What does this syntax in an initializer list in C mean?

Question

I was reading bootloader code of some OS and came up with such syntaxt:

pde_t entry_pgdir[NPDENTRIES] = {
        // Map VA's [0, 4MB) to PA's [0, 4MB)
        [0]
                = ((uintptr_t)entry_pgtable - KERNBASE) + PTE_P,
        // Map VA's [KERNBASE, KERNBASE+4MB) to PA's [0, 4MB)
        [KERNBASE>>PDXSHIFT]
                = ((uintptr_t)entry_pgtable - KERNBASE) + PTE_P + PTE_W
};

What does it mean — [0] = …?

Answer 1

This is C99 array initialization syntax (aka ‘designated initializer’). It lets you mix regular values, e.g.

pde_t entry_pgdir[NPDENTRIES] = {val1, val2}

with [ind1] = val1, [ind2] = val2 syntax. The semantics of this is that the value in square brackets is interpreted as an index, and the value after = is interpreted as the value to store at the corresponding index.

The remaining entries are zeroed out. If you put regular values after an indexed one, the numbering continues at index+1.

This syntax is convenient when the data has gaps, for example

int logval[] = {
    [1<<0] = 1
,   [1<<1] = 2
,   [1<<2] = 3
,   [1<<3] = 4
...
};

This is easier to read than its plain equivalent:

int logval[] = {0, 1, 2, 0, 3, 0, 0, 0, 4, ...}