“:” (colon) in C struct – what does it mean? [duplicate]

by

Those are bit fields. Basically, the number after the colon describes how many bits that field uses. Here is a quote from MSDN describing bit fields:

The constant-expression specifies the width of the field in bits. The
type-specifier for the declarator must be unsigned int, signed int, or
int, and the constant-expression must be a nonnegative integer value.
If the value is zero, the declaration has no declarator. Arrays of bit
fields, pointers to bit fields, and functions returning bit fields are
not allowed. The optional declarator names the bit field. Bit fields
can only be declared as part of a structure. The address-of operator
(&) cannot be applied to bit-field components.

Unnamed bit fields cannot be referenced, and their contents at run
time are unpredictable. They can be used as “dummy” fields, for
alignment purposes. An unnamed bit field whose width is specified as 0
guarantees that storage for the member following it in the
struct-declaration-list begins on an int boundary.

This example defines a two-dimensional array of structures named screen.

struct 
{
    unsigned short icon : 8;
    unsigned short color : 4;
    unsigned short underline : 1;
    unsigned short blink : 1;
} screen[25][80];

Edit: another important bit from the MSDN link:

Bit fields have the same semantics as the integer type. This means a
bit field is used in expressions in exactly the same way as a variable
of the same base type would be used, regardless of how many bits are
in the bit field.

A quick sample illustrates this nicely. Interestingly, with mixed types the compiler seems to default to sizeof (int).

  struct
  {
    int a : 4;
    int b : 13;
    int c : 1;
  } test1;

  struct
  {
    short a : 4;
    short b : 3;
  } test2;

  struct
  {
    char a : 4;
    char b : 3;
  } test3;

  struct
  {
    char a : 4;
    short b : 3;
  } test4;

  printf("test1: %d\ntest2: %d\ntest3: %d\ntest4: %d\n", sizeof(test1), sizeof(test2), sizeof(test3), sizeof(test4));

test1: 4

test2: 2

test3: 1

test4: 4

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