Invalid conversion error on NodeMCU/ESP8266 but not on Arduino

Projects Programming
1

Hello all,
I'm fairly new to C++ and Arduino, and I'm finding the char array hill difficult to climb. It's not made any easier when I have code that compiles and runs as expected on an Arduino Mega, but fails to compile on a "Generic ESP8266 Module".

Can anyone offer some insight as to why I either have perfectly valid code that is not compiling (ESP8266), or I have invalid code that IS compiling (Arduino Mega)?

void parse_metar () {
  Serial.println(F("[PARSING]"));

  // If no metar was returned, print and then get out since there's nothing to parse
  if (strstr(metar, "METAR")) {
    Serial.println(metar);
    return;
  }

  char *ptr = NULL;   // pointer for space-separated chunk of metar data
  char *visibility = 0;     // whole number holder for visibility that includes a fraction (eg. 2 1/2)
  ptr = strtok(metar, " ");
  while (ptr != NULL) {
    if (strstr(ptr, "RMK")) {
      // we can bail here
      break;
    }
    if (strlen(ptr) == 1 && strspn(ptr, "1234567890") == 1) {
      // Fractional visibility so hold on to the whole number
      visibility = ptr;
    } else if (strrchr(ptr, 'SM')-ptr+1 == strlen(ptr)) {
      Serial.print("Visibility: "); 
      if (atoi(visibility) > 0) {
        // Need to recombine the whole number portion of the visibility data (eg. 2 1/2)
        Serial.print(visibility); Serial.print(" "); Serial.println(ptr);
        strcat(visibility, *ptr);  // <<<< ERROR REFERS TO THIS LINE
        check_visibility(*visibility);
       } else {
        check_visibility(*ptr);
      }
    } else if (strstr(ptr, "BKN") || strstr(ptr, "OVC")) {
      Serial.print("Cloud Layer: "); Serial.println(ptr);
        check_ceiling(*ptr);
    }
    ptr = strtok(NULL, " ");
  }
}

The error I get is:
char *strcat (char *__restrict, const char __restrict);
| ^~~~~~~~~~~~
invalid conversion from 'char' to 'const char' [-fpermissive]

If the error is accurate, how/why does compilation succeed on a different device?

Just taking a stab, I removed the * from the second argument and it compiles, but it crashes the ESP when run.

I am apparently too thick to comprehend this business of char arrays so any help is appreciated.

2

strcat() takes two pointers, 'visibility' is a pointer. 'ptr' is a pointer so '*ptr' is a char, not a char *

3

Thanks for your reply! I understand enough of it to see what caused the issue, but I'll have to continue reading about chars and pointers to get a better handle on what they actually mean.

Any idea why it would compile when using the Arduino, though? Perhaps some kind of background "mind-reading"?

And FWIW, the stack crash I was getting after removing the * from ptr was resolved by removing the atoi(visibility) a few lines above, and just checking the value without atoi.

Thanks again. I was really stuck there.

5

That is because you never set visibility to point to anything. If the 'if' statement is true, you set visibility = ptr, but if it is false, you never set visibility and then pass that null pointer to atoi() - not good. Pointers have to point to some valid memory.

6

So this line:

char *visibility = 0;

doesn't count, I guess. Is that because that line sets the pointer to an invalid address?

7

Yes. That lines says visibility is pointing at nothing (nullptr). If you wanted to point to valid memory

char buffer[10];
char *ptr = buffer;

and then if you wrote to the memory location referred to by ptr, it would write into buffer.

Sounds like you might need a bit more reading on C/C++ pointers. We all had to do it at some point.

8

Got it. It is starting to make sense but you are right that I need to do more reading. Thanks for your help!

9 
strrchr(ptr, 'SM')

What is 'SM' between single quotes ?
Is that a single character or a zero-terminated string ?

10

'visibility' is a NULL pointer. The ESP8266 can detect the access error, the ATmega can't.

11

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