How to write a bitfield to an integer value?

Question

I am trying to do this:

typedef struct {
    uint16_t red : 6;
    uint16_t green : 5;
    uint16_t blue : 5;
} color_t

Then I would like to get something like:

color_t clr;
clr.red = 0;
clr.green = 10;
clr.blue = 15;

And write the compound variable clr to an int:

int value = clr; // this does not work

fprintf(draw, "%4X", value);

The reason I am doing this is I want to create colors like orange, purple and so on and draw them on the screen from a file. Inside a file I am writing a color in hex format.

One another thing is that I'd like to do this later in my code:

if (clr == value) { ... }

Or in another words, I'd like to compare the values from struct bitfield and int holding the real hex value of the color.

Answer 1

This is a stand-alone example showing how to use a union to access the value of a struct with bit fields:

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    uint16_t red: 6;
    uint16_t green: 5;
    uint16_t blue: 5;
} color_t;

typedef union {
    uint16_t color_value;
    color_t color_bits;
} color_helper_t;

int main(void) {
    color_helper_t clr;

    clr.color_bits.red = 0;
    clr.color_bits.green = 0;
    clr.color_bits.blue = 15;

    uint16_t value = clr.color_value;

    printf("%04X\n", value);

    if (clr.color_value == value) {
        printf("The values are equal\n");
    }

    return 0;
}

Output

7800
The values are equal