I found some bizarre code in the linux kernel, could some one explain it to me?

Question

I found this in initramfs.c, I haven't seen this syntax before, could some one explain what it is doing?

static __initdata int (*actions[])(void) = {
  [Start]   = do_start,
  [Collect] = do_collect,
  [GotHeader] = do_header,
  [SkipIt]  = do_skip,
  [GotName] = do_name,
  [CopyFile]  = do_copy,
  [GotSymlink]  = do_symlink,
  [Reset]   = do_reset,
};

Source Code (line 366): initramfs.c

Answer 1

Designators in an Array Initialization

The bracketed expressions are called designators, and that's syntax to initialize an array or structure by naming the fields or elements rather than just by ordering the initializers in the same sequence as the declaration.

Answer 2

This is an out-of-sequence array initialization by index. It's like writing

actions[Start] = do_start;
actions[Collect] = do_collect;

except that you can do it as a static initializer.

Answer 3

This is a feature from ISO C99 known as designated initializers. It creates an array and initializes specific elements of that array, not necessarily the first N in order. It's equivalent to the following snippet:

static __initdata int (*actions[SOME_SIZE])(void);
actions[Start]   = do_start;
actions[Collect] = do_collect;
actions[GotHeader] = do_header;
actions[SkipIt]  = do_skip;
actions[GotName] = do_name;
actions[CopyFile]  = do_copy;
actions[GotSymlink]  = do_symlink;
actions[Reset]   = do_reset;

Except that the array will only be as large as it needs to (equal in size to one more than the largest index), and it can be initialized statically at global scope -- you can't run the above code at global scope.

This is not a feature of ANSI C89, but GCC provides this feature as an extension even when compiling code as C89.