In C what does [foo] = bar mean?

Question

I was just reading some code in the st terminal emulator and came across this syntax:

static void (*handler[LASTEvent])(XEvent *) = {
    [KeyPress] = kpress,
    [ClientMessage] = cmessage,
    /* Removed some lines for brevity ... */
};

I have never seen this syntax in C and I am not even sure what to google for. I have a rough idea what it does (defining handler as an array of function pointers), but I would like to understand this syntax better. It seems to be valid at least in C99, but I am looking for some more details why this is correct, how exactly it works and maybe a pointer to the C standard where this syntax is defined.

Answer 1

This is initializing an array of function pointers with enum indexes. See here.

As mentioned in the comments below uses Designated Initializers.

This short example should show how it can be used.

enum indexes {ZERO, ONE, TWO, FOUR=4};
int array[5] = {[FOUR]=1, [TWO]=9};

for(int i = 0; i < 5; i++)
    printf("%d, ", array[i]);

This prints out

0, 0, 9, 0, 1,