How to access a specific byte in a void * ?

rvca · 03-10-2008, 12:44 AM

Hi everybody,

I am writting a C code using the v4l2 API that will track different colored leds. so far I have managed to get my camera workng and reading a frame. However the data is put in a pointer:

void * buffer

However my image is a planar YUV420, because i am tracking color of each pixel i have to get the color value of each pixel by "reconstructing the images" the fact is that i want to access a certain byte of the pointer (lets say the byte 36 of such buffer and the byte 18 to do the appropiate calculations) How do I achieve that? Can I access the memory as if being an array of characters? Something like:

char * imagearray = buffer;

and then access it by:

imagearray[36]

Is that possible? or how can I do this?

Thanx for your help.

Guttorm · 03-10-2008, 02:51 AM

Hi

You can do it yes. But when changing a pointer type like that, you will get a warning from the compiler unless you use a type cast:

char * imagearray = (char*) buffer;

dmail · 03-10-2008, 07:33 AM

Quote:

Originally Posted by Guttorm

Hi

You can do it yes. But when changing a pointer type like that, you will get a warning from the compiler unless you use a type cast:

char * imagearray = (char*) buffer;

This is not correct as the OP is using C and a cast from void pointer to another type pointer is implicit.

ta0kira · 03-10-2008, 07:45 AM

Quote:

Originally Posted by dmail

This is not correct as the OP is using C and a cast from void pointer to another type pointer is implicit.

Actually it's the other way around. Any pointer will implicitly cast to void* but you must explicitly cast void* to turn it into something else. That's because void* is useless and you shouldn't be able to implicitly make it into an arbitrary useful type. Polymorphism doesn't work with void*, either, which is another reason. Take for example

Code:

class A {};
 
class B : virtual public A {};

A B* cast to a void* should only be cast back to B* or to unsigned char*, even if just to further cast to A*. You'll end up with an invalid pointer if you cast the void* directly to A*, hence the requirement for explicit casting.

Technically you should be using unsigned char* because that's the standard binary representation of data. char* may be signed, and as such could have anomalous effects when performing binary operations.
ta0kira

Edit: I think you might be right in a normal C context as far as implicit casting. I guess I'm thinking in terms of C++.

dmail · 03-10-2008, 07:48 AM

Quote:

Originally Posted by ta0kira

Actually it's the other way around. Any pointer will implicitly cast to void* but you must explicitly cast void* to turn it into something else. That's because void* is useless and you shouldn't be able to implicitly make it into an arbitrary useful type. Polymorphism doesn't work with void*, either, which is another reason. Take for example[code]class A {};

class B : virtual public A {};[code]A B* cast to a void* should only be cast back to B* or to unsigned char*, even if just to further cast to A*. You'll end up with an invalid pointer if you cast the void* directly to A*, hence the requirement for explicit casting.

Quote:

Originally Posted by rvca

I am writting a C code ...

Quote:

Originally Posted by dmail

This is not correct as the OP is using C...

Why are you talking about classes?

Edit:
Lol no problem I see your edit now.

And just for confirmation

Quote:

6.2.5 Types...
Pointers to other types coerce silently to and from void* in assignments, function prototypes, comparisons, and conditional expressions, whereas other pointer type clashes are invalid. It is undefined what will happen if a pointer of some type is converted to void*, and then the void* pointer is converted to a type with a stricter alignment requirement.

rvca · 03-10-2008, 01:49 PM

Hi,

Thank you, it compiles without warning. Now how do I access a specific byte, say the 5th byte? I assume that the following is correct?

Code:

void * buffer; //already containing the image data

char * imagearray = buffer;

int pixel_y = atoi(imagearray[5]); // number just as example
printf ("Y Value of 5 pixel is: %d",pixel_y);

However I get a Segmentation Fault when running the app. What am I missing or doing wrong?

Thank you,

rvca · 03-10-2008, 01:57 PM

Hi,

I think I am wrong in using atoi(), because i assume that char just gives me a collection of bytes, so... (I am assuming all this) The next code should give me a correct value?

Code:

char * imagearray = (char*) buffer;
unsigned int pixel_y = imagearray[5]; //5 as example
unsigned int pixel_cr = imagearray[6]; //6 as example
printf ("Pixel 5 has final value of %d\n",pixel_y*pixel_cr); //multiplication just as aritmetic example

Am I right?

Thanx,

osor · 03-10-2008, 06:56 PM

Quote:

Originally Posted by rvca

Am I right?

You are correct in your approach, just remember that array indexing starts at 0.

rvca · 03-11-2008, 12:27 AM

Thanks for your help, got it working.