Arduino 访问数组产生意想不到的结果

Question

我正在尝试读取 Arduino UNO 闪存中一些字符串的长度。数组 string_table 给我带来了问题，如果我用编译器优化为常量的东西得到它的索引，那么我就会得到预期的值。如果我在运行时使用变量访问它，那么每次我都会得到不同的答案。

我认为这不是 Arduino 特有的，因为我似乎没有调用任何 Arduino 特定的功能。

代码：

#include <avr/pgmspace.h>

// Entries stored in flash memory
const char entry_0[] PROGMEM = "12345";
const char entry_1[] PROGMEM = "abc";
const char entry_2[] PROGMEM = "octagons";
const char entry_3[] PROGMEM = "fiver";

// Pointers to flash memory
const char* const string_table[] PROGMEM = {entry_0, entry_1, entry_2, entry_3};

void setup() {
  Serial.begin(115200);
  randomSeed(analogRead(0));

  int r = random(4);
  Serial.print("random key (r) : ");
  Serial.println(r);
  Serial.print("Wrong size for key = ");
  Serial.print(r);
  Serial.print(" : ");
  Serial.println(strlen_P(string_table[r]));
  int i = 1;
  Serial.print("Correct size for key = ");
  Serial.print(i);
  Serial.print(" : ");
  Serial.println(strlen_P(string_table[i]));
  Serial.println("=====");

  Serial.println("Expected Sizes: ");
  Serial.print("0 is: ");
  Serial.println(strlen_P(string_table[0]));
  Serial.print("1 is: ");
  Serial.println(strlen_P(string_table[1]));
  Serial.print("2 is: ");
  Serial.println(strlen_P(string_table[2]));
  Serial.print("3 is: ");
  Serial.println(strlen_P(string_table[3]));
  Serial.println("++++++");

  Serial.println("Wrong Sizes: ");
  for (i = 0; i < 4; i++) {
    Serial.print(i);
    Serial.print(" is: ");
    Serial.println(strlen_P(string_table[i]));
  }
  Serial.println("------");
  delay(500);
}

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

}

结果：

random key (r) : 1
Wrong size for key = 1 : 16203
Correct size for key = 1 : 3
=====
Expected Sizes: 
0 is: 5
1 is: 3
2 is: 8
3 is: 5
++++++
Wrong Sizes: 
0 is: 0
1 is: 11083
2 is: 3
3 is: 3
------

Answer 1

来自avr-libc - source code：

strlen_P() is implemented as an inline function in the avr/pgmspace.h header file, which will check if the length of the string is a constant and known at compile time. If it is not known at compile time, the macro will issue a call to __strlen_P() which will then calculate the length of the string as normal.

这可以解释它（尽管对 __strlen_P() 的调用应该已经解决了）。

也许可以改用以下内容：

strlen_P((char*)pgm_read_word(&(string_table[i])));

或者尝试使用 strlen_PF()。

Arduino 访问数组产生意想不到的结果

Arduino accessing array produces unexpected resuslts

c++

arrays

arduino

avr

progmem

代码：

结果：