOS_CLOSED;
        v->next = NULL;
        v->parent = parent;
        v->class_memory = (unsigned char *)VirtualAlloc(NULL, 512, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
        luastmt_class_init(v);
        return v;
    }

    void luaodbc_tran_begin(odbcType *v)
    {
        SQLSetConnectAttrA(v->hdbc, SQL_ATTR_AUTOCOMMIT, (SQLPOINTER)SQL_FALSE, NULL);
        v->status |= LOS_TRAN;
    }

    void luaodbc_tran_end(odbcType *v, int ct)
    {
        SQLEndTran(SQL_HANDLE_DBC, v->hdbc, (short)ct);
        SQLSetConnectAttrA(v->hdbc, SQL_ATTR_AUTOCOMMIT, (SQLPOINTER)SQL_TRUE, NULL);
        v->status ^= LOS_TRAN;
    }

    void luaodbc_close(odbcType *v)
    {
        if(v->status & LOS_CONNECTIONED)
        {
            stmtType *s = v->stmt_list;
            do{
                luastmt_close(s);
                s = s->next;
            }while(s);
            if(v->status & LOS_TRAN)
                luaodbc_tran_end(v, SQL_ROLLBACK);
            SQLDisconnect(v->hdbc);
            SQLFreeHandle(SQL_HANDLE_DBC, v->hdbc);
            v->status ^= LOS_CONNECTIONED;
            v->hdbc = NULL;
        }
    }

    BOOL luaodbc_open(odbcType *v, const char *dsn)
    {
        luaodbc_close(v);
        SQLHANDLE hdbc;
        SQLRETURN rc = SQLAllocHandle(SQL_HANDLE_DBC, v->henv, &hdbc);
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
            return FALSE;
        rc = SQLConnectA(hdbc, (SQLCHAR*)dsn, SQL_NTS, NULL, 0, NULL, 0);
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
        {
            SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
            return FALSE;
        }
        v->hdbc = hdbc;
        v->status |= LOS_CONNECTIONED;
        return TRUE;
    }

    luastmt_class *luaodbc_exec(odbcType *v, const char *sql)
    {
        stmtType *s = v->stmt_list;
        stmtType *b = s;
        do{
            if(!(s->status & LOS_STMTOPENED))
                break;
            b = s;
            s = s->next;
        }while(s);

        if(!s)
        {
            s = luaodbc_newstmt(v);
            b->next = s;
        }

        SQLHANDLE hstmt;
        SQLRETURN rc;
        if(!(v->status & LOS_CONNECTIONED))
            return NULL;

        rc = SQLAllocHandle(SQL_HANDLE_STMT, v->hdbc, &hstmt);
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
            return FALSE;
        SQLCancel(hstmt);
#if(_MSC_VER < 1300)
        SQLSetStmtOption(hStmt, SQL_CONCURRENCY, SQL_CONCUR_VALUES);
        SQLSetStmtOption(hStmt, SQL_CURSOR_TYPE, SQL_CURSOR_KEYSET_DRIVEN);
#else
        SQLSetStmtAttr(hstmt, SQL_ATTR_CONCURRENCY, (SQLPOINTER)SQL_CONCUR_VALUES, 0);
        SQLSetStmtAttr(hstmt, SQL_ATTR_CURSOR_TYPE, (SQLPOINTER)SQL_CURSOR_KEYSET_DRIVEN, 0);
#endif
        rc = SQLExecDirectA(hstmt, (SQLCHAR*)sql, SQL_NTS);
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
        {
            //printf("ERROR:[ODBC::Connection::Execute]->%s\n", sql);
            SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
            return FALSE;
        }
        s->hstmt = hstmt;
        luastmt_colinfo_init(s);
        s->status |= LOS_STMTOPENED;
        return &(s->thisobj);
    }

    void luaodbc_class_init(odbcType *v)
    {
        BYTE *psc = v->class_memory;
#define make_luaodbc_shellcode(_member_, _sc_t_, _pup_, _proc_) *(DWORD*)&(v->thisobj._member_) = (DWORD)psc; \
    memcpy_s(psc, sizeof(_sc_t_), _sc_t_, sizeof(_sc_t_)); \
    *(DWORD*)(psc + _pup_) = (DWORD)v; \
    *(DWORD*)(psc + _pup_ + 5) = (DWORD)_proc_ - (DWORD)(psc + _pup_ + 5 - 1) - 5; \
    psc += sizeof(_sc_t_)
        make_luaodbc_shellcode(open, scT2, SCTP2, luaodbc_open);
        make_luaodbc_shellcode(close, scT1, SCTP1, luaodbc_close);
        make_luaodbc_shellcode(exec, scT2, SCTP2, luaodbc_exec);
        make_luaodbc_shellcode(tran_begin, scT1, SCTP1, luaodbc_tran_begin);
        make_luaodbc_shellcode(tran_end, scT2, SCTP2, luaodbc_tran_end);
    }

    __declspec(dllexport) luadbc_class *luadbc_init()
    {
        odbcType *v = new odbcType;
        memset(v, 0, sizeof(odbcType));
        SQLRETURN rc;
        rc = SQLAllocHandle(SQL_HANDLE_ENV, NULL, &(v->henv));
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
            return NULL;
        rc = SQLSetEnvAttr(v->henv, SQL_ATTR_ODBC_VERSION, (SQLPOINTER)SQL_OV_ODBC3, SQL_IS_INTEGER);
        if(rc != SQL_SUCCESS && rc != SQL_SUCCESS_WITH_INFO)
        {
            SQLFreeHandle(SQL_HANDLE_ENV, v->henv);
            v->henv = NULL;
            return NULL;
        }
        v->stmt_list = luaodbc_newstmt(v);
        v->status = LOS_INITILZED;
        v->class_memory = (unsigned char *)VirtualAlloc(NULL, 512, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
        luaodbc_class_init(v);
        v->thisobj.dbc_obj = v;
        EnterCriticalSection(&g_safe_luaodbc_cs);
        g_safe_luaodbc_list.push_back((int)&(v->thisobj));
        LeaveCriticalSection(&g_safe_luaodbc_cs);
        return (luadbc_class*)&(v->thisobj);
    }

    __declspec(dllexport) void luadbc_exit(luadbc_class *obj)
    {
        EnterCriticalSection(&g_safe_luaodbc_cs);
        auto it = std::find(g_safe_luaodbc_list.begin(), g_safe_luaodbc_list.end(), (int)obj);
        if(it == g_safe_luaodbc_list.end())
        {
            LeaveCriticalSection(&g_safe_luaodbc_cs);
            return;
        }
        g_safe_luaodbc_list.erase(it);
        LeaveCriticalSection(&g_safe_luaodbc_cs);
        odbcType *v = (odbcType*)(((luadbc_class_ex*)obj)->dbc_obj);
        VirtualFree(v->class_memory, 0, MEM_RELEASE);
        luaodbc_close(v);
        stmtType *s = v->stmt_list;
        stmtType *b;
        do{
            b = s;
            s = s->next;
            luastmt_release(b);
        }while(s);
        SQLFreeHandle(SQL_HANDLE_ENV, v->henv);
        delete v;
    }
}

--luajit中这样调用
local ffi = require("ffi")
ffi.cdef[[
    typedef struct{
        void (*close)();
        bool (*set)(int i, int type, char *data, int size);
        bool (*get)(int i, int type);
        void *(*data)();
        bool (*next)();
        char *(*getstr)(int i);
        int (*getint)(int i);
        char *(*getstr_fn)(const char *name);
        char *(*getint_fn)(const char *name);
        bool (*setstr)(int i, const char *str);
        bool (*setint)(int i, int num);
        bool (*setstr_fn)(const char *name, const char *str);
        bool (*setint_fn)(const char *name, int num);
    }luastmt_class;

    typedef struct{
        bool (*open)(const char*dsn);
        void (*close)();
        luastmt_class *(*exec)(const char *sql);
        void (*tran_begin)();
        void (*tran_end)(int ct);
    }luadbc_class;

    luadbc_class *luadbc_init();
    void luadbc_exit(luadbc_class *obj);
]]

local conn = ffi.C.luadbc_init();

if conn.open("USERDSN") ~= false then
    print("已连接!")
    local rs = conn.exec("SELECT * FROM characters")
    if rs ~= nil then
        print("開始遍历数据!")
        while rs.next() do
            print(ffi.string(rs.getstr(3)))
        end
        rs.close()
    end
    conn.close()
end