c++ - Arduino progmem 读回乱码数据

Question

我正在开发一个小型 HTTP 服务器。我正在构建一个路由器，因为可能有很多路由，所以我想将它们放入闪存中，这样我就不必使用宝贵的 SRAM。然而，要么我没有正确理解某些事情，要么发生了一些奇怪的事情，因为我似乎无法从闪存中读回我存储的数据。

我有一个包含函数指针和字符指针的结构。我想将这些结构的数组存储到闪存中并读回它们。但是，通过一个小的调试打印，我可以看到我无法正确读回 char 指针。它将乱码打印到串口。

这是一个小例子。

#include <avr/pgmspace.h>

typedef struct {
    void (*func)();
    const char *URI;
} Route;

void test1() {
    Serial.println("Executed testfunc1");
}

void test2() {
    Serial.println("Executed testfunc2");
}

const char route1URI[] PROGMEM = "/route1";
const Route route1 PROGMEM = {
    test1,
    route1URI
};

const char route2URI[] PROGMEM = "/route2";
const Route route2 PROGMEM = {
    test2,
    route2URI
};

const Route routingTable[] PROGMEM = {
    route1,
    route2
};

void (*getRoute(char *URI))() {
    Route *r = (Route *)pgm_read_word(routingTable + 0);
    char *f = (char *)pgm_read_word(r->URI);

    Serial.println(f);

    return r->func;
}
void setup() {
    Serial.begin(9600);
    while (!Serial) { }

    Serial.println("started setup");
    void (*fn)() = getRoute("sometest");
    // will cause errors if called
    //fn();
    Serial.println("ended setup");
}

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

}

Answer 1

PROGMEM 不是那么容易使用。它可以稍微简化一下:

#include <avr/pgmspace.h>

struct Route {
    void (*func)();
    const char *URI;
};

void test1() {
    Serial.println(F("Executed testfunc1")); // if you are using progmem, why not for string literals?
}

void test2() {
    Serial.println(F("Executed testfunc2"));
}

const char route1URI[] PROGMEM = "/route1";
const char route2URI[] PROGMEM = "/route2";

const Route routingTable[] PROGMEM = {
    {test1,route1URI},
    {test2,route2URI}
};

void (*getRoute(char *URI))() {
    Route r;
    memcpy_P((void*)&r, routingTable, sizeof(r)); // read flash memory into the r space. (can be done by constructor too)

    Serial.println((__FlashStringHelper*)r.URI); // it'll use progmem based print
    // for comparing use: strcmp_P( URI, r.URI)

    return r.func; // r.func is already pointer to the function
}

void setup() {
    Serial.begin(57600);
    while (!Serial) { }

    Serial.println("started setup");
    void (*fn)() = getRoute("sometest");
    // will cause errors if called
    //fn();
    Serial.print((uint16_t)test1, HEX); Serial.print(' ');
    Serial.print((uint16_t)test2, HEX); Serial.print(' ');
    Serial.println((uint16_t)fn, HEX);

    Serial.println("ended setup");
}

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

}

我想 route1 和 route2 可能会导致所有问题，因为它们被用于复制到 routingTable 中。如果像我一样初始化 routingTable 的元素，它会工作得更好。而且 getRoute 也经常坏掉。

无论如何，如果你有 flash 字符串，你也可以使用 String str {(__FlashStringHelper*)r.URI}; 然后使用比较运算符:str == URI:

#include <avr/pgmspace.h>

// get size of array[]
template<typename T, int size> int GetArrLength(T(&)[size]){return size;} 

struct Route {
    void (*func)();
    const char *URI;
};

void test1() {
    Serial.println(F("Executed testfunc1")); // if you are using progmem, why not for string literals?
}

void test2() {
    Serial.println(F("Executed testfunc2"));
}
void test3() {
    Serial.println(F("Executed testfunc3"));
}

const char route1URI[] PROGMEM = "/route1";
const char route2URI[] PROGMEM = "/route2";
const char route3URI[] PROGMEM = "/route3";

const Route routingTable[] PROGMEM = {
    {test1,route1URI},
    {test2,route2URI},
    {test3,route3URI}
};

void (*getRoute(char *URI))() {
  for (int8_t i = 0; i < GetArrLength(routingTable); ++i) {
    Route r;
    memcpy_P((void*)&r, routingTable+i, sizeof(r)); // read flash memory into the r space. (can be done by constructor too)

    String uri {(__FlashStringHelper*)r.URI};
    if (uri == URI) {
      return r.func; // r.func is already pointer to the function
    }
  }

  return nullptr;
}

void setup() {
    Serial.begin(57600);
    while (!Serial) { }

    Serial.println("started setup");
    void (*fn)() = getRoute("/route3");
    // will cause errors if called
    //fn();
    Serial.print((uint16_t)test1, HEX); Serial.print(' ');
    Serial.print((uint16_t)test2, HEX); Serial.print(' ');
    Serial.print((uint16_t)test3, HEX); Serial.print(' ');
    Serial.println((uint16_t)fn, HEX);

    Serial.println("ended setup");
}