如何设置 C 语言中的联合（union）值？

huangapple
go评论122阅读模式
英文:

cgo how to set a C union value

问题

给定以下的C结构体，我可以使用不安全指针读取数据联合的值，但是我不知道如何设置联合值的数据。

  1. typedef struct val {
  2.     char *var1;
  3.     type type;
  4.     union {
  5.         char *binary_val;
  6.         char *bits_val;
  7.         bool bool_val;
  8.         double decimal64_val;
  9.         char *enum_val;
  10.         char *identityref_val;
  11.         char *instanceid_val;
  12.         int8_t int8_val;
  13.         int16_t int16_val;
  14.         int32_t int32_val;
  15.         int64_t int64_val;
  16.         char *string_val;
  17.         uint8_t uint8_val;
  18.         uint16_t uint16_val;
  19.         uint32_t uint32_val;
  20.         uint64_t uint64_val;
  21.     } data;
  22. } val_t;

在C语言中，有没有一种方法可以在不创建辅助方法的情况下设置联合值？一个设置string_val的示例将非常完美。我知道Go语言将联合表示为具有最长类型的字节数组，并且该类型的长度设置为字节数组的长度。

英文:

Given I have a C structure as below. I am able to read the data union values using unsafe pointers but am unable to work out how I can set the union value data?

  1.  typedef struct val {
  2.     char *var1;
  4.     type type;
  6.     union {
  7.         char *binary_val;      
  8.         char *bits_val;        
  9.         bool bool_val;         
  10.         double decimal64_val;  
  11.         char *enum_val;        
  12.         char *identityref_val; 
  13.         char *instanceid_val;   
  14.         int8_t int8_val;       
  15.         int16_t int16_val;    
  16.         int32_t int32_val;     
  17.         int64_t int64_val;     
  18.         char *string_val;   
  19.         uint8_t uint8_val;      
  20.         uint16_t uint16_val;   
  21.         uint32_t uint32_val;   
  22.         uint64_t uint64_val;   
  23.     } data;
  24. } val_t;

Is there a way without creating helper methods in C? An example of setting string_val would be perfect? I am aware that Go represents a union as a byte array with the longest type setting the length of the byte array.

答案1

得分: 1

似乎cgo将联合值转换为字节数组。因此，您可以使用encoding/binary来设置它们：

  1. v._type = 0
  2. binary.LittleEndian.PutUint16(v.data[:], 42)
  3. C.print_val(v)
  5. v._type = 1
  6. cstr := C.CString("hello world")
  7. defer C.free(unsafe.Pointer(cstr))
  8. binary.LittleEndian.PutUint64(v.data[:], uint64(uintptr(unsafe.Pointer(cstr))))
  9. C.print_val(v)

这段代码（配合适当的print_val函数）会输出：

  1. int16: 42
  2. string: hello world

这种方法是不安全和不可移植的，如果有更好的方法，我会很乐意学习。

英文:

It seems that cgo turns the union values into byte arrays. As such, you can set them using encoding/binary:

  1. v._type = 0
  2. binary.LittleEndian.PutUint16(v.data[:], 42)
  3. C.print_val(v)
  5. v._type = 1
  6. cstr := C.CString("hello world")
  7. defer C.free(unsafe.Pointer(cstr))
  8. binary.LittleEndian.PutUint64(v.data[:], uint64(uintptr(unsafe.Pointer(cstr))))
  9. C.print_val(v)

This (with an appropriate print_val function) prints:

  1. int16: 42
  2. string: hello world

This is unsafe and non-portable though, so if anyone has a better way, I'll be glad to learn it.

huangapple
  • 本文由 发表于 2016年9月6日 17:21:36
  • 转载请务必保留本文链接：https://go.coder-hub.com/39345285.html
  • c
  • cgo
  • go
  • unions
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