Iterating through elements in an array by memory address. (C)

Question

Im new to C. I have a const unsigned short array of 1024 hex numbers. each hex number represents is 8 bits and represents bits to be turned on and off when displaying an image to a GBA screen. But nevermind all that and the DMA syntax I have below just for reference!!

My main question is...how can I iterate through elements in an array BY ADDRESS, grab those contents, then continue incrementing through addresses? Also, if you could give a stare to the below code and maybe see why Im getting:

"Program.c:(.text+0xe8): undefined reference to `myimg'"

on the line that calls "drawImage3" and that would be rad.

(in the main of program.C):

const unsigned short *pt;  
pt = &myimg[0]; 
int size = 5;
drawImage3(15,15,img_WIDTH,img_HEIGHT, pt);

(defined elsewhere):

void drawImage3(int x, int y, int width, int height, const u16* image)
{
    int r;
    for (r=0; r<height; r++)
    {   
        DMA[3].src = &image;
        DMA[3].dst = &videoBuffer[OFFSET(x+width, y, 240)];
        DMA[3].cnt = width | DMA_SOURCE_FIXED | DMA_ON |   DMA_DESTINATION_INCREMENT;  
        image++;    
    }
}

unpythonic · Accepted Answer · 2011-07-10 06:06:12Z

1

You're setting DMA[3].src to the address of a pointer, which is probably not what you want to do. For clarity's sake, here's what these references mean:

*image    -- the value of the thing which image points to
 image[0] -- same as *image
 image    -- the location in memory of your thing
&image    -- the location in memory that is storing your pointer
&image[0] -- same as image
&image[n] -- the location of the nth element in your thing

So instead of DMA[3].src = &image;, you probably want one of these two:

DMA[3].src = &image[r];    # If you do this do NOT increment image

or

DMA[3].src = image;        # And continue to increment image

If you choose the latter, then

DMA[3].src = image;
image++;

Can be better written as:

DMA[3].src = image++;

answered Jul 10, 2011 at 6:06

unpythonic

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beatgammit · Accepted Answer · 2011-07-10 05:52:40Z

0

From the code provided, myimg is never defined (line two in the second code block).

As for looping by address, arrays are already pointers, so doing a simple for loop is the same as looping over addresses. I'm not sure what you're trying to accomplish with the 'looping by address' business though, because that's what C is already doing.

EDIT:

AFAIK, arrays don't really exist in C, but they do in C++, so indexing into an 'array' is just saying, 'give me a chunk of memory starting at this address and this many bytes * size into it'.

For example, an int array (4 bytes per index) is just a chunk of memory that is 4 bytes * number of indexes. Getting an index into this 'array' is just getting a memory offset x bytes * 4 (sizeof int) into the memory chunk.

Simply put, you shouldn't have to worry about it much.

edited Jul 10, 2011 at 5:52

answered Jul 10, 2011 at 5:43

beatgammit

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9 Comments

beatgammit Over a year ago

I added a little info about what I understand arrays to be.

Perception Over a year ago

Not sure what all this new-fangled terminology is, but arrays in c are pretty much pointers.

Karl Knechtel Over a year ago

@Perception that is a very misleading and inaccurate thing to say. @tjameson arrays are basically the same in C and in C++; the only meaningful difference is that in C++ you can create a reference to an array. Everyone should read c-faq.com/aryptr/index.html thoroughly.

Perception Over a year ago

@Jeff Mercado - I beg to disagree. Arrays (const pointers) and pointers are much more similar in C/C++ then arrays in other languages that support real properties like for example, the length attribute.

beatgammit Over a year ago

I think arrays are a little more array-like in c++ because you can use the new operator to declare a 'new' array. Since new doesn't exist in c, and since 'new' is just malloc or something similar under the hood, there really isn't a big difference. I just think C++ makes a clearer distinction between a memory block and an 'array'. Sure, arrays in C++ aren't an object, just a memory chunk, but it's a little clearer distinction in my mind.

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