问题

本地代码中的结构体:
```c
struct s_xl_daio_data {  /* 32字节 */
         unsigned short    flags;                 // 2
         unsigned int      timestamp_correction;  // 4
         unsigned char     mask_digital;          // 1
         unsigned char     value_digital;         // 1
         unsigned char     mask_analog;           // 1
         unsigned char     reserved0;             // 1
         unsigned short    value_analog[4];       // 8
         unsigned int      pwm_frequency;         // 4
         unsigned short    pwm_value;             // 2
         unsigned int      reserved1;             // 4
         unsigned int      reserved2;             // 4
};

我的Java类:

@FieldOrder ({"flags", "timestamp_correction", "mask_digital", 
"value_digital", "mask_analog", "reserved0", "value_analog",
"pwm_frequency", "pwm_value", "reserved1", "reserved2"})
public class s_xl_daio_data extends Structure {	
    public short    flags;                 // 2
    public int      timestamp_correction;  // 4
    public byte     mask_digital;          // 1
    public byte     value_digital;         // 1
    public byte     mask_analog;           // 1
    public byte     reserved0;             // 1
    public short[]    value_analog= new short[4];       // 8
    public int      pwm_frequency;         // 4
    public short    pwm_value;             // 2
    public int      reserved1;             // 4
    public int      reserved2;             // 4

	public s_xl_daio_data() {
		super();
	}
}

本地结构体的大小为32字节。如果我使用Structure的.size()操作打印出结构体的大小，结果是36字节。

那4个额外的字节是什么？

<details>
<summary>英文:</summary>

the struct in my native code:

struct s_xl_daio_data { /* 32 Bytes */
unsigned short flags; // 2
unsigned int timestamp_correction; // 4
unsigned char mask_digital; // 1
unsigned char value_digital; // 1
unsigned char mask_analog; // 1
unsigned char reserved0; // 1
unsigned short value_analog[4]; // 8
unsigned int pwm_frequency; // 4
unsigned short pwm_value; // 2
unsigned int reserved1; // 4
unsigned int reserved2; // 4
};

my Java class:

@FieldOrder ({"flags", "timestamp_correction", "mask_digital",
"value_digital", "mask_analog", "reserved0", "value_analog",
"pwm_frequency", "pwm_value", "reserved1", "reserved2"})
public class s_xl_daio_data extends Structure {
public short flags; // 2
public int timestamp_correction; // 4
public byte mask_digital; // 1
public byte value_digital; // 1
public byte mask_analog; // 1
public byte reserved0; // 1
public short[] value_analog= new short[4]; // 8
public int pwm_frequency; // 4
public short pwm_value; // 2
public int reserved1; // 4
public int reserved2; // 4

public s_xl_daio_data() {
	super();
}

}

The native Struct is 32 Bytes. If I print out the size of the struct with the `.size()`operation from Structure it is 36 Bytes.

What are the 4 additional bytes?

</details>


# 答案1
**得分**: 1

映射是正确的，但是[默认结构对齐][1]在JNA中创建了额外的填充，JNA假定使用默认值，除非另有说明。

您可以确认这一点并调试将来的结构大小/对齐不匹配问题，方法是使用默认的`Structure`类的`toString()`方法。它会打印每个字段的偏移和值，这样您就可以查找偏移与您预期不匹配的情况。

```java
s_xl_daio_data foo = new s_xl_daio_data();
System.out.println(foo.toString());

在我的系统上检查该代码的输出显示，在第一个short之后有两个额外的字节，因为int timestamp_correction必须从4字节边界开始（0x4而不是0x2）：

short flags@0x0=0x00
int timestamp_correction@0x4=0x0000

在最后一个short之后还有两个额外的字节，因为int reserved1必须从4字节边界开始（0x1C而不是0x1A）：

short pwm_value@0x18=0x00
int reserved1@0x1C=0x0000

默认的对齐通常适用于系统DLL，这些DLL在许多情况下会考虑结构对齐并使用显式填充字段。然而，有时其他DLL可能不需要对齐，在这种情况下，在实例化结构时您可以在JNA中指定这一点。

public s_xl_daio_data() {
    super(Structure.ALIGN_NONE);
}

通过使用前面提到的相同toString()方法，确认这是否为您提供了预期的数据在适当的字段中将会很有用。

s_xl_daio_data foo = new s_xl_daio_data();
System.out.println(foo.toString());

short flags@0x0=0x00
int timestamp_correction@0x4=0x0000

short pwm_value@0x18=0x00
int reserved1@0x1C=0x0000

public s_xl_daio_data() {
    super(Structure.ALIGN_NONE);
}