How to convert a (u32 *) to (u8 *)???

[relaxe]

Posted on November 14, 2008 at 02:29

How to convert a (u32 *) to (u8 *)???

[relaxe]

Posted on May 17, 2011 at 12:52

Ok, this question is rather simple.

the SDIO+SDCard example's functions give their result in a 32bit pointer, like: u32 *readbuff.

Now, I want to implement a filesystem library like dosfs or efsl. They all require to have a read/write function wich use a 8 bit pointer, like: u8 *buffer.

Now, is there an easy way to pass/convert one to another, without have to transfer the u32* buffer to another u8*buffer 8bits at a time?

I was looking at something like ''u8 *buffer8 = (u8 *) buffer32'' ....

Is it doable?

Thanks!

-Relaxe

[sword_82]

Posted on May 17, 2011 at 12:52

Hi Relaxe,

u8 *buffer8;

 

u32 *buffer32;

 

buffer8 = (u8*)buffer32;

sword_82

[relaxe]

Posted on May 17, 2011 at 12:52

Thanks Sword!

Now, this actually works:

u32 Buffer32[2] = {0x33323130, 0x37363534};

u32 *PtrBuffer32 = Buffer32;

u8 *PtrBuffer8;

PtrBuffer8 = (u8*) PtrBuffer32;

printf(''%d\n'', *(PtrBuffer8+0));

printf(''%d\n'', *(PtrBuffer8+1));

printf(''%d\n'', *(PtrBuffer8+2));

printf(''%d\n'', *(PtrBuffer8+3));

printf(''%d\n'', *(PtrBuffer8+4));

printf(''%d\n'', *(PtrBuffer8+5));

And as you can see, I'm not recreating a whole new buffer, just changing the pointer :p

-Relaxe

[ This message was edited by: relaxe on 13-11-2008 16:33 ]

[picguy]

Posted on May 17, 2011 at 12:52

You may find this general purpose cast macro to be useful

/* Thanks to SM Ryan who provided the

* LV macro in news:comp.lang.c

* As given, the macro name was lvaluecast

* Message-ID:

*/

#define LV(type,lvalue) (*((type*)((void*)(&(lvalue)))))

I used it to store an int to a *int like this

LV(int*,sp) -= extraStackSpace;

[st3]

Posted on May 17, 2011 at 12:52

Quote:

I'm not recreating a whole new buffer, just changing the pointer

You probably don't even need to change the pointer - just the cast should do it.

However, this is not so simple in the other direction; ie, from a u8* to a u32* pointer - because of the alingment requirements of multi-byte objects...

[ This message was edited by: st7 on 14-11-2008 01:52 ]