Arduino progmem 读回乱码数据

Question

我正在开发小型 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

char *f = (char *)pgm_read_word(r->URI);
Serial.println(f);

f是一个指向PROGMEM中字符数组的指针，但是Serial.println不知道！它最终试图从 RAM 中读取字符串，而实际上它不是。

Arduino Serial 库似乎不支持 PROGMEM 中的字符串。您将需要遍历字符串，一次打印一个字符，使用另一个库，或将字符串存储在 RAM 中。

Answer 2

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 可能会引起所有的麻烦，因为它被用于复制到 routingTable。如果像我一样初始化 routingTable 的元素，效果会更好。而且getRoute也坏了很多。

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");
}

Answer 3

正如@KIIV 所指出的，最好直接在 routingTable 的声明中指定 Route。作为替代解决方案，您可以将结构 Route 重新定义为

typedef struct {
    void (*func)();
    char URI[16];  //adjust the size to your need
} Route;

这样，从闪存中读取URI和function地址可以通过一次调用memcpy_P来完成。完整代码：

typedef struct {
    void (*func)();
    char URI[16];  //adjust the size to your need
} Route;

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

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

const Route routingTable[] PROGMEM = {
    {test1, "/route1"},
    {test2, "/route2"}
};

void (*getRoute(char *URI, int idx))() {
    Route r;
    memcpy_P(&r, &routingTable[idx], sizeof(Route));

    Serial.print(idx); Serial.println(". -----------------------------");
    Serial.print("Route: "); Serial.println(r.URI);
    Serial.print("fn address: "); Serial.println((uint16_t)r.func, HEX);
    Serial.print("test1 address: "); Serial.println((uint16_t)test1, HEX);
    Serial.print("test2 address: "); Serial.println((uint16_t)test2, HEX);

    return r.func;
}

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

    Serial.println("started setup");
    void (*fn)();

    const int n = sizeof(routingTable) / sizeof(Route);
    for (int i = 0; i < n; i++) {
      fn = getRoute("sometest", i);
      fn();
    }
    Serial.println("ended setup");
}

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

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