C++浮点数和字节数组的联合问题

C++ Union of a Float and a Byte Array Issue

本文关键字：问题 数组 字节数 浮点数 字节 C++      更新时间：2023-10-16

我正在研究一个arduino(基于AVR平台(，我有一个方法可以接收浮点数并将其写入EEPROM。我必须将浮点数转换为字节数组才能与EEPROM交互。我有两个功能如下：

void WriteFloatToEEPROM(int address, float value) {
union {
byte byteVal[4];
float floatVal;
} data;
data.floatVal = value;
for (int i = 0; i < 4; i++) {
EEPROM.update(address + i, data.byteVal[i]);
}   
}
float ReadFloatFromEEPROM(int address) {
union {
byte byteVal[4];
float floatVal;
} data;
for (int i = 0; i < 4; i++) {
uint8_t readValue = EEPROM.read(address + i);
data.byteVal[i] = readValue;        
}
return data.floatVal;
}

当我打印出结果时，我得到以下几个例子：

Read value at address 50 for float read 0
Read value at address 51 for float read 0
Read value at address 52 for float read 0
Read value at address 53 for float read 0
Returned float val for address 50:0.00
Read value at address 90 for float read 0
Read value at address 91 for float read 0
Read value at address 92 for float read 0
Read value at address 93 for float read 160
Returned float val for address 90:-0.00
Read value at address 130 for float read 44
Read value at address 131 for float read 113
Read value at address 132 for float read 61
Read value at address 133 for float read 138
Returned float val for address 130:-0.00
Read value at address 170 for float read 0
Read value at address 171 for float read 0
Read value at address 172 for float read 0
Read value at address 173 for float read 0
Returned float val for address 170:0.00

我是否错误地使用了联合/向后写入EEPROM还是什么？此外，如果有人有更好的方法，我愿意接受建议。提前致谢

读取不是联合的"活动"字段是UB(未定义的行为(。

您必须使用memcpy：

void WriteFloatToEEPROM(int address, float value) {
byte byteVal[sizeof(float)];
memcpy(byteVal, &value, sizeof(float));
for (int i = 0; i < sizeof(float); i++) {
EEPROM.update(address + i, byteVal[i]);
}   
}
float ReadFloatFromEEPROM(int address) {
byte byteVal[sizeof(float)];
for (int i = 0; i < sizeof(float); i++) {
byteVal[i] = EEPROM.read(address + i);
}
float f;
memcpy(&f, byteVal, sizeof(float));
return f;
}

Arduino 为对象提供了 .put(( 和 .get(( 方法EEPROM因此您不必重新发明轮子。您的代码可以像以下那样简单：

void WriteFloatToEEPROM(int address, float value)
{
EEPROM.put(address, value);
}
float ReadFloatFromEEPROM(int address)
{
float value;
EEPROM.get(address, value);
return value;
}

.put()的行为方式也与.update()类似

此函数使用EEPROM.update()来执行写入，因此不会 如果值没有更改，则重写该值。

这是结构和联合的常见问题。您需要调查"对齐"。默认通常为"word"(2 个字节(，因此您的byte byteVal[4];可以占用 8 个字节(byte, empty, byte, empty ...(。您必须使用__attribute__ ((packed))来避免它。

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