Thread: Double Pointer Memory Allocation: Problems

Ok i have this:

Code:

void array_dump();
void print_array();
void free_color_array();

int width = 256;
int height = 256;

struct RGB
{
	unsigned char red;
	unsigned char green;
	unsigned char blue;
};

struct RGB **color_array;

main()
{
	array_dump();
	print_array();
	free_color_array();
	
}

void print_array()
{
	int i;
	int j;

	for(i = 0; i < height; i++)
	{
		for(j = 0; j < width; j++)
		{
			printf("(%d ", (unsigned char)color_array[i][j].blue);
			printf("%d ", (unsigned char)color_array[i][j].green);
			printf("%d) ", (unsigned char)color_array[i][j].red);
		}
		printf("\n");
	}
}

void array_dump()
{
	int i;
	int j;
		
	color_array = (struct RGB**)malloc(sizeof(unsigned char)*3*height);
	for(i = 0; i < height; i++)
	{
		color_array[i] = (struct RGB*)malloc(sizeof(unsigned char)*3*width);
	}
	
	for(i = 0; i < height; i++)
	{
		for(j = 0; j < width; j++)
		{
			color_array[i][j].blue = (unsigned char)20;
			color_array[i][j].green = (unsigned char)20;
			color_array[i][j].red = (unsigned char)20;
		}
	}
}

void free_color_array()
{
	int i;
	int j;
	
	for(i = 0; i < height; i++)
	{
		free(color_array[i]);
	}
	free(color_array);
}

Very simple right.

The problem is that it seg-falts, but the real problem is where i found it seg-falting

if i change my array_dump() function to this:

Code:

void array_dump()
{
	int i;
	int j;
		
	color_array = (struct RGB**)malloc(sizeof(unsigned char)*3*height);
	for(i = 0; i < height; i++)
	{
		color_array[i] = (struct RGB*)malloc(sizeof(unsigned char)*3*width);
	}
}

*Notice that it is the exact same code but the two bottom for loops are cut out, so i am just allocating the array and not setting the values

This will run fine and print out the system junk gathered durning allocation when print_array() is called.
If you will also Notice that the two for loops that where cut out of array_dump() are the exact same in structure to the ones in print_array(), the only difference is the what is happening inside the for loops.

So i tell my self, maybe my color_array is not being allocated correctly, which could in turn allow my print_array() function to be wayward and just look at any thing it likes to. So i add one to width and height before i enter the for loops in print_array() and low in behold it prints all the way to the end and then seg-falt. This should mean that my malloc code in array_dump is working correctly. SO WHY ARE THE LAST FOR LOOPS IN array_dump SEG-FALTING?

Thanks for any help

Thanks alot Salem, i havent used C in a while.