Thread: 24 bit pointer from a 4-byte field

On the processor I am programming on, there can be 64-bit, 31-bit and 24-bit pointers.

I'm trying to extract a 24-bit pointer out of a 4-byte word. The first 8 bits in the 4-byte word can be flags that are set. The last 3 bytes will be a pointer, addressable for up to 16mb.

For example, the 4-byte value might be 0x7F000010 which would point to location 0x00000010 in storage.

I've accomplished my goal of extracting it via this code sequence:

Code:

#include <stdio.h>                                
int main(void) {                                  
   void * my_32_ptr = (void *) 0x7F000010 ;
   void * my_24_ptr  ;                     
   struct {                                       
      int       : 8 ;                             
      int int24 : 24 ;                            
   } addr24 ;                                     
                                                  
   #define int24 addr24.int24                     
   int24 = (int) my_32_ptr ;                      <--- line 11 
   my_24_ptr = (void *) int24 ;            
   printf("my_24_ptr = %08X\n", my_24_ptr) ;      
   return 0 ;                                      
}

It works, however, I get an informational message for line 11 as follows:

INFORMATIONAL CCN3451 MISC.C(PTR24):11 The target integral type cannot hold all possible values of the source integral type.

At runtime, the first byte gets lopped off as desired.

What's a better method to extract the last 3 bytes of my_32_ptr to avoid the compiler's informational message?

My processor is little endian, so no concerns about the ordering of values in the 4-byte word.

Thanks.

I see that this works.

Code:

   my_24_ptr = (void *) ((int) my_32_ptr & (int) 0x00FFFFFF) ;

stdint.h has types of specific sizes. In particular, it has uintptr_t which is an unsigned integer sized to hold a pointer (presumably the largest pointer type on your system). This compiles with no warnings on gcc with warnings maximized:

Code:

#include <stdio.h>
#include <stdint.h>
 
int main() {
    void *p32 = (void*)0x12345678;
 
    void *p24 = (void*)((uintptr_t)p32 & (uintptr_t)0xffffff);
    unsigned flags = (unsigned)((uintptr_t)p32 >> 24);
 
    printf("ptr  : %p\n", p24);
    printf("flags: 0x%x (", flags);
    for (unsigned bit = 0x80; bit; bit >>= 1)
        putchar(flags & bit ? '1' : '0');
    printf(")\n");
 
    return 0;
}

Yep, thanks. That's what I came up with.