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.

Answer 2

These are neither labels nor bitfields.

This is a syntax to initialize struct members dating back to the days before C99. It is not standardized but available in e.g. gcc.

typedef struct { int y; int x; } POINT; POINT p = { x: 1, y: 17 }; 

In C99, syntax for initializing specific struct members has been introduced for the first time in a standard, but it looks a little differently:

typedef struct { int y; int x; } POINT; POINT p = { .x = 1, .y = 17 };