Convert byte* to int in Arduino

Question

I am trying to convert byte* value to int here is how I have it.

void mqttCallback(char* topic, byte* payload, unsigned int length) {
 String topicStr = topic;
 int* payload_value;
 int updates_cal;
 payload_value = (int*)payload;
 //updates_cal = *payload_value;
 Serial.print((int)payload_value);
 delay(1);
 Serial.println();
 Serial.println("-----------------------");
}

I am getting MQTT payload as Integer value. Which I want to receive in my NodeMCU (ESP8266).

Is there any way to do this?

Thank you!

• AnT stands with Russia: If you want to convert a byte * to int, then you already have it, assuming that int is large enough to store an address on your platform. But what would be the point of this? Your payload_value is an address.
• user3201500: @AnT I just want integer (without pointer). How can I Serial.print it?
• AnT stands with Russia: What specific integer do you want? Taken from where?
• user3201500: @AnT the integer is published by MQTT as a message.
• AnT stands with Russia: The description of the format of MQQT message suggest that the second part of my answer (as well as Duncan C's) answer is the proper way to do it.

Answer 1

Judging by the comments, you are actually receiving your integer data as text, as a sequence of ASCII characters. This is what you send yourself using mosquitto_pub. The buffer is apparently not zero-terminated.

If these characters represent an integer value, then the way to retrieve it as an integer value would be

char buffer[128];
// Make sure here that `length` is smaller than the above buffer size. 
// Otherwise, you'd need a bigger buffer
// Form a C-string from the payload
memcpy(buffer, payload, length);
buffer[length] = '0円';
// Convert it to integer
char *end = nullptr;
long value = strtol(buffer, &end, 10);
// Check for conversion errors
if (end == buffer || errno == ERANGE)
 ; // Conversion error occurred
else
 Sterial.println(value);

---

Another approach, which avoids using a separate large[ish] buffer just for the purposes of zero-termination, would be

// Build a `scanf` format string that will read no more than `length`
// characters without relying on zero-termination of the payload
char format[16];
snprintf(format, sizeof format, "%%%ud", length);
// Convert the payload
int payload_value = 0;
if (sscanf((const char *) payload, format, &payload_value) == 1)
 Serial.println(payload_value);
else
 ; // Conversion error occurred

However, this approach is less protected from integer overflow than the previous one.

---

Note that most of the above "jumping through the hoops" is dedicated to adding a zero-terminator to the input buffer. If instead you opt for ensuring zero-termination on the sender's side, then it would all reduce to a simple

// Convert C-string to integer
char *end = nullptr;
long value = strtol((const char *) payload, &end, 10);
// Check for conversion errors
if (end == buffer || errno == ERANGE)
 ; // Conversion error occurred
else
 Sterial.println(value);

No extra buffers required.

• Majenko: Comments are not for extended discussion; this conversation has been moved to chat.

Answer 2

Don't know if you found an answer yet, but I had the exact same issue and eventually came up with this:

 payload[length] = '0円'; // Add a NULL to the end of the char* to make it a string.
 int aNumber = atoi((char *)payload);

Pretty simple in the end!

• Steve2955: Keep in mind that payload[length] is not part of the memory dedicated to the payload variable. The last item in the memory space of an array always is length-1. Your approach may work most of the time, but you could potentialy overwrite other variables located at that place in memory.

Answer 3

byte* means "pointer to byte(s). It is usually used to point to a byte buffer, and usually will also have a length value so you know how many bytes are in the buffer. It looks like your function does have a length value.

If you're certain that the data in the payload parameter is a single int, and the length is correct for an int (2 bytes on Arduino I believe) then you should be able to cast the pointer to int* type and then fetch the value from the buffer:

void mqttCallback(char* topic, byte* payload, unsigned int length) {
 String topicStr = topic;
 int payload_value;
 int updates_cal;
 if (length >= 2) {
 //Cast payload to an int pointer and fetch the value
 intValue = *((int*)payload); 
 }
}

• user3201500: I tried this, but there is some error. Its resetting my NodeMCU. Exception number is 9 is througing.