Is this tricky Pointer casting ?

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MaxK
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Posted by MaxK: Sun. Jun 24, 2007 - 10:10 PM
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Hello,

ive been writing a few C functions in some other ide for developing. (non-gnu).
All code ran fine, compiled good, in short it was working according to expectations.
Today (2007-06-24) Downloaded the most recent WinAVR-20070525 package.
All looked good till I added the a function with the following line:

Buffer is of type void*

(u_int8*)FA->C.Buffer += (FA->C.Volume->BytesPerSec);
[error: invalid lvalue in assignment]

And managed to make it accept the code by omitting the type cast :shock: 

/*(u_int8*)*/ FA->C.Buffer += (FA->C.Volume->BytesPerSec);

My question is, is this behaviour is known? Or is it me lacking on C.

advise is highly appreciated.

//Steven

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Tools, Compilers and General Programming
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dl8dtl
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Posted by dl8dtl: Mon. Jun 25, 2007 - 11:43 AM
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> Or is it me lacking on C.

Yes, it is.

GCC used to accept the typecast on the left-hand side until version
4, but it's never been correct C. A typecast does not produce an
lvalue.

Jörg Wunsch

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MaxK
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Posted by MaxK: Mon. Jun 25, 2007 - 12:52 PM
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Hi Jörg,

Ok im eager to know the right way to increase the address where the void pointer is pointing to.

Without is the right was as gcc automatically type-casts it? but to which type?

where can I read this up in detail.

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dl8dtl
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Posted by dl8dtl: Mon. Jun 25, 2007 - 02:52 PM
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The full form would be

*(u_int8*)FA->C.Buffer = *(u_int8*)FA->C.Buffer + FA->C.Volume->BytesPerSec;

I don't think this can be simplified in legal C. If FA->C.Buffer is
meant to represent a pointer to uint8_t objects, why not simply
declare it that way?

Jörg Wunsch

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MaxK
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Posted by MaxK: Mon. Jun 25, 2007 - 09:41 PM
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Thanks Jörg,

is it ok to state "Don't use += with void pointers" ?

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kmr
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Posted by kmr: Mon. Jun 25, 2007 - 10:22 PM
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MaxK, you can generalize (like Jörg aleady stated): never typecast the left side of an assignment.
Thus, you'd need to do something like:

u_int8* bptr = (u_int8*) FA->C.Buffer;
*bptr += FA->C.Volume->BytesPerSec;

Edit
As for adding void pointers, I'd have to review the C89 specs. But, I believe you're allowed to do so. Incrementing a void pointer, IIRC, increases its value by 1. So,

void *a = (void*) 1;
a += 2;
// a should now contain 3

as opposed to

u_int16* a = (u_int16*) 1;
a += 2;
// c should now contain 5

Kevin Rosenberg
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dl8dtl
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Posted by dl8dtl: Mon. Jun 25, 2007 - 10:51 PM
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> As for adding void pointers, I'd have to review the C89 specs.

void pointers don't have any defined arithemetics.

It's simply a GCC extension to treat void pointer arithmetics
same as byte pointer arithmetics, but that's obviously not
portable.

Jörg Wunsch

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kmr
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Posted by kmr: Mon. Jun 25, 2007 - 11:05 PM
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Thanks for the clarification, Jörg!

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Lajon
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Posted by Lajon: Mon. Jun 25, 2007 - 11:06 PM
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Quote:
The full form would be 

*(u_int8*)FA->C.Buffer = *(u_int8*)FA->C.Buffer + FA->C.Volume->BytesPerSec;


Not sure what that would do exactly, are you sure about that Jörg?

Would not just

FA->C.Buffer = (u_int8*)FA->C.Buffer + FA->C.Volume->BytesPerSec;

be ok?

/Lars

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kmr
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Posted by kmr: Mon. Jun 25, 2007 - 11:27 PM
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Lajon wrote:
Would not just

FA->C.Buffer = (u_int8*)FA->C.Buffer + FA->C.Volume->BytesPerSec;

be ok?

Definitely not. That assigns the value of your sum to your value of your pointer. You need a pointer dereference on the left side to assign the sum to the memory location that your pointer is pointing to.

Edit
Lars pointed out the above statement is incorrect.

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Last Edited: Tue. Jun 26, 2007 - 12:00 AM
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Lajon
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Posted by Lajon: Mon. Jun 25, 2007 - 11:53 PM
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What? The original problem is about incrementing the pointer, no pointer dereferencing going on here:

(u_int8*)FA->C.Buffer += (FA->C.Volume->BytesPerSec);

/Lars

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kmr
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Posted by kmr: Mon. Jun 25, 2007 - 11:59 PM
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Lars, right you are. I was tripped up by C's syntatic density and had thought the original problem was:

*(u_int8*)FA->C.Buffer += (FA->C.Volume->BytesPerSec);

Kevin Rosenberg
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MaxK
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Posted by MaxK: Tue. Jun 26, 2007 - 07:00 AM
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Hmm never knew/realized type-casting on the left side was depreciated.

Ok, as what Jörg has been opted before,
to sort this (elegant) is to change the Buffer from void* to unsigned char* and '+=' can be used without the need of typecasting the left side of the assignment.

No?

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