CareerCup

// Assuming N rows and M columns
void change_pixels(char *grid, int N, int M, int x, int y, char new_pixel)
{
	// stride per row in bits
	int stride = M * 3;
	int bit_offset = y * stride + x * 3;
	int byte_offset = bit_offset / 8;
	int shift = bit_offset % 8;
	
	// read two bytes in case pixels are on byte boundary
	short int *tmp = (short *)(grid + byte_offset);
	short int tmp_pixels = *tmp;
	
	// store surrounding pixel bits (shift, one's complement and AND), and insert (OR) our new pixel bits
	tmp_pixels = tmp_pixels & ~(7 << shift);
	tmp_pixels = tmp_pixels | ((short int)new_pixel << shift);
}

// Assuming N rows and M columns
void change_pixels(char *grid, int N, int M, int x, int y, char new_pixel)
{
	// stride per row in bits
	int stride = M * 3;
	int bit_offset = y * stride + x * 3;
	int byte_offset = bit_offset / 8;
	int shift = bit_offset % 8;
	
	// read two bytes in case pixels are on byte boundary
	short int *tmp = (short *)(grid + byte_offset);
	short int tmp_pixels = *tmp;
	
	// store surrounding pixel bits (shift, one's complement and AND), and insert (OR) our new pixel bits
	tmp_pixels = tmp_pixels & ~(7 << shift);
	tmp_pixels = tmp_pixels | ((short int)new_pixel << shift);
        *tmp = tmp_pixels;
}

#include <stdio.h>

int main(void) {
	char mat[3][2] = {
				'1','2', '3','4', '5', '6'
		};

	char *a = mat;
	int x=1;
	int y=1;
	
	int col = 2;
	char c = *((a + x*col) + y);
	printf(" original value : %c\n", c);
	c = c & 0x00;
	char given = '3';
	c = c | given;
	printf(" modified value : %c\n", c);
	return 0;
}

typedef unsigned char b:3 pixel;
pixel *start; //pointer to beginning of grid
pixel newpix;
changepix(start,N,M,x,y,pix); // N = rows, M = columns

void changepix(pix* start,int N,int M,int x,int y,pixel pix) {
    start[Mx+y] = pix;
}

Why are you multiplying with "N". I think it should be " start[Mx+y] = pix; "