The initializer of a structure variable can include a reference to itself

structureinit.c

/*
	LLVM reports a warning as follows;
	"variable 's' is uninitialized when used within its own initialization [-Wuninitialized]"
	But it shows the answer "3" correctly. 
*/
#include <stdio.h>
typedef struct {
	int a, b, c;
} S;
int sum(S s) {
	return s.a + s.b;
}
int main() { 
	S s = {1, 2, sum(s)};
	printf("%d %d %d\n", s.a, s.b, s.c);
	return 0;
}

Assembled code is here;
https://gist.github.com/yyasuda/c5633cea0e9c9de303f15a3af4c5d324