[fpc-pascal] Converting C's bit field struct

[Jonas Maebe]

On 13 Aug 2008, at 11:59, Vinzent Höfler wrote:

> leledumbo wrote:
>> Since no one answers on message board, I'll post it here.
>> C struct:
>> typedef unsigned int u32int;
>> typedef struct page
>> {
>>  u32int present    : 1;   // Page present in memory
>>  u32int rw         : 1;   // Read-only if clear, readwrite if set
>>  u32int user       : 1;   // Supervisor level only if clear
>>  u32int accessed   : 1;   // Has the page been accessed since last
>> refresh?
>>  u32int dirty      : 1;   // Has the page been written to since last
>> refresh?
>>  u32int unused     : 7;   // Amalgamation of unused and reserved bits
>>  u32int frame      : 20;  // Frame address (shifted right 12 bits)
>> } page_t;
>
> Hmm. That's nasty. ;)w
>
> > Pascal record please...
>
> Should be something like that:
>
> type
>  Unsigned_7  = 0 .. (1 shl 7)  - 1;
>  Unsigned_20 = 0 .. (1 shl 20) - 1;
>
> type
>  page_t = bitpacked record

Yes and no: the layout of bitpacked records is by no means guaranteed  
to be the same as that of a C record, regardless of the definition of  
the C record. The internal format of bitpacked records is opaque and  
must not be used in any case where data has to be interchanged or  
where a particular layout is desired.


Jonas