Golang CGo：将联合字段转换为Go类型

Question

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.

Answer 1

From the CGO documentation:

To access a struct, union, or enum type directly, prefix it with struct_, union_, or enum_, as in C.struct_stat.

So I guess (not tested) the code might be something similar to:

myUint32var := somePtrTo_GNetSnmpVarBind.union_guint32

for accessing the guint32 member of the union of the struct pointed to by somePtrTo_GNetSnmpVarBind