在 ESP8266 上读取 c 中的 const* 时崩溃

Question

我正在制作一个系统，通过 SoftwareSerial 从 Arduino Uno 读取传感器值并通过 MQTT 发布它。但是，我认为我面临的问题更普遍，我必须承认我是 c 的新手。

我正在读取数据，并将其拆分为在程序顶部定义的两个 const* 变量。

当我读回从串行连接解析的已保存 "data" 和 "topic" 变量时，我只得到垃圾输出，并且通常是重启设备的崩溃。

在read-from-serial函数中打印成功，但后面无法正确读取。它与数据的保存方式有关吗？我可以显式地为变量分配一些内存吗？

我使用的是 ESP8266 (ESP07) 芯片，具有较低的波特率和适当的电压供应。好像运行很好很稳定。

#include <StringSplitter.h>
#include <PubSubClient.h>
#include <ESP8266WiFi.h>
#include <time.h>

//const char* ssid = "xxxx";
//const char* password =  "xxxx";
const char* ssid = "xxxx";
const char* password =  "xxxx";
const char* mqttServer = "xxxx;
const int mqttPort = xxxx;
const char* mqttUser = "xxxx";
const char* mqttPassword = "xxxx";
int timezone = 1;
int dst = 0;

数据存储在这里：

char* data;
char* topic;
boolean newData = false;
boolean unpublishedData = false;

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {

  Serial.begin(19200);
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.println("Connecting to WiFi..");
  }
  Serial.println("Connected to the WiFi network");
  configTime(timezone * 3600, dst * 0, "pool.ntp.org", "time.nist.gov");


  client.setServer(mqttServer, mqttPort);
  client.setCallback(callback);

  while (!client.connected()) {
    Serial.println("Connecting to MQTT...");

    if (client.connect("ESP8266Client", mqttUser, mqttPassword )) {

      Serial.println("connected");

    } else {

      Serial.print("failed with state ");
      Serial.print(client.state());
      delay(2000);

    }
    // wait and determine if we have a valid time from the network.
    time_t now = time(nullptr);
    Serial.print("Waiting for network time.");
    while (now <= 1500000000) {
      Serial.print(".");
      delay(300); // allow a few seconds to connect to network time.
      now = time(nullptr);
    }
  }
  Serial.println("");

  time_t now = time(nullptr);
  Serial.println(ctime(&now));

  String datatext = "val: ";
  String timetext = ", time: ";
  String dataToBeSent = "test";
  String timeToBeSent = ctime(&now);

  String publishString = datatext + dataToBeSent + timetext + timeToBeSent;
  Serial.println("Attempting to publish: " + publishString);
  client.publish("trykk/sensor0", (char*) publishString.c_str());
  client.subscribe("trykk/sensor0");

}

void callback(char* topic, byte* payload, unsigned int length) {

  Serial.print("Message arrived in topic: ");
  Serial.println(topic);

  Serial.print("Message:");
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }

  Serial.println();
  Serial.println("-----------------------");

}

void loop() {
  client.loop();
  recvWithStartEndMarkers();
  showNewData();
  publishReceived();
}

void publishReceived() {
  if (unpublishedData) {
    Serial.println("Hello from inside the publisher loop!");
    time_t now = time(nullptr);
    char* timeNow = ctime(&now);

这里失败，正在读取数据：

    char publishText[30]; //TODO: make it JSON
    strcpy( publishText, data );
    strcat( publishText, " " );
    strcat( publishText, timeNow );

    Serial.print("publishText: ");
    Serial.println(publishText);
    Serial.print("topic: "); 
    Serial.println(topic);
    client.publish(topic, publishText);
    client.subscribe(topic);
    unpublishedData = false;
  } else if (!data) {
    Serial.println("No data saved to array.");
  } else if (!topic) {
    Serial.println("No topic saved to array.");
  }
}

void recvWithStartEndMarkers() {
  int numChars = 32;
  char receivedChars[numChars];
  static boolean recvInProgress = false;
  static byte ndx = 0;
  char startMarker = '<';
  char endMarker = '>';
  char rc;

  if (Serial.available() > 0) {
    Serial.println("Hello from inside the receive loop!");
    delay(100);
    while (Serial.available() > 0 && newData == false) {
      rc = Serial.read();
      Serial.println("Reading from data line.");

      if (recvInProgress == true) {
        if (rc != endMarker) {
          receivedChars[ndx] = rc;
          ndx++;
          if (ndx >= numChars) {
            ndx = numChars - 1;
          }
        }
        else {
          Serial.println("Found the end marker.");
          receivedChars[ndx] = '[=12=]'; // terminate the string
          recvInProgress = false;
          ndx = 0;
          newData = true;
          unpublishedData = true;

这部分将正确的值打印回给我：

          //Split the string
          Serial.print("ESP debug: read: ");
          Serial.println(receivedChars);
          const char s[2] = ":";
          *data = strtok(receivedChars, s);
          Serial.print(data);
          Serial.print(" ");
          *topic = strtok(NULL, s);
          Serial.println(topic);
        }
      }

      else if (rc == startMarker) {
        recvInProgress = true;
        Serial.println("Found start marker");
      }
    }
  }
}

//This is gutted as it gave me problems reading the variables
void showNewData() {
  if (newData == true) {
    Serial.print("This just in ... ");
    Serial.print("Topic: ");
    Serial.print("stuff");
    Serial.print(", data: ");
    Serial.println("more stuff");
    newData = false;
  }
}

Answer 1

来自您的代码：

char* data;
...
*data = strtok(receivedChars, s);

strtok return a char* 但你做 *data = strtok(...) 而 data 本身就是一个（未初始化）char *，这是不一致的，并且您首先 'chance' 发生崩溃，因为您在随机地址写入。

如果您没有崩溃并且您的程序可以继续数据不会自行修改并保持未初始化状态。

在

strcpy( publishText, data );
...
Serial.print(data);

当您使用 data 作为 char* 执行 Serial.print(data); 和 strcpy( publishText, data ); 时，您从一个随机（当然是无效的）地址读取，产生你的崩溃。

要更正，只需将 *data = strtok(receivedChars, s); 替换为 data = strtok(receivedChars, s);

Answer 2

如 bruno 的回答所示，在修复了 strtok 的结果分配给 data 之后，还有另一个可能导致崩溃的错误。

您的函数 loop() 首先调用 recvWithStartEndMarkers()，然后 publishReceived()。

void loop() {
  client.loop();
  recvWithStartEndMarkers();
  showNewData();
  publishReceived();
}

在函数 recvWithStartEndMarkers 中，您将一些数据读入局部数组 receivedChars，将其输入 strtok 并将从 strtok 返回的指针写入全局变量 data.

void recvWithStartEndMarkers() {
  int numChars = 32;
  char receivedChars[numChars]; /* this is a local variable with automatic storage */
  /* ... */

    while (Serial.available() > 0 && newData == false) {
      /* ... */
          receivedChars[ndx] = rc;
          ndx++;
          if (ndx >= numChars) {
            ndx = numChars - 1;
          }
          /* ... */
          receivedChars[ndx] = '[=11=]'; // terminate the string
          /* Now there is a terminated string in the local variable */
          /* ... */

          //Split the string
          /* ... */
          const char s[2] = ":";
          data = strtok(receivedChars, s); /* strtok modifies the input in receivedChars and returns a pointer to parts of this array. */ 
          /* ... */
}

离开函数后，receivedChars 的记忆不再有效。这意味着 data 将指向堆栈上的这个无效内存。

稍后您想在函数 publishReceived() 中访问全局变量 data。访问此内存是未指定的行为。你可能仍然得到数据，你可能得到其他东西或者你的程序可能会崩溃。

void publishReceived() {
  /* ... */
    char publishText[30]; //TODO: make it JSON
    strcpy( publishText, data ); /* This will try to copy whatever is now in the memory that was part of receivedChars inside recvWithStartEndMarkers() but may now contain something else, e.g. local data of function publishReceived(). */
  /* ... */

要解决此问题，您可以在 recvWithStartEndMarkers():

中使用 strdup

data = strtok(receivedChars, s);
if(data != NULL) data = strdup(data);

那么当您不再需要数据或再次调用 recvWithStartEndMarkers() 之前，您必须 free(data) 某个地方。

或者使data成为一个数组并在recvWithStartEndMarkers()中使用strncpy。

在 ESP8266 上读取 c 中的 const* 时崩溃

Crash when reading const* in c on ESP8266

c

arduino

software-serial

esp8266