sizeof() char array

Projects Programming
1

After some wrangling, this little bit of code compiles but gives a curious result. The size reported for char* posnText[] is 12. Why not 6, or 48 or 17? Is it because pointers are two bytes?

char* posnText[] = {"topline1",
                          "topline2",
                          "topline3",
                          "topline4",
                          "topline5",
                          "topline6"
                         };
struct menuItems {
  char* topLineText;
  char* bottomLineText;
  byte numOfParms;
};
menuItems screenText[2] = {
  "top1", "bottom1", 4,
  "top2", "bottom2", sizeof(posnText)/2,
};


void setup() { 
Serial.begin(9600);
Serial.println (screenText[1].bottomLineText);
Serial.print("size ");
Serial.println(sizeof(posnText));
}

void loop() {
  // put your main code here, to run repeatedly:

}
1 Like
2

Is it because pointers are two bytes?

It is because pointers are the same size as ints, and ints are two bytes on some Arduinos.

3

Aah. Thanks!

4

Each element posnText is a pointer to a char. The size of a pointer is indeed 2 bytes on most 8-bit Arduinos, and there are 6 elements in the array, so the total size will be 12 bytes.
To get the number of strings in posnText, use:

const size_t nb_strings = sizeof(posnText) / sizeof(posnText[0]);

posnText[0] is a pointer, so sizeof(posnText[0]) will return 2.

Each of the six char pointers points to a string of characters in static memory. To get the number of characters in one of these strings, you can use the strlen function.

Pieter

5

You can expand PieterP's concept to:

#define ELEMENTS(x) (sizeof(x) / sizeof(x[0]))

which allows you to do things like:

   for (int i = 0; i < ELEMENTS(posnText); i++) {
      // rest of code

The advantage of the macro over Pieter's version is that the macro is "typeless"; you can use it with any array.

6

And, have no fear, the calculation is done at compile time, so there is little to no performance penalty due to the division.

7

Good stuff! Thread bookmarked.