How to use Struct with Bit Fields

Question

I am using a file shared between multiple programmers which has this struct:

typedef struct _APPLE{
   ULONG appleID;
   struct{
       ULONG isBig: 1;
       ULONG isRed: 1;
       ULONG isFresh: 1;
       ULONG isGood: 1;
       ULONG bReserved: 28;
   };
}APPLE;

As the file is shared I cant edit it.I want to use this APPLE struct in my code and want to provide values to each of its members. How can I do this?

What is the issue you are facing in creating its instances? Is there any problem in doing Apple a; a.isBig=0;? — Sadique
– Sadique, Commented Nov 22, 2013 at 5:43
I can do that. but when i do this a.isBig=0; does the memory is shared like a union here or i can give values for each of the member separately? — mohit jain
– mohit jain, Commented Nov 22, 2013 at 5:49

Some programmer dude · Accepted Answer · 2013-11-22 05:53:38Z

2

First of all you can't have anonymous nested structures in standard C, it's an extension used by some compilers.

So you have to name your bit-field structure:

typedef struct _APPLE{
   ULONG appleID;
   struct{
       ULONG isBig: 1;
       ULONG isRed: 1;
       ULONG isFresh: 1;
       ULONG isGood: 1;
       ULONG bReserved: 28;
   } flags;
}APPLE;

Then just use normal dot-notation to access the fields:

APPLE apple;
apple.appleID = 5;
apple.flags.isBig = 1;
apple.flags.isRed = 0;

And while multiple members of a bit-field may share the same int, they are still separate from each other. So changing one member of a bit-field will not change any other.

edited Nov 22, 2013 at 5:53

answered Nov 22, 2013 at 5:46

Some programmer dude

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1 Comment

mohit jain Over a year ago

I am using Visual Studio 2012, i think its supported here. becoz other programmers are using it.

Dayal rai · Accepted Answer · 2013-11-22 06:03:38Z

0

According to ISO/IEC 9899:2011, section §6.7.2.1 Structure and union specifiers

An implementation may allocate any addressable storage unit large enough to hold a bitfield. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified.

answered Nov 22, 2013 at 6:03

Dayal rai

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