escape quotes in a c-string

Projects Programming
1

I have a project where I need to send a JSON string in an MQTT message.

Using the ArduinoJson library makes generating the JSON string fairly easy, but to send the JSON string as an MQTT message, I have to escape the quotes.

{"temperature":"65.43"} has to be sent as: {"temperature":"65.43"}

I couldn't find a function or method to escape the quotes in the string, so I wrote one.

// Demo of a function to escape quotes in a c-string.

char fooBufer[150];             //Holds the results of escapeQuotes()


// ======= setup() =======
void setup(void)
{
  Serial.begin(115200);
  delay(10);
  Serial.println();
  Serial.println();


  char foo[] = R"(He said: "The Quick Brown Fox".)";
  Serial.print(F("foo= "));
  Serial.println(foo);
  escapeQuotes(foo);
  Serial.print(F("fooBufer = "));
  Serial.println(fooBufer);

}


// ======= loop() =======
void loop(void) {
}


// ======= escapeQuotes() =======
void escapeQuotes(char *foos) {
  //Function to take a c-string containing quotes and fill a c-string with the quotes escaped.
  //For example:
  //  (He said "The Quick Brown Fox".)
  //becomes:
  //  (He said \"The Quick Brown Fox\".)
  //
  char *p = foos;
  int j = 0;

  for (int i = 0; i < strlen(p); i++) {
    if (p[i] == 34) {
      fooBufer[j++] = 92;       // Backslash character
      fooBufer[j++] = 34;       // Quote character
    } else {
      fooBufer[j++] = p[i];     // Not a quote, keep it.
    }
  }
}

Do you see any problems with the function escapeQuotes() , or is there a better way?

Q2: I am still learning to use c-strings wherever possible, but I still haven't figured out how to return a c-string from a function. Tips here would be appreciated.

2

But wouldn't that write past the scope of the char array?

3

SteveMann:
...I still haven't figured out how to return a c-string from a function.

You can return a *pointer *to a C-string.

4

dougp:
You can return a *pointer *to a C-string.

Even if that string is defined in the function? Doesn't that make it out of scope?

5

Maybe this will help. You can definitely return a pointer to a C-string as long as it's properly allocated (i.e. via malloc()) without having to worry about scope.

6

Power_Broker:
Maybe this will help. You can definitely return a pointer to a C-string as long as it's properly allocated (i.e. via malloc()) without having to worry about scope.

If a function uses malloc() it is a good practice to also use free() before returning. A static variable is safer. You can also pass an array by reference, if it's a char string you don't have to pass the array length because it is zero terminated. That way, the calling function can allocate the memory and it can have an identifier that is in the same scope.

7

A part of your code is looking to complex, this is simpler and does the same:

for (int i = 0; i < strlen(p); i++) {
  if (p[i] == 34) fooBufer[j++] = 92; //Add backslash before quote
  fooBufer[j++] = p[i];
}
8

"but to send the JSON string as an MQTT message, I have to escape the quotes."

Have you tried using a ' single quote instead of a " double quote? This works with html applications. If needed you might later convert the ' back to a ".