What are the most useful new features in C99? [closed]

Question

I'm so used to typing

for (int i = 0; i < n; ++i) { ... }

in C++ that it's a pain to use a non-C99 compiler where I am forced to say

int i;
for (i = 0; i < n; ++i ) { ... }

stdint.h, which defines int8_t, uint8_t, etc. No more having to make non-portable assumptions about how wide your integers are.

uint32_t truth = 0xDECAFBAD;

I think that the new initializer mechanisms are extremely important.

struct { int x, y; } a[10] = { [3] = { .y = 12, .x = 1 } };

OK - not a compelling example, but the notation is accurate. You can initialize specific elements of an array, and specific members of a structure.

Maybe a better example would be this - though I'd admit it isn't hugely compelling:

enum { Iron = 26, Aluminium = 13, Beryllium = 4, ... };

const char *element_names[] =
{
    [Iron]      = "Iron",
    [Aluminium] = "Aluminium",
    [Beryllium] = "Beryllium",
    ...
};

Support for one-line comments beginning with //.

Variable length arrays:

int x;
scanf("%d", &x);
int a[x];
for (int i = 0; i < x; ++i)
    a[i] = i * i;
for (int i = 0; i < x; ++i)
    printf("%d\n", a[i]);

Being able to declare variables at locations other than the start of a block.