在PROGMEM (Arduino)中创建一个链接列表

Question

我正在研究一个问题，我认为这个问题基本上可以通过一个(多个)链表很好地解决。但是，我的平台是一个具有非常有限的SRAM的Arduino，因此我希望将其全部保存在PROGMEM中(使用avr/pgmspace.h库)。

引用具有指针的结构的字段时有问题。或者，换句话说，我在跟踪我的链接列表时遇到了困难。

下面是一些代码(我试图缩短代码)：

#include <avr/pgmspace.h>

typedef struct list_item
{ 
   const prog_char * header;
   const struct list_item *next_item;
}; 

// declarations 
extern const list_item PROGMEM first_item; 
extern const list_item PROGMEM second_item; 
extern const list_item PROGMEM third_item; 

// name
const prog_char first_header[] PROGMEM = "Foo";
const prog_char second_header[] PROGMEM = "Bar";
const prog_char third_header[] PROGMEM = "Baz";

// instantiation & initialization 
const list_item first_item = { &first_header[0], &second_item }; 
const list_item second_item = { &second_header[0], &third_item }; 
const list_item third_item = { &second_header[0], &first_item }; 

// pointers to our items, just for testing
list_item const * const pointer_to_first_item = &first_item;
list_item const * const pointer_to_second_item = &second_item;
list_item const * const pointer_to_third_item = &third_item;

// prints the address of the pointer passed to it
void print_pointer_address( char * description, const void * pointer)
{
    Serial.print(description);
    Serial.println((unsigned int) pointer,HEX);
}

// a test
void setup()
{
    Serial.begin(57600);

    Serial.println("\n--addresses of everything--");

    print_pointer_address("pointer to first_item = ", pointer_to_first_item);
    print_pointer_address("pointer to second_item = ", pointer_to_second_item);
    print_pointer_address("pointer to third_item = ", pointer_to_third_item);

    Serial.println("\n--go through list via pointers--");

    list_item const * the_next_item;
    the_next_item = pointer_to_first_item;
    print_pointer_address("item 1 = ", the_next_item);

    the_next_item = the_next_item->next_item;
    print_pointer_address("item 2 = ", the_next_item);

    the_next_item = the_next_item->next_item;
    print_pointer_address("item 3 = ", the_next_item);

    the_next_item = the_next_item->next_item;
    print_pointer_address("item 4 = ", the_next_item);

}

void loop()
{
}

它的产出如下：

--addresses of everything--
pointer to first_item = 68
pointer to second_item = 6C
pointer to third_item = 70

--go through list via pointers--
item 1 = 68
item 2 = 6C
item 3 = 1
item 4 = 5350

我的问题是:为什么第3项不等于"70"?

我认为有可能，我应该使用其中一个pgmspace函数来读取结构，但我不明白为什么它似乎适用于第一项。谢谢你的帮助或指点，我应该读一些东西来理解这一点。

Answer 1

好了，解决了。在PROGMEM中获取struct字段中的指针的正确方法是：

the_next_item = (list_item*) pgm_read_byte(&(the_next_item->next_item));

我确实很困惑。而且，它似乎适用于前几个例子的事实是一条让我走错了路的红鲱鱼。我还是不能完全解释-可能只是运气？我没有把握。

有关指针和PROGMEM的最有用的资源，请参见此处：http://www.nongnu.org/avr-libc/user-manual/pgmspace.html