Passing a pointer to an array in a different function in C - c

Question

I am trying to pass a pointer rgb that is initialized with memset to 0 and then looped through to place a 32 bit integer only in the bounds that I create with height and width input (h and w) as well as offset from the top left corner of the 2d array (x and y). after compiling, I seem to have the value with printf of the pointer just after it was made which gives the correct value (in my case 0xFFFFFF with and input of 255 255 255 for r g b) but after it is passed through to rgb2yuv function, it is set to 0 when I printf there.
Any suggestions would be awesome!
#include <stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<fcntl.h>
#include<errno.h>
#include<string.h>
/*global definitions*/
#define WIDTH 480
#define HEIGHT 240
/*global declarations*/
int w,h,scrn, bytewrite;
unsigned char red, green, blue;
static unsigned long rgb[WIDTH][HEIGHT];
/*function declarations*/
void colorq();
void rgb_rectdraw(int x, int y, int w, int h, unsigned char red ,
unsigned char green, unsigned char blue, unsigned long *rgb);
void rgb2yuv(unsigned long *rgb);
/*
Function Name: main
Purpose: main function
*/
int main(int argc, char** argv){
printf("\n");
int x, y;
/*call colorq to create a 32bit number of RGB*/
colorq();
/ *call rgb_rectdraw to draw a rectangle RGB array*/
rgb_rectdraw(x, y, w, h, red, green, blue, rgb);
/*call rgb2yuv to take the RGB array and covert it to a YUV array*/
rgb2yuv(rgb);
return 0;
}
/*
Function name: color q
Purpose: asks user to input colors from 0 to 255 in RGB format
*/
void colorq(){
printf("Please enter a color for Red Green and Blue from 0 to 255:\n");
scanf("%hu", &red);
scanf("%hu", &green);
scanf("%hu", &blue);
printf("\n");
return;
}
/*
Function name: rectdraw
Purpose: Draws a rectangle array
*/
void rgb_rectdraw(int x, int y, int w, int h,unsigned char red,
unsigned char green, unsigned char blue,unsigned long *rgb){
unsigned long rgbpixel;
/* testing only take out when
finished debugging why red is always 0 after scanf */
red = 255;
printf("red set to 255 for debugging\n");
/*creates a 32-bit number of RGB*/
rgbpixel = (red<<16)|(green<<8)|blue;
printf("%#x\n",rgbpixel);
/*create array of width w height h*/
/*initialize array*/
memset (rgb, 0,sizeof(HEIGHT*WIDTH));
int i, j, startx, stopx, starty, stopy;
printf("enter width and height of rectangle in pixels\n");
scanf("%d %d", &w, &h);
printf("enter offset x pixels and y pixels of rectangle:\n");
scanf("%d %d", &x, &y);
startx=x;
starty=y;
stopx=x+w;
stopy=y+w;
/* creates array of w and h of int rgh */
for(i=startx; i <= stopx; i++){
for(j=starty; j <= stopy; j++){
rgb = rgbpixel;
}
j = 0;
}
printf("original rgb %#x\n",rgb);
return ;
}
/*
*Function Name: rgb2yuv
*Purpose: convert the RGB array to a YUV array
*/
void rgb2yuv(unsigned long *rgb){
int i,j;
printf("ptrpassed = %#x\n",*rgb);
for(i=0; i<=WIDTH;i++){
for(j=0; j<=HEIGHT; j++){
}
}
printf("\n");
return;
}

Answer 1

I went through and basically sorted out all the warts, and explained why. A lot of it amounts to the fact that if your compiler spits out warnings, you have to listen to them.
/* Changed: Code formatted for my sanity */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
/*global definitions*/
#define WIDTH 480
#define HEIGHT 240
/*global declarations*/
int w,h,scrn, bytewrite;
/* Changed: to a short because I don't like the fact that I might be overwriting
memory on accident (in colorq). */
unsigned short red, green, blue;
static unsigned long rgb[WIDTH][HEIGHT];
/* Changed: Some format strings have been changed to get rid of compiler
warnings. */
/*function declarations*/
void colorq();
/* Changed: Specify the second dimension when you're passing an array. */
void rgb_rectdraw(int x, int y, int w, int h, unsigned char red,
unsigned char green, unsigned char blue,
unsigned long rgb[][HEIGHT]);
/* Changed: always pass an array of arrays. */
void rgb2yuv(unsigned long rgb[][HEIGHT]);
/*
Function Name: main
Purpose: main function
*/
int main(int argc, char** argv)
{
printf("\n");
int x, y;
/*call colorq to create a 32bit number of RGB*/
colorq();
/* call rgb_rectdraw to draw a rectangle RGB array */
rgb_rectdraw(x, y, w, h, red, green, blue, rgb);
/* call rgb2yuv to take the RGB array and covert it to a YUV array */
rgb2yuv(rgb);
return 0;
}
/*
Function name: color q
Purpose: asks user to input colors from 0 to 255 in RGB format
*/
void colorq(){
/* Suggestion: restructure this method to just take in all its input
locally, then return a rgbpixel. */
printf("Please enter a color for Red Green and Blue from 0 to 255:\n");
scanf("%hu", &red);
scanf("%hu", &green);
scanf("%hu", &blue);
printf("\n");
return;
}
/*
Function name: rectdraw
Purpose: Draws a rectangle array
*/
void rgb_rectdraw(int x, int y, int w, int h,unsigned char red,
unsigned char green, unsigned char blue,
unsigned long rgb[][HEIGHT])
{
unsigned long rgbpixel;
/* testing only take out when
finished debugging why red is always 0 after scanf */
red = 255;
printf("red set to 255 for debugging\n");
/*creates a 32-bit number of RGB*/
/* Changed: Added the extra 0xFF masking because of shortness rather than
charness. */
rgbpixel = ((red & 0xFF) << 16) | ((green & 0xFF) << 8) | (blue & 0xFF);
printf("%#lx\n",rgbpixel);
/*create array of width w height h*/
/*initialize array*/
/* Changed: fill the size of one element times the number of elements */
memset(rgb, 0, sizeof(unsigned long) * HEIGHT * WIDTH);
int i, j, startx, stopx, starty, stopy;
printf("enter width and height of rectangle in pixels\n");
scanf("%d %d", &w, &h);
printf("enter offset x pixels and y pixels of rectangle:\n");
scanf("%d %d", &x, &y);
startx=x;
starty=y;
stopx=x+w;
stopy=y+w;
/* creates array of w and h of int rgh */
for(i=startx; i <= stopx; i++){
for(j=starty; j <= stopy; j++){
rgb[i][j] = rgbpixel;
}
j = 0;
}
printf("original rgb %#lx\n", (long unsigned int) rgb);
return ;
}
/*
*Function Name: rgb2yuv
*Purpose: convert the RGB array to a YUV array
*/
void rgb2yuv(unsigned long rgb[][HEIGHT]){
int i,j;
/* Changed: You can't just dereference rgb twice -- you have to use array
notation here. */
printf("ptrpassed = %#lx\n", rgb[0][0]);
for(i=0; i<=WIDTH; i++){
for(j=0; j<=HEIGHT; j++){
}
}
printf("\n");
return;
}

Answer 2

This actually has a number of bugs, but your first issue is assigning the pixel value to the array:
for(i=startx; i <= stopx; i++){
for(j=starty; j <= stopy; j++){
rgb = rgbpixel;
}
j = 0;
}
You probably meant something like this:
rgb[i][j] = rgbpixel;
You don't need to reset j to 0 -- the inner for-loop will immediately reset j to starty anyhow.
Also, you're misusing sizeof(). You probably want sizeof(rgb) instead.

Answer 3

Why not use a structure? If the code has no intention of being portable you could easily get away with:
struct rgb_bits {
int red : 8;
int green : 8;
int blue : 8;
};
union rgb {
rgb_bits colour;
long array;
}
Blanking out the pixel then simply becomes:
union rgb pixel;
pixel.array = 0;
and setting individual colours becomes:
union rgb pixel;
pixel.colour.red = ...;
pixel.colour.green = ...;
pixel.colour.blue = ...;

Answer 4

Looks like you're storing your single pixel value in the pointer to your data to me:
rgb = rgbpixel;