":" (colon) in C struct - what does it mean? [duplicate]

Question

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

I also ran into the colon notation but in my context bit fields didn't make sense. So I did some digging. This notation is also used for assigning values - in my specific situation pointers to functions.

Source: http://www.tldp.org/LDP/lkmpg/2.4/html/c577.htm

Below is a sample and an excerpt to explain.

"There is a gcc extension that makes assigning to this structure more convenient. You'll see it in modern drivers, and may catch you by surprise. This is what the new way of assigning to the structure looks like:"

struct file_operations fops = {
   read: device_read,
   write: device_write,
   open: device_open,
   release: device_release
};

The C99 (old, compatible) way looks like:

struct file_operations fops = {
   .read = device_read,
   .write = device_write,
   .open = device_open,
   .release = device_release
};

It defines bit-fields of width 1 and 8.