How to string compare const uint8* in a ESP32 microcontroller chip

Question

I'm sending data from an ionic app (typescript) to an ESP32chip. Data from the app uses protocol buffer format. Here's the code:

this.provisionMgrAction.setAction('stop');
let bytesOfStuff = await this.provisionMgrAction.serializeBinary();
BleClient.write(this.deviceID, serviceID, characteristicID, bytesOfStuff);

On the ESP chip side (code in C), data is received as "stop". The log (ESP_LOGI line below) prints value of inbuf as "stop" (without quotation marks).

esp_err_t custom_prov_data_handler(uint32_t session_id, const uint8_t *inbuf, ssize_t inlen,
                                          uint8_t **outbuf, ssize_t *outlen, void *priv_data)
{
    if (inbuf) {
        ESP_LOGI(TAG, "Received data: %.*s", inlen, (char *)inbuf);
        if (strcmp(inbuf, "stop") == 0) {
            ESP_LOGI(TAG, "print something");
        }
    }
}

But strcmp doesn't seem to work. I'm presuming it has something to do with the fact that when "stop" is sent over BLE from the client side app to the ESP32 chip, it comes without a null character, though I'm not sure how to confirm that. In any event, how do I make sure that strcmp equates the value of inbuf to "stop"? Do I need to add a null character to inbuf? If yes, how do I accomplish that? Or do I need to compare inbuf to "stop\0"? The length of inbuf (inlen) is printed out to be 2 more than the number of characters in the message that is transmitted to the chip over BLE (in this case, 6).

Answer 1

Yes the problem is the '\0' character that is missing.

You could use strncmp and specify the length (here it would be inlen -2 I guess, the 2 more might be checksum or something like that)

Like

if (strncmp(inbuf, "stop", inlen - 2) == 0)

That should work.

Also, be cautious if you sometime answer from ESP32. Depending on the app side you would also need a termination character (for example on Android Studio Java I needed to add '\n' because I called ReadLine function).

So based on the values extracted in hexa from the buffer you can get your info using

if (strncmp((char *)&inbuf[2], "stop", inlen - 2) == 0)

Answer 2

1. Count the amount of received characters in the payload. If they aren't 4, you can stop there because the input is obviously wrong.
2. If there is no null termination, you don't have a C string but just a raw data array. If you don't need this to become a C string, then the fastest and easiest is memcmp(inbuf, "stop", 4);. This doesn't rely on null termination.
3. In case you need to re-use the data later as a C string, then just do inbuf[4] = '\0' to append a null terminator manually to the 5th character. Now you could use strcmp but there's no real point of using it over memcmp when you already know the data size. memcmp is always faster than strcmp.

I can't think of any scenario where it would make sense for you to use strncmp.