String concat (+) problem

Projects Programming
1

Hi,

I am trying to concatenate an Arduino String and a const string. It causing random characters as a result. Have you encountered a problem like this?
Thanks

String mac2String(byte ar[])
{
    String s;
    for (byte i = 0; i < 6; ++i)
    {
        char buf[3];
        sprintf(buf, "%02X", ar[i]); // J-M-L: slight modification, added the 0 in the format for padding
        s += buf;
        if (i < 5)
            s += ':';
    }
    return s;
}

uint64_t chipid = ESP.getEfuseMac();
topic = mac2String((byte *)&chipid) + "/hello";
Serial.println("now subscribing: " + topic);

and the outputs look like:
Screenshot 2020-11-20 140913.jpg

Screenshot 2020-11-20 140913.jpg

2

I guess topic is defined as a String.
I'd try instead of this:

s += buf ;

this :

s += String( buf ) ;

and also set s explicitly to "" with String s = "" ;

Also, look at snprintf() instead of sprintf(). It is generally safer.

3

I'm not seeing any problems with your code, appears to print properly here. With Strings, I'd suspect a memory corruption problem, particularly if your sketch is low on available ram to begin with.

4

With "MQTT" visible in the output, where only hex digits and colons would be expected, does indicate some corruption.
If declaring char buf[3] as static char buf[3] makes the problem appear to go away, then a RAM problem is very plausible.

5

I'm not used to using String, does the compiler actually return a String from the function, or a reference to the local variable that goes out of scope?

6

david_2018:
I'm not used to using String, does the compiler actually return a String from the function, or a reference to the local variable that goes out of scope?

The local String s is effectively preserved because the return statement assigns the String back to the function.
It would be different if, within the function, the user built up a local String s, then assigned s.c_str (or something similar) to a global variable. Once the function ends, any global variables left pointing at (non static) variables, local to that function, will be in an undefined state.

7

you shouldn't use String on a microcontroller at all. Microcontrollers are low on RAM

This is the reason why

c++ is a very famous programming-language. But it has its flaws and "bugs" if you don't know much about it

Use the library PString or SafeStrings instead

best regards Stefan