Arduino （ESP32） - Struct-Size 和输出混淆-解网

问：

我尝试在 EPS32 上将结构转换为字节流。在我的结构/联合下面。

#include <Arduino.h>

typedef struct {
  uint16_t header;
  float pi;
  uint16_t crc;
} BlackBoxStruct;

typedef union {
  BlackBoxStruct data;
  uint8_t bytes [sizeof(BlackBoxStruct)];
} BBD; 

BBD bbd;
uint8_t size, size2, size3;
char buf[100];

void setup() {
  Serial.begin(115200);
  bbd.data.header=0xFEEF;
  bbd.data.pi = 1.234;
  bbd.data.crc=0x1122;

  size = sizeof(BlackBoxStruct);
  size2 = sizeof(bbd);
  size3 = sizeof(BBD);
  sprintf(buf, "\n\nArraySize: %d, StructSize: %d, BBD: %d", size, size2, size3);
  Serial.println(buf);
  for (uint8_t i=0; i < size; i++) {
    sprintf(buf, "byte %03d is %02x", i, bbd.bytes[i]);
    Serial.println(buf);
  }

}

void loop() {
  // put your main code here, to run repeatedly:

}

我仍然想知道for-loop

ArraySize: 12, StructSize: 12, BBD: 12
byte 000 is ef <= header uint16_t 2 Bytes little endian 0xFEEF
byte 001 is fe
byte 002 is 00 <== ??? - seems to be a uint16_t or 2 uint8_t
byte 003 is 00
byte 004 is b6 <== float 4 bytes
byte 005 is f3
byte 006 is 9d
byte 007 is 3f
byte 008 is 22 <== CRC, uint16_t little endian 0x1122
byte 009 is 11
byte 010 is 00 <== ??? 2 bytes 
byte 011 is 00

我无法解释这 4 个标有 ??? 的字节

有什么想法吗？

我期望数组的大小为 8 字节，其他 4 个字节 - 我不知道。

这种行为更令人困惑。我更改了我的结构并包含两个数组。

typedef struct {
  uint16_t header;
  long ldata[2];
  float pid[2];
  uint16_t crc;
} BlackBoxStruct;

和输出

ArraySize: 24, StructSize: 24, BBD: 24
byte 000 is ef
byte 001 is fe
byte 002 is 00 <== long are 4 bytes - 1. LONG
byte 003 is 00 
byte 004 is 44 
byte 005 is 33 <== 0x3344
byte 006 is 00 <== long are 4 bytes - 2. LONG
byte 007 is 00  
byte 008 is cd 
byte 009 is ab <== 0xabcd
byte 010 is 00 <== ???
byte 011 is 00 <== ???
byte 012 is b6 <== 1. float
byte 013 is f3
byte 014 is 9d
byte 015 is 3f
byte 016 is 1b <== 2. float
byte 017 is 2f
byte 018 is 5d
byte 019 is 40
byte 020 is 22 <== crc 0x1122
byte 021 is 11
byte 022 is 00 <== ???
byte 023 is 00 <== ???

struct arduino esp32 sizeof Arduino-C++

ESP32 作为 32 位 MCU，其数据结构对齐为 4，因此即使您根据其数据类型在结构中添加所有元素，也支持为 8 字节，但为这两个元素添加了填充，使其长度为 4 字节，以便与 4 字节内联。编译器这样做既是出于性能原因，也是出于数据对齐的目的（请参阅下面的链接以获取进一步阅读）。这可以通过以下简单示例来观察。uint16_tfloat

#include <Arduino.h>

typedef struct{
  uint16_t head{0x1234};
  float f{1000.0};
  uint16_t tail{0x5678};
} Foo_t;

Foo_t foo;

void setup() {
  Serial.begin(115200);

  // since you are using an esp32, so Serial.printf() is available
  Serial.printf("The size of uint16_t head %d bytes\n", sizeof(foo.head));
  Serial.printf("The size of float f %d bytes\n", sizeof(foo.f));
  Serial.printf("The size of uint16_t tail %d bytes\n", sizeof(foo.tail));
  Serial.printf("The size of foo %d\n", sizeof(foo));

  uint8_t *ptr = (uint8_t*) &foo;
  for(int i=0; i<sizeof(foo); i++) {
      Serial.printf("byte %d=%02x\n", i, *ptr++);
  }

}

void loop() {

}

这将生成以下输出：

The size of uint16_t head 2 bytes
The size of float f 4 bytes
The size of uint16_t tail 2 bytes
The size of foo 12
byte 0=34   // uint16_t 0x1234 stored in little endian
byte 1=12
byte 2=00   // padding for 0x1234
byte 3=00   // padding for 0x1234
byte 4=00   // float 1000.0 stored accoding to IEEE754 as 0x447a00000
byte 5=00
byte 6=7a
byte 7=44
byte 8=78   // uint16_t 0x5678 in little endian
byte 9=56
byte 10=00  // padding
byte 11=00  // padding

现在将数据结构更改为以下内容，看看你得到了什么？

typedef struct{
  uint16_t head1{0x1234};
  uint16_t head2{0x5678};
  float f{1000.0};
  uint16_t tail{0x9abc};
} Foo_t;

或

typedef struct{
  uint8_t head1{0x12};
  uint16_t head2{0x5678};
  float f{1000.0};
  uint16_t tail{0x9abc};
} Foo_t;

所有这些结构的大小为 12 字节。

延伸阅读

数据结构对齐和 C++ 结构大小和数据对齐。

Arduino （ESP32） - Struct-Size 和输出混淆

Arduino (ESP32) - Struct-Size and output obfuscated

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