ESP8266使用PubSubClient时将byte*转换为字符串的问题

Hey there! I see exactly what's tripping you up here. Let's break down the issues and fix your callback function step by step.

First, let's call out the key problems in your current code:

• You’re using the assignment operator = instead of the comparison operator == in your if checks—this just sets payload to the string literal instead of checking if they match.
• The byte* payload from PubSubClient isn’t a null-terminated C string, so you can’t directly compare it to literals like "dragon" with ==, or even print it as a full string with (char)payload.

Here’s the corrected approach: #

1. Convert the payload to a null-terminated string
   MQTT payloads don’t automatically include a \0 terminator, so we need to create a proper C string by copying the payload bytes and adding the terminator manually.

2. Use strcmp() for string comparison
   Direct pointer comparison (==) checks if two variables point to the same memory address, not if their string contents match. strcmp() compares the actual character values in the strings.

Modified Callback Function: #

#include <string.h> // Required for memcpy() and strcmp()

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  // Print the full payload correctly (your loop here was right!)
  for (int i = 0; i < length; i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println("]");

  // Convert payload to a safe, null-terminated string
  // Add a buffer limit to prevent overflow
  #define MAX_PAYLOAD_LENGTH 64
  char payloadStr[MAX_PAYLOAD_LENGTH];
  
  if (length < MAX_PAYLOAD_LENGTH) {
    memcpy(payloadStr, payload, length);
    payloadStr[length] = '\0'; // Critical: add null terminator to make it a valid C string
  } else {
    Serial.println("Error: Payload too large for buffer!");
    return;
  }

  // Now we can safely compare the string contents
  if (strcmp(payloadStr, "dragon") == 0) {
    Serial.println("Received dragon!");
  } else if (strcmp(payloadStr, "boa") == 0) {
    Serial.println("Received boa!");
  }

  // Your LED control logic was already correct (it only checks the first character)
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW); // Active low LED on ESP-01
  } else {
    digitalWrite(BUILTIN_LED, HIGH);
  }
}

Quick Notes: #

• Make sure to include <string.h>—most Arduino environments include this by default, but adding it explicitly avoids surprises.
• Adjust MAX_PAYLOAD_LENGTH based on the largest payload size you expect to handle, to prevent buffer overflow.
• Your original LED control code worked because it only checks the first character, which doesn’t require a null-terminated string.

That should get your payload comparisons working as expected! Let me know if you hit any other roadblocks.

内容的提问来源于stack exchange，提问作者Thøger Abildgaard