[Info-vax] DEC C 64/32 bit pointer conversion question

[Bob Gezelter]

On Thursday, September 12, 2019 at 12:13:39 PM UTC-4, Arne Vajhøj wrote:
> On 9/12/2019 9:45 AM, John E. Malmberg wrote:
> > I have a case where a system service only has accepts a 32 bit pointer 
> > in an item list.
> > 
> > When I am compiling with 64 bit pointers available and the value can be 
> > referenced in a 32 bit pointer, I would like to just extract out the 
> > lower 32 bits.
> > 
> > However on the test program I get:
> > 
> > #if __INITIAL_POINTER_SIZE == 64
> > # pragma pointer_size save
> > # pragma pointer_size 32
> > #endif
> > 
> > #ifndef VMS_PTR32_TYPEDEF
> > #define VMS_PTR32_TYPEDEF
> > 
> > typedef char * char_ptr32;
> > typedef unsigned long * long_ptr32;
> > typedef void * void_ptr32;
> > 
> > #endif
> > 
> > str32 = (char_ptr32)((unsigned long int)str_64 & 0xFFFFFFFF);
> > 
> > str64 = 2060834088 or 0x000000007ad5d528
> > str32 = -2147483536 or 0x80000070
> > 
> > Which of course does not work.
> > 
> > What am I doing wrong?
> 
> I would just do a simple case.
> 
> #include <stdio.h>
> 
> #pragma pointer_size 64
> typedef char *char_ptr64;
> 
> #pragma pointer_size 32
> typedef char *char_ptr32;
> 
> int main(int argc,char *argv[])
> {
>     char_ptr64 str64;
>     char_ptr32 str32;
>     printf("%d %d\n", (int)sizeof(str64), (int)sizeof(str32));
>     str64 = (char_ptr64)0x000000007AD5D528LL;
>     str32 = (char_ptr32)str64;
>     printf("%016llX %08X\n", str64, str32);
>     if(str64 != str32)
>     {
>        printf("Houston we have a problem\n");
>        return 0;
>     }
>     str64 = (char_ptr64)0x000000017AD5D528LL;
>     str32 = (char_ptr32)str64;
>     printf("%016llX %08X\n", str64, str32);
>     if(str64 != str32)
>     {
>        printf("Houston we have a problem\n");
>        return 0;
>     }
>     return 0;
> }
> 
> produce what I expect:
> 
> 8 4
> 000000007AD5D528 7AD5D528
> 000000017AD5D528 7AD5D528
> Houston we have a problem
> 
> Arne

Arne,

Bravo for including the equality check. 

I have seen what happens when that check (and analogous checks) are omitted. They are very difficult to identify and exterminate. The symptoms can be bizarre in the extreme.

- Bob Gezelter, http://www.rlgsc.com