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

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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.

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