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我创建了一个如下结构：
typedef union __attribute__ ((__packed__)) adcs_measurements_t
{
unsigned char raw[72];
struct __attribute__ ((__packed__)) //191
{
int magneticFieldX : 16;
int magneticFieldY : 16;
int magneticFieldZ : 16;
int coarseSunX : 16;
int coarseSunY : 16;
int coarseSunZ : 16;
int sunX : 16;
int sunY : 16;
int sunZ : 16;
int nadirX : 16;
int nadirY : 16;
int nadirZ : 16;
int XAngularRate : 16;
int YAngularRate : 16;
int ZAngularRate : 16;
int XWheelSpeed : 16;
int YWheelSpeed : 16;
int ZWheelSpeed : 16;
int star1BX : 16;
int star1BY : 16;
int star1BZ : 16;
int star1OX : 16;
int star1OY : 16;
int star1OZ : 16;
int star2BX : 16;
int star2BY : 16;
int star2BZ : 16;
int star2OX : 16;
int star2OY : 16;
int star2OZ : 16;
int star3BX : 16;
int star3BY : 16;
int star3BZ : 16;
int star3OX : 16;
int star3OY : 16;
int star3OZ : 16;
} fields;
} adcs_measurements_t;
我通过调用以下函数填充结构：
void adcsTM191_measurements(adcs_measurements_t* dataOut)
{
int pass;
unsigned char TMID = 191;
unsigned char readBuff[72] = {0};
pass = I2C_write(ADCS_ADDR, &TMID, 1);
if(pass != 0)
{
printf("write error %d\n", pass);
}
pass = I2C_read(ADCS_ADDR, readBuff, 72);
if(pass != 0)
{
printf("read error %d\n", pass);
}
dataOut->fields.magneticFieldX = (readBuff[1] & 0x00FF) << 8 | (readBuff[0] & 0x00FF);
dataOut->fields.magneticFieldY = (readBuff[3] & 0x00FF) << 8 | (readBuff[2] & 0x00FF);
dataOut->fields.magneticFieldZ = (readBuff[5] & 0x00FF) << 8 | (readBuff[4] & 0x00FF);
dataOut->fields.coarseSunX = (readBuff[7] & 0x00FF) << 8 | (readBuff[6] & 0x00FF);
dataOut->fields.coarseSunY = (readBuff[9] & 0x00FF) << 8 | (readBuff[8] & 0x00FF);
dataOut->fields.coarseSunZ = (readBuff[11] & 0x00FF) << 8 | (readBuff[10] & 0x00FF);
dataOut->fields.sunX = (readBuff[13] & 0x00FF) << 8 | (readBuff[12] & 0x00FF);
dataOut->fields.sunY = (readBuff[15] & 0x00FF) << 8 | (readBuff[14] & 0x00FF);
dataOut->fields.sunZ = (readBuff[17] & 0x00FF) << 8 | (readBuff[16] & 0x00FF);
dataOut->fields.nadirX = (readBuff[19] & 0x00FF) << 8 | (readBuff[18] & 0x00FF);
dataOut->fields.nadirY = (readBuff[21] & 0x00FF) << 8 | (readBuff[20] & 0x00FF);
dataOut->fields.nadirZ = (readBuff[23] & 0x00FF) << 8 | (readBuff[22] & 0x00FF);
dataOut->fields.XAngularRate = (readBuff[25] & 0x00FF) << 8 | (readBuff[24] & 0x00FF);
dataOut->fields.YAngularRate = (readBuff[27] & 0x00FF) << 8 | (readBuff[26] & 0x00FF);
dataOut->fields.ZAngularRate = (readBuff[29] & 0x00FF) << 8 | (readBuff[28] & 0x00FF);
dataOut->fields.XWheelSpeed = (readBuff[31] & 0x00FF) << 8 | (readBuff[30] & 0x00FF);
dataOut->fields.YWheelSpeed = (readBuff[33] & 0x00FF) << 8 | (readBuff[32] & 0x00FF);
dataOut->fields.ZWheelSpeed = (readBuff[35] & 0x00FF) << 8 | (readBuff[34] & 0x00FF);
dataOut->fields.star1BX = (readBuff[37] & 0x00FF) << 8 | (readBuff[36] & 0x00FF);
dataOut->fields.star1BY = (readBuff[39] & 0x00FF) << 8 | (readBuff[38] & 0x00FF);
dataOut->fields.star1BZ = (readBuff[41] & 0x00FF) << 8 | (readBuff[40] & 0x00FF);
dataOut->fields.star1OX = (readBuff[43] & 0x00FF) << 8 | (readBuff[42] & 0x00FF);
dataOut->fields.star1OY = (readBuff[45] & 0x00FF) << 8 | (readBuff[44] & 0x00FF);
dataOut->fields.star1OZ = (readBuff[47] & 0x00FF) << 8 | (readBuff[46] & 0x00FF);
dataOut->fields.star2BX = (readBuff[49] & 0x00FF) << 8 | (readBuff[48] & 0x00FF);
dataOut->fields.star2BY = (readBuff[51] & 0x00FF) << 8 | (readBuff[50] & 0x00FF);
dataOut->fields.star2BZ = (readBuff[53] & 0x00FF) << 8 | (readBuff[52] & 0x00FF);
dataOut->fields.star2OX = (readBuff[55] & 0x00FF) << 8 | (readBuff[54] & 0x00FF);
dataOut->fields.star2OY = (readBuff[57] & 0x00FF) << 8 | (readBuff[56] & 0x00FF);
dataOut->fields.star2OZ = (readBuff[59] & 0x00FF) << 8 | (readBuff[58] & 0x00FF);
dataOut->fields.star3BX = (readBuff[61] & 0x00FF) << 8 | (readBuff[60] & 0x00FF);
dataOut->fields.star3BY = (readBuff[63] & 0x00FF) << 8 | (readBuff[62] & 0x00FF);
dataOut->fields.star3BZ = (readBuff[65] & 0x00FF) << 8 | (readBuff[64] & 0x00FF);
dataOut->fields.star3OX = (readBuff[67] & 0x00FF) << 8 | (readBuff[66] & 0x00FF);
dataOut->fields.star3OY = (readBuff[69] & 0x00FF) << 8 | (readBuff[68] & 0x00FF);
dataOut->fields.star3OZ = (readBuff[71] & 0x00FF) << 8 | (readBuff[70] & 0x00FF);
}
最后我打印，例如YWheelSpeed。
adcsTM191_measurements(&temp);
printf("structure y wheel speed is: %d \n", temp.fields.YWheelSpeed);
此值应打印一个负值，并且可以：
structure y wheel speed is: -97
现在这是事情，如果我打印(readBuff[27] & 0x00FF) << 8 | (readBuff[26] & 0x00FF)，它对应于Y车轮速度变量中填充的内容，则adcsTM191_measurements(adcs_measurements_t* dataOut)内的任何地方都不会打印此负值。而是打印无符号字符的最大值（65,535）。
int y = (int) (readBuff[33] & 0x00FF) << 8 | (readBuff[32] & 0x00FF);
printf("inside struct y is: %d", y);
我期望存储在结构内部会进行某种隐式强制转换，因此它将按预期输出负值。怎么样了？在不使用结构的情况下如何打印正确的值？
答案 0 :(得分：1)
您可能有32位int，所以初始化永远不会设置符号位。但是结构字段只有16位，并且在将其int调用转换为printf()时将进行符号扩展。
答案 1 :(得分：1)
根据C 2018脚注128，由int定义的位域（如int YWheelSpeed中定义的）是实现定义的还是未签名的。由于您的实现为此显示一个负值，因此大概是带符号的，因此，作为一个16位带符号整数，它可以表示从-32,768到32,767的值。
我们还可以推断出，您实现中的int多于16位，可能是32位（这是因为在一种情况下，当int y上打印有“％d”时，会打印“ 65535” ”）。
考虑此作业：
dataOut->fields.YWheelSpeed = (readBuff[33] & 0x00FF) << 8 | (readBuff[32] & 0x00FF);`
在此表达式中，常规促销将readBuff[33]和readBuff[32]转换为int。 0x00FF也是int。
如果我们假设readBuff[33]为255而readBufff[32]为159（即2 8 −97），则表达式{的右边的值{1}}为65439（即2 16 −97）。在分配中，右操作数将转换为左操作数的类型，即16位带符号整数。在这种情况下，值65,439不能用16位带符号整数表示。 C 2018 6.3.1.3 3告诉我们“结果是实现定义的，还是引发实现定义的信号。”
此转换的常见实现是产生模2 16 的结果，或者等效地将=的16个低位重新解释为2的补码16位整数。这产生−97。由于您的实现随后将值显示为-97，因此大概就是您的实现了。
因此，int被分配值为-97。以后再打印时：
dataOut->fields.YWheelSpeed然后使用默认参数推广（其中包括通常的整数提升），将printf("structure y wheel speed is: %d \n", temp.fields.YWheelSpeed);
从带符号的-97的16位整数转换为值为-97的temp.fields.YWheelSpeed 97，并打印“ -97”。
相反，假设int用(readBuff[33] & 0x00FF) << 8 | (readBuff[32] & 0x00FF)打印。正如我们在上面看到的，该表达式的值是65439，因此应打印“ 65439”。
问题指出：
现在这是事情，如果我打印
%d，它对应于Y车轮速度变量中填充的内容，…它将打印无符号字符的最大值（65,535）。
但是，(readBuff[27] & 0x00FF) << 8 | (readBuff[26] & 0x00FF)不是分配给(readBuff[27] & 0x00FF) << 8 | (readBuff[26] & 0x00FF)的值，它可能是“ Y车轮速度变量”。 YWheelSpeed是由YWheelSpeed元素32和33分配的，而不是26和27分配的。因此，如果打印出一些不同的值而不是65439，我们就不会感到惊讶。