Thread: Casting 2D arrays

I'm trying to cast a 2D array of unsigned shorts to a 2D array of unsigned chars. I've done this with 1D arrays, but I can't seem to get this to work. What I have kind of works in that a few values print out correctly, but most are wrong.

Code:

	int i,j, count;

	USHORT **test;

	test = (USHORT **)malloc(sizeof(USHORT)*4);

	count=0;
	for(i=0;i<4;i++)
	{
		for(j=0;j<4;j++)
		{
			test[i] = (USHORT *)malloc(sizeof(USHORT)*4);
			count++;
			test[i][j] = count;
			count++;
			test[i][j] = test[i][j] | count<<8;
		}
	}

	for(i=0;i<4;i++)
	{
		for(j=0;j<8;j++)
		{
			printf("%d\n", ((UCHAR **)test)[i][j]);
		}
	}

Basically, I'm trying to avoid doing the right shift in the print out. Does anyone see what this issue is?

Ha! Oh man, silly error. I should have seen that. Thanks.

Is there anyway to do this with test being declared as test[X]N]?

Recommended idiom is:

Code:

p = malloc( sizeof(*p) );

Originally Posted by Bayint Naung

Recommended idiom is:

Code:

p = malloc( sizeof(*p) );

I see this said on this forum a lot. I've never had it compile. My compiler always says, "cannot convert from void * to <insert type here>".

Originally Posted by homer_3

Ha! Oh man, silly error. I should have seen that. Thanks.

Is there anyway to do this with test being declared as test[X]N]?

Yes you can get rid of all the malloc and just define

USHORT test[4][4];

Originally Posted by homer_3

I see this said on this forum a lot. I've never had it compile. My compiler always says, "cannot convert from void * to <insert type here>".

Because you aren't actually using a C compiler...
That's a C++ requirement.

What are you using?

Check your IDE and compiler settings... if it cannot be set for C compliance (C-99 in particular) you need to dump the braindead thing and find a proper C compiler.

Originally Posted by CommonTater

Because you aren't actually using a C compiler...
That's a C++ requirement.

Interesting, didn't know that. I'm using MSVSC++, changed the compiler to C and it didn't complain.

USHORT test[4][4];

No, I tried that. I get bad memory access using both the C and C++ compiler.

The printout part has to change:

Code:

for(i=0;i<32;i++)
    {
        printf("%d\n", ((UCHAR *)&test)[i]);
    }

Originally Posted by Salem

> No, I tried that. I get bad memory access using both the C and C++ compiler.
Probably because the 2nd pair of loops run off into the weeds.

> for(i=0;i<4;i++)
> for(j=0;j<8;j++)

No it doesn't. In the second set of loops I'm looping 1 byte at a time because I casted to a char. The 1st set of loops goes at 2 bytes at a time because it's working on shorts.

Using malloc works because the "elements" in the 2nd dimension of the array are pointers. So I can offset to the 1st dimension pointer and then offset again off that pointer.

[][] doesn't work because a continuous block of memory is allocated for the 2d array and element [i][j] is really i*Length+j. I was hoping someone knew of any tricks to get this to work how I have above.

But I don't get why I get a bad memory access error when i=0 and j=0. That should still be valid because it's the 1st element.

Cool, that works nicely.

Congtrats on the use of
unsigned char (*arr)[8] = (unsigned char (*)[8])a;

I'll have to bend my head around that. I don't see why you need a '*'. There are no array of pointer. I would have tried char [][8]. Plus, it should be illegal to define a variable whose first use is inside parenthesis (except function pointers - and those are a headache too)

I think you are taking advantage of undocumented compiler oversights.