Hi ,
in so many interview i facing one question , like " can u give one realtime example for union use instead of structer". we working onlly on structure concepts.
i know basic functionalites of unions and structures. i know differences also b/w them.
can any body give example for the use of unions in realtime senario
One common real life application of unions is, to use generate device handles or similar entities. Given below is an example, where an 8-bit demux handle is constructed bitwise, and once it is constructed it is passed on to the demux APIs using the single 8-bit handler value.
Code:typedef union Handle_s { unsigned char uiHandle; struct { unsigned char source:3; unsigned char sink:3; unsigned char mode:2; } HandleBits; } Handle_t; Handle_t DemuxHandle; DemuxHandle.HandleBits.source = 0x01; DemuxHandle.HandleBits.sink = 0x11; DemuxHandle.HandleBits.mode = 0x10; DemuxOpen (DemuxHandle.uiHandle);
"realtime" really makes little sense here but if you are looking for a "real-world" use I use it like the previous poster in that it makes bit-packing mucho easier. I recently had to work on a 128-bit system that used 128bit values as messages. Using a union I was able to pack the message sender, recv., msg ID (type) and the handle to a shared memory window into a single 128 bit integer. Another use that was in a similar vein was the use of unions to "cast" a 16-byte memory block into 4 integers, two doubles, etc. This was used in an event logging system (look in the Linux forum under multithreaded event logging) that used no text....
A word of caution here is that if you initialize uiHandle later on, this will overwrite what you have initialized your struct members to. From my understanding here is that now uiHandle and your struct share the same memory footprint of the largest data declaration.
????
Can someone clarify if you can still do this(below), even though defined within the union?
Code:DemuxHandle.HandleBits = {0x1, 0x11, 0x10};
Last edited by slingerland3g; 12-21-2009 at 04:37 PM.
Did you try it and find out? It's not that hard to throw a few lines of code together to see.
Quzah.
Hope is the first step on the road to disappointment.
Originally Posted by quzah
Sorry, no I did not, looked at the clock and I had to catch my bus! Guess I should test before I post huh?
Sorry, there is a small correction in my sample code. It struck to me just now. I gave that example without trying out (still did not try out!!), but the below assignment was what I had in mind when I posted first.
In my earlier example of 0x11 and 0x10, the values would've got clipped off, and would've been actually 1 instead of 17 (due to 3-bit limit) and 0 instead of 16 (due to 2-bit limit).Code:DemuxHandle.HandleBits.source = 1; /* 001(binary) */ DemuxHandle.HandleBits.sink = 3; /* 011(binary), NOT 0x11(17 or 0001 0001) */ DemuxHandle.HandleBits.mode = 2; /* 10(binary), NOT 0x10(16 or 0001 0000) */
Last edited by technam; 12-23-2009 at 01:14 AM. Reason: binary expansion (for better understanding)
i cant analysing the above code.i cant understanding any thing
