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我想使用MAVLink C 库来解析 dart 中的 MAVLink 数据包，但是 Dart FFI 生成的 mavlink_message_t 是一个不透明的类，而像 mavlink_system_t 这样的其他结构通常是用它们的所有属性生成的。这种行为的原因是什么，我该如何解决？

mavlink_message_t C 结构

MAVPACKED(
typedef struct __mavlink_message {
    uint16_t checksum;      ///< sent at end of packet
    uint8_t magic;          ///< protocol magic marker
    uint8_t len;            ///< Length of payload
    uint8_t incompat_flags; ///< flags that must be understood
    uint8_t compat_flags;   ///< flags that can be ignored if not understood
    uint8_t seq;            ///< Sequence of packet
    uint8_t sysid;          ///< ID of message sender system/aircraft
    uint8_t compid;         ///< ID of the message sender component
    uint32_t msgid:24;      ///< ID of message in payload
    uint64_t payload64[(MAVLINK_MAX_PAYLOAD_LEN+MAVLINK_NUM_CHECKSUM_BYTES+7)/8];
    uint8_t ck[2];          ///< incoming checksum bytes
    uint8_t signature[MAVLINK_SIGNATURE_BLOCK_LEN];
}) mavlink_message_t;

mavlink_system_t C 结构

MAVPACKED(
typedef struct __mavlink_system {
    uint8_t sysid;   ///< Used by the MAVLink message_xx_send() convenience function
    uint8_t compid;  ///< Used by the MAVLink message_xx_send() convenience function
}) mavlink_system_t;

FFI 生成的 Dart 类

class mavlink_message_t extends ffi.Opaque {}

@ffi.Packed(1)
class mavlink_system_t extends ffi.Struct {
  @ffi.Uint8()
  external int sysid;

  @ffi.Uint8()
  external int compid;
}
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1 回答 1

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感谢@Richard Heap 的评论，我发现这个问题的根源是msgid:24字段定义。由于它是一个位字段，目前Dart FFI 不支持，所以 ffigen 会从中生成一个不透明的类定义。删除msgid:24声明后，生成了一个 ffi.Struct 而不是 ffi.Opaque

@ffi.Packed(1)
class mavlink_message_t extends ffi.Struct {
  @ffi.Uint16()
  external int checksum;

  @ffi.Uint8()
  external int magic;

  @ffi.Uint8()
  external int len;

  @ffi.Uint8()
  external int incompat_flags;

  @ffi.Uint8()
  external int compat_flags;

  @ffi.Uint8()
  external int seq;

  @ffi.Uint8()
  external int sysid;

  @ffi.Uint8()
  external int compid;

  @ffi.Array.multi([33])
  external ffi.Array<ffi.Uint64> payload64;

  @ffi.Array.multi([2])
  external ffi.Array<ffi.Uint8> ck;

  @ffi.Array.multi([13])
  external ffi.Array<ffi.Uint8> signature;
}

This of course is not enough since I need all the fields, but there is a workaround for this which I'll test.

于 2021-06-15T09:38:29.310 回答