C struct initialization using labels. It works, but how?

Question

I found some struct initialization code yesterday that threw me for a loop. Here's an example:

typedef struct { int first; int second; } TEST_STRUCT;
void testFunc() {
    TEST_STRUCT test = {
        second: 2,
        first:  1
    };
    printf("test.first=%d test.second=%d\n", test.first, test.second);
}

Surprisingly (to me), here's the output:

-> testFunc
test.first=1 test.second=2

As you can see, the struct gets initialized properly. I wasn't aware labeled statements could be used like that. I've seen several other ways of doing struct initialization, but I didn't find any examples of this sort of struct initialization on any of the online C FAQs. Is anybody aware of how/why this works?

Answer 1

Here is the section of the gcc manual which explains the syntax of designated initializers for both structs and arrays:

In a structure initializer, specify the name of a field to initialize with '.fieldname =' before the element value. For example, given the following structure,

 struct point { int x, y; };

the following initialization

 struct point p = { .y = yvalue, .x = xvalue }; 

is equivalent to

 struct point p = { xvalue, yvalue }; 

Another syntax which has the same meaning, obsolete since GCC 2.5, is 'fieldname:', as shown here:

 struct point p = { y: yvalue, x: xvalue };

The relevant page can be found here.

Your compiler should have similar documentation.