Macro to iterate over struct members - c

Question

I use a struct of bit fields to access each colour channel in a pixel, the problem is that quite often I have code that applies in the same way to each channel, but because I cannot just iterate over the members of a struct in C I end up having 3 copies of the same code for each member, or more inconveniently have to use switch-case statements.
I figured it would be more elegant if I could use a macro so that I can access a member by providing a number, ideally a macro that would make .CHAN(i) become either .r, .g or .b depending on whether the integer variable i contains a 0, 1 or 2. Except I have no idea how one would make such a macro or even if that's possible.
A detail but each member is something like 12 bits, not 8 as one might expect, so I cannot just turn it into an array or have a union with a pointer. Also X-Macros won't do as I often need to do many things to each channel before doing the same to another channel, in other words the for loop for going through each member can contain a lot more than just one line.
EDIT: Here's some code, first the struct:
typedef struct
{
uint32_t b:12;
uint32_t g:12;
uint32_t r:12;
uint32_t a:12;
} lrgb_t;
Now an example of what my problem looks like in code:
for (ic=0; ic<3; ic++)
{
for (i=0; i<curvecount; i++)
{
curve[i].p0.x = (double) i;
curve[i].p3.x = (double) i+1.;
switch (ic) // this is what I'm trying to eliminate
{
case 0:
curve[i].p0.y = pancol[i].r / 4095.;
curve[i].p3.y = pancol[i+1].r / 4095.;
break;
case 1:
curve[i].p0.y = pancol[i].g / 4095.;
curve[i].p3.y = pancol[i+1].g / 4095.;
break;
case 2:
curve[i].p0.y = pancol[i].b / 4095.;
curve[i].p3.y = pancol[i+1].b / 4095.;
break;
}
// Ideally this would be replaced by something like this, CHAN() being an hypothetical macro
// curve[i].p0.y = pancol[i].CHAN(ic) / 4095.;
// curve[i].p3.y = pancol[i+1].CHAN(ic) / 4095.;
}
... // more stuff that ultimately results in a bunch of pixels being written, channel after channel
}

Answer 1

as pointed out in the comments, this doesn't really address the OP's problem because the members on his struct are bitfields that wouldn't align with an array. I'll keep the answer here though, in hopes it can still be useful to someone.
I think a union is what you want.
You can write your struct such as
union
{
struct
{
float r;
float g;
float b;
}rgb;
float channel[3];
} color;
This way the struct will be in the same place in memory as the float[3], and you can effectively access the same members as either a struct member or as an element in the array.
You might have to look up the exact syntax, but you get the idea.

Answer 2

One possibility might be to wrap the repeated code into a function, and then call it for each of the channels:
typedef struct {
int r:12;
int g:12;
int b:12;
} Pixel;
int inc(int val) {
return val + 1;
}
int main(void) {
Pixel p = {0, 0, 0};
p.r = inc(p.r);
p.g = inc(p.g);
p.b = inc(p.b);
return 0;
}

Answer 3

After reading the code that you added I made some changes to my suggested macro
#define CHAN(ic) \
(ic == 1) ? curve[i].p0.y = pancol[i].r / 4095; curve[i].p3.y = pancol[i+1].r / 4095; : \
(ic == 2) ? curve[i].p0.y = pancol[i].g / 4095; curve[i].p3.y = pancol[i+1].g / 4095; : \
curve[i].p0.y = pancol[i].b / 4095; curve[i].p3.y = pancol[i+1].b / 4095;
The macro CHAN(ic) will evaluate 'ic' in order to decided which member to manipulate. If 'ic' is 1 then the member '.r' will be manipulated if 'ic' is 2 then '.g' will be manipulated, and if 'ic' is neither 1 or 2 then '.b' will be manipulated because of this assumption you must make sure that 'ic' is properly set otherwise you could screw with the value of panco[i].b and pancol[i+1].b . You code should look something like the following but you will most likely need to tweak the macro a bit let me know if you have any questions.
//#define CHAN(ic) here
for (ic=0; ic<3; ic++)
{
for (i=0; i<curvecount; i++)
{
curve[i].p0.x = (double) i;
curve[i].p3.x = (double) i+1.;
CHAN(ic)
}
... // more stuff that ultimately results in a bunch of pixels being written, channel after channel
}
Also please note that my macro will do exactly the same thing as your switch case. The only difference is that it is defined in a macro the point I am trying to make is that the difference between the switch case and the macro is purely visual.