指针和PROGMEM的问题

问题描述

我在Arduino UNO上编程，遇到了非常烦人的路障。

我在PROGMEM中遇到了一个非常奇怪的问题，我将其范围缩小到了指针数组的问题。

当我用变量j索引PROGMEM数组时。根据天气的不同，它会返回不同的结果，变量j是由静态数字设置还是由另一个变量设置。

程序号是存储在对象中的整数，并且只能按以下方式设置，对内存进行分配并且变量保留其值。我已经测试过了。

*(this->program) = 0;

将j设置为0直接产生正确的地址

uint16_t j = 0;
Serial.println(j);    
//printed 0 as expected

address = (uint16_t)*(pgmStringTable + (uint16_t)j); 
Serial.println(address);   
//gave address = 105,which is correct and PROGMEM reads the correct bytes

但是，如下使用progam变量设置j会产生错误的地址

uint16_t j = *(this->program);
Serial.println(j);    
//printed 0 as expected

address = (uint16_t)*(pgmStringTable + (uint16_t)j); 
Serial.println(address);   
//gave address = 4,which isnt right and results in PROGMEM read errors

即使尝试解决此设置问题，例如对*（this-> program）进行算术运算或将其发送到返回相同值的整数的函数也无济于事。唯一有效的方法是遍历所有整数，如下所示：

//loops through most unsigned integer values
for(uint16_t j = 0; j < 65000; j++){
  if(j == *(this->program)){
     address = (uint16_t)*(pgmStringTable + (uint16_t)j); 
     //address is correct and PROGMEM works correctly
  }
}

上述工作甚至是确认malloc正常工作的更多原因，因为*（this-> program）在与j比较时返回正确的值。

这是一个非常糟糕的解决方法，因为它效率很低，并且会导致我的程序浪费大量的时间，这对时间很敏感。

任何指向解决方案的指针或该指针表现为滑稽的原因都将非常有用。

问题出在一个非常大的项目中，因此为了缩小错误范围，我在一个更小的.ino文件中重新创建了该问题

整个代码是这个。上传我的arduino UNO时，它产生了相同的错误

编辑：

在进一步研究中，将j设置为在我的主项目中不是恒定的任何变量（因此该变量不能在项目中的其他任何地方）似乎以与上述相同的方式破坏了pgm。



const char pgmString1[] PROGMEM = "String 1__";
const char pgmString2[] PROGMEM = "2nd String";
const char *const pgmStringTable[] PROGMEM = {pgmString1,pgmString2};




class testObject{

    private:
        uint16_t* program;

    public:


        testObject(){
            program = (uint16_t*) malloc(sizeof(uint16_t));
            *(this->program) = 0;
        }

        void read(){
            if(*(this->program) == 0){
                //the console prints the below,meaning the value is definitly 0
                Serial.println("it was 0");
                *(this->program) = 0;
            }else{
                Serial.println("it wasnt 0");
                
                //if this is uncommeneted,this program outputs the correct values but the above comment will not be dispalyed
                //*(this->program) = 0;
            }

            char temp[10];
            uint16_t j = 0;
            uint16_t address = 0;   

////////////////////////////////////////////// indirectly setting j
            //sets the j value to be 0,which is what is stored in this->program
            j = *(this->program);

            //prints j to make sure its 0,then finds the address of the progmem string
            Serial.print(j);
            Serial.print("  1 Address: |"); 
            address = (uint16_t)*(pgmStringTable + (uint16_t)j);   
            Serial.print(address);     //the address printed is incorrect
            Serial.print("   ");

            //reads byte by byte from the address,it reads the wrong bytes
            for(int i = 0; i < 10; i++){    
                char myChar = pgm_read_byte( address + i );
                Serial.print(myChar);
            }
            Serial.println("\n");
                    
////////////////////////////////////////////// directly setting j
            //but just setting j = 0 works fine
            j=0;

            //so directly setting j as 0,same code as above but it produces the correct output
            //SAME AS ABOVE,CouldNT SEPERATE INTO ITS ONW FUNCTION
            Serial.print(j);
            Serial.print("  2 Address: |");
            address = (uint16_t)*(pgmStringTable + (uint16_t)j);
            Serial.print(address);
            Serial.print("   ");
            for(int i = 0; i < 10; i++){    
                char myChar = pgm_read_byte( address + i );
                Serial.print(myChar);
            }
            Serial.println();
            //

        }
};





testObject pgmReader;

void setup(){
    Serial.begin(9600);
}

void loop(){
    Serial.println("\n\n\n\n\n\n\n\n");
    delay(1000);
    pgmReader.read();

}

解决方法

不仅pgmString1和pgmString2也是pgmStringTable都是PROGMEM。因此，您必须使用pgm_read_函数来读取pgmStringTable。

address = pgm_read_ptr(pgmStringTable + (uint16_t)j);

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