In C, what does a colon mean inside a declaration? [duplicate]

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.

Answer 2

It means it's a bitfield - i.e. the size of dumpable is a single bit, and you can only assign 0 or 1 to it. Normally used in old code to save space, or in low-level code that interfaces with hardware (even though the packing is non-portable). See here for more information