In C, what does a colon mean inside a declaration?

Question

Possible Duplicate:
What does ‘unsigned temp:3’ means

I'm learning some kernel code, and came along the following line (in linux 2.4, sched.h, struct mm_struct):

unsigned dumpable:1;

What does this mean?

Answer 1

It's a bitfield member. Your code means dumpable occupies exactly 1 bit in the structure.

Bitfields are used when you want to pack members in bit-level. This can greatly reduce the size of memory used when there are a lot of flags in the structure. For example, if we define a struct having 4 members with known numeric constraint

0 < a < 20
    b in [0, 1]
0 < c < 8
0 < d < 100

then the struct could be declared as

struct Foo {
   unsigned a : 5;   // 20 < 2^5 = 32
   unsigned b : 1;   // 
   unsigned c : 3;   // 
   unsigned d : 7;   // 100 < 2^7 = 128
};

then the bits of Foo may be arranged like

                      ddddddd c  cc b aaaaa
---------  ---------  ---------  ----------
                       octet 1     octet 0
===========================================
                uint32

instead of

struct Foo {
   unsigned a;
   unsigned b;
   unsigned c;
   unsigned d;
};

in which many bits are wasted because of the range of values

# wasted space which is not used by the program
# v                                     v
                             ddddddd                                  ccc
------------------------------------ ------------------------------------
            uint32                                 uint32


                                   b                                aaaaa
------------------------------------ ------------------------------------
            uint32                                 uint32

so you can save space by packing many members together.

Note that the C standard doesn't specify how the bitfields are arranged or packed within an "addressable storage unit". Also, bitfields are slower compared with direct member access.