Wrong pointer after copying all data from array to a structure - c

Question

When I run the following code
#include <stdio.h>
typedef unsigned char uint8_t;
typedef unsigned short int uint16_t;
const uint8_t Symbols[] = {
0xCA,0x04,// size
0x0B, // width
0x0B, // height
0x00, // first char
0x04, // char count
0x01, 0x02, 0x03, 0x04,// char widths
// font data
0x00, 0x00, 0x20, 0x00, 0xF0, 0x07, 0x08, 0x04, 0x84, 0x07, 0x84, 0x07, 0x84, 0x07, 0x0E, 0x04, 0xF0, 0x07, 0x20, 0x00, 0x00, 0x00, // 0
0x10, 0x01, 0x08, 0x01, 0x04, 0x01, 0x04, 0x01, 0x08, 0x01, 0x10, 0x01, 0x10, 0x01, 0x08, 0x01, 0x04, 0x01, 0x04, 0x01, 0x00, 0x00, // 1
0x18, 0x00, 0xFF, 0x07, 0x18, 0x00, 0x00, 0x00, 0x80, 0x01, 0xFF, 0x07, 0x80, 0x01, 0x00, 0x00, 0x0C, 0x00, 0xFF, 0x07, 0x0C, 0x00, // 2
0x00, 0x02, 0x80, 0x05, 0x60, 0x04, 0x18, 0x04, 0x04, 0x04, 0x72, 0x05, 0x04, 0x04, 0x18, 0x04, 0x60, 0x04, 0x80, 0x05, 0x00, 0x02 // 3
};
typedef struct
{
uint16_t size; //2
uint8_t width; //1
uint8_t height; //1
uint8_t first_char; //1
uint8_t char_count; //1
uint8_t *font_widths;
uint8_t *font_data;
} _graphics_font;
_graphics_font* test;
uint8_t* font_st;
uint8_t temp;
int main(void)
{
test = (_graphics_font*)&Symbols;
font_st = (uint8_t*)&test->font_widths;
temp = font_st[0]+font_st[1]+font_st[2]+font_st[3]; //1+2+3+4 = 10
printf("temp=%d",temp);
return 0;
}
in C, I'm expecting that the pointer font_st will point exactly after the 6-th Byte of the Symbols array. Therefore the result printed should be 10. Instead of that the pointer is allocated to the 9-th byte, missing every time 2 bytes and wrongly to my expectations is printing 7 as a result. Why and how could that be?
Important update is that the using instead the line:
temp = font_st[-2]+font_st[-1]+font_st[0]+font_st[1]; //1+2+3+4 = 10
works.

Answer 1

This behaviour is most likely caused by struct padding.
Try the following:
#include <stdio.h>
#pragma pack(push, 1)
.... (all other code)
#pragma pack(pop)

Answer 2

As to the location of single elements in a struct, read about padding and packing, e.g. Structure padding and packing.
You could try it like this:
typedef struct glyph {
uint8_t char_width;
uint8_t[] glyph_data;
} glyph_t;
typedef struct font_t {
uint16_t size; //2
uint8_t width; //1
...
glyph_t[] glyphs;
} font_t;
That way, you can avoid some issues you are facing due to the dynamic addresses of the data, i.e. those which depend on the value of char_count.
Of course, if the size of glyph_data is not a constant, you have to provide your own access logic, like
uint8_t* p_glyph = (uint8_t*)(font->glyph_data[0]);
for ( int i = 0; i < requiredCharIndex; i++ ) {
p_glyph += total_size_of_glyph_in_bytes( (glyph_t*)p_glyph );
}
glyph_t* result = (glyph_t*)p_glyph;

Answer 3

test = (_graphics_font*)Symbols;
font_st = (uint8_t*)test->font_widths;
I didn't check that, but at simple sight I would say that &s was wrong in that context. Symbols and test are already pointers, so you are referencing to the address to that pointers.

Answer 4

You are not initializing correctly the two pointers in your struct.
test = (_graphics_font*)&Symbols;
On this line you are copying byte by byte from Symbols to test, which means that font_widths will point to the address made up by combining 0x01, 0x02, 0x03, 0x04, which contains garbage data.
Why 4 bytes? because every pointer in C has a size of 4 bytes regardless of the data it is pointing to.
Moreover, font_data will have the same behaviour.
Solution:
You have to use memcpy:
test->font_widths = (uint8_t*)memcpy(test->font_widths, Symbols + 6, 4*sizeof(uint8_t));
The same goes for font_data.