Golang CGo：将union字段转换为Go类型 [英] Golang CGo: converting union field to Go type

问题描述

我正在64位平台上使用此C结构，试图访问值联合中的 ui32v 字段：

  struct _GNetSnmpVarBind {
 guint32 * oid; / *变量的名称* / 
 gsize oid_len; / *名称长度* / 
 GNetSnmpVarBindType类型; / *变量类型/异常* / 
 union {
 gint32 i32; / * 32位有符号* / 
 guint32 ui32; / * 32位无符号* / 
 gint64 i64; / * 64位有符号* / 
 guint64 ui64; / * 64位无符号* / 
 guint8 * ui8v; / * 8位无符号向量* / 
 guint32 * ui32v; / * 32位无符号矢量* / 
}值; / *变量的值* / 
 gsize value_len; / *向量的长度，以字节为单位* / 
}; 
  

我可以为每个union元素编写C wrapper函数，但为了教学目的，我宁愿工作走。以下是我试图访问 ui32v 字段的方式：

  func union_to_guint32_ptr（cbytes [8]字节）（结果* _Ctype_guint32）{
 buf：= bytes.NewBuffer（cbytes [：]）
 var ptr uint64 
如果err：= binary.Read（buf，binary.LittleEndian，& amp ; PTR）; err == nil {
 return（* _Ctype_guint32）（unsafe.Pointer（ptr））
} 
 return nil 
} 
  

然而，这给出了一个错误无法将ptr（类型uint64）转换为键入unsafe.Pointer

那么如何 do 我将 uint64 转换为指向C guint32的Go类型？我已经尝试了各种组合投射到一个uintptr，然后投射到一个* _Ctype_guint32，投射到一个uintptr，然后使用unsafe.Pointer，...

我的推理是：我传递了一个8字节的数组。将其转换为uint64，即内存地址。将它转换为一个指向guint32的指针（即一个guint32的C数组），并返回结果 - 这就是联合字段value作为guint32 *。

< hr>

上下文

后来我想将guint32的C数组转换为字符串利用value_len字段，使用我知道的函数已经有效：

  guint32_star：= union_to_guint32_ptr（data.value）
结果+ = OidArrayToString（guint32_star，data.value_len）
  

C代码来自gsnmp 。

解决方案

解决方案首先投射到 uintptr ，然后投射到 unsafe.Pointer 即两个单独的演员阵容：

<$ p $ b $ func union_to_guint32_ptr（cbytes [8] byte）（result * _Ctype_guint32）{
buf：= bytes.NewBuffer（cbytes [：]）
var ptr uint64
如果err：= binary.Rea d（buf，binary.LittleEndian，& ptr）; err == nil {
uptr：= uintptr（ptr）
return（* _Ctype_guint32）（unsafe.Pointer（uptr））
}
return nil
}


我通过比较结果和命令行工具来检查它，并且它返回正确的结果。

---

背景

  // gsnmp ._Ctype_gpointer  - > * gsnmp._Ctype_GNetSnmpVarBind 
 data：=（* C.GNetSnmpVarBind）（out.data）
 
 switch VarBindType（data._type）{
 case GNET_SNMP_VARBIND_TYPE_OBJECTID：
 result + =GNET_SNMP_VARBIND_TYPE_OBJECTID+：
 guint32_star：= union_to_guint32_ptr（data.value）
 result + = OidArrayToString（guint32_star，data.value_len）
  

I'm working with this C struct on a 64 bit platform, trying to access the ui32v field in the value union:

struct _GNetSnmpVarBind {
  guint32       *oid;       /* name of the variable */
  gsize     oid_len;    /* length of the name */
  GNetSnmpVarBindType   type;       /* variable type / exception */
  union {
    gint32   i32;           /* 32 bit signed   */
    guint32  ui32;          /* 32 bit unsigned */
    gint64   i64;           /* 64 bit signed   */
    guint64  ui64;          /* 64 bit unsigned */
    guint8  *ui8v;          /*  8 bit unsigned vector */
    guint32 *ui32v;         /* 32 bit unsigned vector */
  }         value;      /* value of the variable */
  gsize     value_len;  /* length of a vector in bytes */
};

I could write a C wrapper function for each union element but for didactic purposes I'd rather work in Go. Here's how I'm trying to access the ui32v field:

func union_to_guint32_ptr(cbytes [8]byte) (result *_Ctype_guint32) {
  buf := bytes.NewBuffer(cbytes[:])
  var ptr uint64
  if err := binary.Read(buf, binary.LittleEndian, &ptr); err == nil {
    return (*_Ctype_guint32)(unsafe.Pointer(ptr))
  }
  return nil
}

However this gives an error cannot convert ptr (type uint64) to type unsafe.Pointer

So how do I convert a uint64 to a Go type that points to a C guint32? I've tried various combinations of casting to a uintptr then casting to a *_Ctype_guint32, casting to a uintptr then using unsafe.Pointer, ...

My reasoning is: I'm passed an array of 8 bytes. Convert that to a uint64, that's the memory address. Cast that to a pointer to a guint32 (ie a C array of guint32's), and return that as a result - that is the union field "value" as a guint32 *.

---

Context

Later I'll want to convert the C array of guint32's to a string utilising the value_len field, using a function I know already works:

guint32_star := union_to_guint32_ptr(data.value)
result += OidArrayToString(guint32_star, data.value_len)

The C code is from gsnmp.

解决方案

The solution was first to cast to uintptr, then cast to unsafe.Pointer ie two separate casts:

func union_to_guint32_ptr(cbytes [8]byte) (result *_Ctype_guint32) {
    buf := bytes.NewBuffer(cbytes[:])
    var ptr uint64
    if err := binary.Read(buf, binary.LittleEndian, &ptr); err == nil {
        uptr := uintptr(ptr)
        return (*_Ctype_guint32)(unsafe.Pointer(uptr))
    }   
    return nil 
}                

I checked this by comparing results with a command line tool, and it's returning correct results.

---

Context

// gsnmp._Ctype_gpointer -> *gsnmp._Ctype_GNetSnmpVarBind
data := (*C.GNetSnmpVarBind)(out.data)

switch VarBindType(data._type) {
case GNET_SNMP_VARBIND_TYPE_OBJECTID:
    result += "GNET_SNMP_VARBIND_TYPE_OBJECTID" + ":"
    guint32_star := union_to_guint32_ptr(data.value)
    result += OidArrayToString(guint32_star, data.value_len)

这篇关于Golang CGo：将union字段转换为Go类型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！