This is a Designated Initializer, which is syntax added for C99.
Relevant excerpt:
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 };