What is this C syntax (used in Linux drivers/net/bonding/bond_main.c)?

Question

I've written plenty of C before, but I don't recognize this syntax:

static const char *names[] = {
    [BOND_MODE_ROUNDROBIN] = "load balancing (round-robin)",
    [BOND_MODE_ACTIVEBACKUP] = "fault-tolerance (active-backup)",
    [BOND_MODE_XOR] = "load balancing (xor)",
    [BOND_MODE_BROADCAST] = "fault-tolerance (broadcast)",
    [BOND_MODE_8023AD] = "IEEE 802.3ad Dynamic link aggregation",
    [BOND_MODE_TLB] = "transmit load balancing",
    [BOND_MODE_ALB] = "adaptive load balancing",
};

The [...] = part is what looks weird to me. (By the way, BOND_MODE_ROUNDROBIN and the others are macros which just expand to integers.)

Answer 1

It's called designated initializers, which is introduced in C99. GCC also supports it as an extension.

It's used to initialize structures and arrays, see Designated Initializers for detail.

Answer 2

That is a designated initializer. It allows you to intialize the contents of the array in an arbitrary order.