bmiColors field of BITMAPINFO structure

Question

The BITMAPINFO structure has the following declaration

typedef struct tagBITMAPINFO {
    BITMAPINFOHEADER bmiHeader;
    RGBQUAD bmiColors[1];
         


        

Answer 1

There's no static keyword in the declaration. It's a completely normal struct member. It's used to declare a variable-sized struct with a single variable-sized array at the end

The size of the array is only known at compile time, but since arrays of size 0 are forbidden in C and C++ so we'll use array[1] instead. See the detailed explanation from MS' Raymond Chen in Why do some structures end with an array of size 1?

On some compilers like GCC zero-length arrays are allowed as an extension so Linux and many other platforms usually use array[0] instead of array[1]

Declaring zero-length arrays is allowed in GNU C as an extension. A zero-length array can be useful as the last element of a structure that is really a header for a variable-length object:

struct line {
  int length;
  char contents[0];
};

struct line *thisline = (struct line *)
  malloc (sizeof (struct line) + this_length);
thisline->length = this_length;

Arrays of Length Zero

In C99 a new feature called flexible array member was introduced. Since then it's better to use array[] for portability

struct vectord {
    size_t len;
    double arr[]; // the flexible array member must be last
};

See also

• What is the purpose of a zero length array in a struct?
• What's the need of array with zero elements?
• What is the advantage of using zero-length arrays in C?