Finding the size of Hexadecimal char array in C from PROGMEM

Question

I'm working on a AT90 micro controller and I'm working on displaying different fonts on the screen. As part of my font definition, I've got a char array of hexadecimal below. For the purpose of this question, I've shorten the char array to only represent 4 characters.

const char font5x7[] PROGMEM = {
    0x00, 0x00, 0x00, 0x00, 0x00, // SPACE
    0x00, 0x00, 0x5F, 0x00, 0x00, // !
    0x00, 0x03, 0x00, 0x03, 0x00, // "
    0x14, 0x3E, 0x14, 0x3E, 0x14  // #
}

Now if I run sizeof(font5x7), it gives me the correct answer which is 20. However when I run my real font5x7, it returns 475 instead of the correct number 225. Here is the real one:

const char font5x7[] PROGMEM = {
    0x00, 0x00, 0x00, 0x00, 0x00, // SPACE
    0x00, 0x00, 0x5F, 0x00, 0x00, // !
    0x00, 0x03, 0x00, 0x03, 0x00, // "
    0x14, 0x3E, 0x14, 0x3E, 0x14, // #
    0x24, 0x2A, 0x7F, 0x2A, 0x12, // $
    0x43, 0x33, 0x08, 0x66, 0x61, // %
    0x36, 0x49, 0x55, 0x22, 0x50, // &
    0x00, 0x05, 0x03, 0x00, 0x00, // '
    0x00, 0x1C, 0x22, 0x41, 0x00, // (
    0x00, 0x41, 0x22, 0x1C, 0x00, // )
    0x14, 0x08, 0x3E, 0x08, 0x14, // *
    0x08, 0x08, 0x3E, 0x08, 0x08, // +
    0x00, 0x50, 0x30, 0x00, 0x00, // ,
    0x08, 0x08, 0x08, 0x08, 0x08, // -
    0x00, 0x60, 0x60, 0x00, 0x00, // .
    0x20, 0x10, 0x08, 0x04, 0x02, // /
    0x3E, 0x51, 0x49, 0x45, 0x3E, // 0
    0x00, 0x04, 0x02, 0x7F, 0x00, // 1
    0x42, 0x61, 0x51, 0x49, 0x46, // 2
    0x22, 0x41, 0x49, 0x49, 0x36, // 3
    0x18, 0x14, 0x12, 0x7F, 0x10, // 4
    0x27, 0x45, 0x45, 0x45, 0x39, // 5
    0x3E, 0x49, 0x49, 0x49, 0x32, // 6
    0x01, 0x01, 0x71, 0x09, 0x07, // 7
    0x36, 0x49, 0x49, 0x49, 0x36, // 8
    0x26, 0x49, 0x49, 0x49, 0x3E, // 9
    0x00, 0x36, 0x36, 0x00, 0x00, // :
    0x00, 0x56, 0x36, 0x00, 0x00, // ;
    0x08, 0x14, 0x22, 0x41, 0x00, // <
    0x14, 0x14, 0x14, 0x14, 0x14, // =
    0x00, 0x41, 0x22, 0x14, 0x08, // >
    0x02, 0x01, 0x51, 0x09, 0x06, // ?
    0x3E, 0x41, 0x59, 0x55, 0x5E, // @
    0x7E, 0x09, 0x09, 0x09, 0x7E, // A
    0x7F, 0x49, 0x49, 0x49, 0x36, // B
    0x3E, 0x41, 0x41, 0x41, 0x22, // C
    0x7F, 0x41, 0x41, 0x41, 0x3E, // D
    0x7F, 0x49, 0x49, 0x49, 0x41, // E
    0x7F, 0x09, 0x09, 0x09, 0x01, // F
    0x3E, 0x41, 0x41, 0x49, 0x3A, // G
    0x7F, 0x08, 0x08, 0x08, 0x7F, // H
    0x00, 0x41, 0x7F, 0x41, 0x00, // I
    0x30, 0x40, 0x40, 0x40, 0x3F, // J
    0x7F, 0x08, 0x14, 0x22, 0x41, // K
    0x7F, 0x40, 0x40, 0x40, 0x40, // L
    0x7F, 0x02, 0x0C, 0x02, 0x7F, // M
    0x7F, 0x02, 0x04, 0x08, 0x7F, // N
    0x3E, 0x41, 0x41, 0x41, 0x3E, // O
    0x7F, 0x09, 0x09, 0x09, 0x06, // P
    0x1E, 0x21, 0x21, 0x21, 0x5E, // Q
    0x7F, 0x09, 0x09, 0x09, 0x76, // R
    0x26, 0x49, 0x49, 0x49, 0x32, // S
    0x01, 0x01, 0x7F, 0x01, 0x01, // T
    0x3F, 0x40, 0x40, 0x40, 0x3F, // U
    0x1F, 0x20, 0x40, 0x20, 0x1F, // V
    0x7F, 0x20, 0x10, 0x20, 0x7F, // W
    0x41, 0x22, 0x1C, 0x22, 0x41, // X
    0x07, 0x08, 0x70, 0x08, 0x07, // Y
    0x61, 0x51, 0x49, 0x45, 0x43, // Z
    0x00, 0x7F, 0x41, 0x00, 0x00, // [
    0x02, 0x04, 0x08, 0x10, 0x20, // slash 
    0x00, 0x00, 0x41, 0x7F, 0x00, // ]
    0x04, 0x02, 0x01, 0x02, 0x04, // ^
    0x40, 0x40, 0x40, 0x40, 0x40, // _
    0x00, 0x01, 0x02, 0x04, 0x00, // `
    0x20, 0x54, 0x54, 0x54, 0x78, // a
    0x7F, 0x44, 0x44, 0x44, 0x38, // b
    0x38, 0x44, 0x44, 0x44, 0x44, // c
    0x38, 0x44, 0x44, 0x44, 0x7F, // d
    0x38, 0x54, 0x54, 0x54, 0x18, // e
    0x04, 0x04, 0x7E, 0x05, 0x05, // f
    0x08, 0x54, 0x54, 0x54, 0x3C, // g
    0x7F, 0x08, 0x04, 0x04, 0x78, // h
    0x00, 0x44, 0x7D, 0x40, 0x00, // i
    0x20, 0x40, 0x44, 0x3D, 0x00, // j
    0x7F, 0x10, 0x28, 0x44, 0x00, // k
    0x00, 0x41, 0x7F, 0x40, 0x00, // l
    0x7C, 0x04, 0x78, 0x04, 0x78, // m
    0x7C, 0x08, 0x04, 0x04, 0x78, // n
    0x38, 0x44, 0x44, 0x44, 0x38, // o
    0x7C, 0x14, 0x14, 0x14, 0x08, // p
    0x08, 0x14, 0x14, 0x14, 0x7C, // q
    0x00, 0x7C, 0x08, 0x04, 0x04, // r
    0x48, 0x54, 0x54, 0x54, 0x20, // s
    0x04, 0x04, 0x3F, 0x44, 0x44, // t
    0x3C, 0x40, 0x40, 0x20, 0x7C, // u
    0x1C, 0x20, 0x40, 0x20, 0x1C, // v
    0x3C, 0x40, 0x30, 0x40, 0x3C, // w
    0x44, 0x28, 0x10, 0x28, 0x44, // x
    0x0C, 0x50, 0x50, 0x50, 0x3C, // y
    0x44, 0x64, 0x54, 0x4C, 0x44, // z
    0x00, 0x08, 0x36, 0x41, 0x41, // {
    0x00, 0x00, 0x7F, 0x00, 0x00, // |
    0x41, 0x41, 0x36, 0x08, 0x00, // }
    0x02, 0x01, 0x02, 0x04, 0x02};// ~

Answer 1

This code prints "Array len: 20" on my system.

#include <stdio.h>

#define PROGMEM

const char font5x7[] PROGMEM = {
    0x00, 0x00, 0x00, 0x00, 0x00, // SPACE
    0x00, 0x00, 0x5F, 0x00, 0x00, // !
    0x00, 0x03, 0x00, 0x03, 0x00, // "
    0x14, 0x3E, 0x14, 0x3E, 0x14  // #
};

#define ARRAY_LEN(a) (sizeof(a) / sizeof((a)[0]))

const font5x7_len = ARRAY_LEN(font5x7);
int main(void) {
    printf("Array len: %d\n", font5x7_len);
}

Answer 2

In C programming, arrays are little more than pointers. While you are still in the scope where you declared the array you can use sizeof(font5x7) / sizeof(char) to determine the length of the array.

However, when an array is passed into a function, it decays into a pointer. Pointers have no associated length so you need to keep another variable describing how long the array.

In other words, the function you want is impossible without some kind of sentinel value. For instance if you could guarantee that your font array will never contain a 0xff, then you could end every array with 0xff. Your function would then search through the array until it finds that value and return the length.