c# - 如何将带有 const char* 的 C union 映射到 C# 结构？

Question

在本地库的回调函数中，我需要访问一个 espeak_EVENT 数组。问题出在原C代码中的UNION语句:

typedef struct {
    espeak_EVENT_TYPE type;
    unsigned int unique_identifier; // message identifier (or 0 for key or character)
    int text_position;    // the number of characters from the start of the text
    int length;           // word length, in characters (for espeakEVENT_WORD)
    int audio_position;   // the time in mS within the generated speech output data
    int sample;           // sample id (internal use)
    void* user_data;      // pointer supplied by the calling program
    union {
        int number;        // used for WORD and SENTENCE events. For PHONEME events this is the phoneme mnemonic.
        const char *name;  // used for MARK and PLAY events.  UTF8 string
    } id;
} espeak_EVENT;

我有

[StructLayout(LayoutKind.Explicit)]
        public struct espeak_EVENT
        {
            [System.Runtime.InteropServices.FieldOffset(0)]
            public espeak_EVENT_TYPE type;

            [System.Runtime.InteropServices.FieldOffset(4)]
            public uint unique_identifier;  // message identifier (or 0 for key or character)

            [System.Runtime.InteropServices.FieldOffset(8)]
            public int text_position;    // the number of characters from the start of the text

            [System.Runtime.InteropServices.FieldOffset(12)]
            public int length;           // word length, in characters (for espeakEVENT_WORD)

            [System.Runtime.InteropServices.FieldOffset(16)]
            public int audio_position;   // the time in mS within the generated speech output data

            [System.Runtime.InteropServices.FieldOffset(20)]
            public int sample;           // sample id (internal use)

            [System.Runtime.InteropServices.FieldOffset(24)]
            public IntPtr user_data;      // pointer supplied by the calling program

            [System.Runtime.InteropServices.FieldOffset(32)]
            public int number;

            [System.Runtime.InteropServices.FieldOffset(32)]
            [System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.LPStr)]
            public string name; 
        }

然后

public static Int32 SynthCallback(IntPtr wav, Int32 numsamples, IntPtr eventsParameter)
        {
            if (wav == IntPtr.Zero) 
                return 0;

            int j=0;
            while(true)
            {
                System.IntPtr ptr = new IntPtr( 
                                                (
                                                    eventsParameter.ToInt64() 
                                                    + (j *
                                                        System.Runtime.InteropServices.Marshal.SizeOf(typeof(cEspeak.espeak_EVENT))
                                                      ) 
                                                )
                                              );
                if(ptr == IntPtr.Zero)
                    Console.WriteLine("NULL");

                cEspeak.espeak_EVENT events = (cEspeak.espeak_EVENT) System.Runtime.InteropServices.Marshal.PtrToStructure(ptr, typeof(cEspeak.espeak_EVENT));

                if(events.type == cEspeak.espeak_EVENT_TYPE.espeakEVENT_SAMPLERATE)
                {
                    Console.WriteLine("Heureka");
                }
                break;


                //Console.WriteLine("\t\t header {0}: address={1}: offset={2}\n", j, info.dlpi_phdr, hdr.p_offset);
                ++j;
            }


            if(numsamples > 0)
            {
                byte[] wavbytes = new Byte[numsamples * 2];
                System.Runtime.InteropServices.Marshal.Copy(wav, wavbytes, 0, numsamples*2);
                bw.Write(wavbytes, 0, numsamples*2);
            }
            return 0;
        }

但总是失败

cEspeak.espeak_EVENT events = (cEspeak.espeak_EVENT) System.Runtime.InteropServices.Marshal.PtrToStructure(ptr, typeof(cEspeak.espeak_EVENT));

但是，当我删除

[System.Runtime.InteropServices.FieldOffset(32)][System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.LPStr)]
            public string name; 

从 espeak_event 开始，然后就可以了。

如何在不删除 union 中的字符串的情况下完成这项工作？我需要在回调函数中访问它。

编辑:顺便说一句，如果我让它在 x64 上运行，字段偏移量会发生什么变化，以及“public IntPtr user_data;”的大小。从 32 位更改为 64 位？

嗯，想一想，fieldoffset 32​​ 是否正确？似乎我在考虑 x64 时混淆了指针大小。这很可能是另一个错误。

嗯， union int 和 char*，我猜他们从未为 x64 编译它。因为 sizof(int) 在 x64 Linux 系统上是 32 位的。

Answer 1

将 name 声明为 IntPtr 而不是 string 然后使用 Marshal.PtrToStringAnsi 将其转换为字符串变量。

我忽略了字符串内容是 UTF-8 的事实。如果您的文本是纯 ASCII，那很好。如果不是，则需要复制到 byte[] 数组，然后使用 Encoding.UTF8.GetString 从 UTF-8 转换。