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Structures and subroutines

Structures can be used within subroutines, both as return types and within argument lists, in a straightforward way. For instance, here is a program to add two fractions:
#include <stdio.h>
typedef struct {
  int n, d;
} frac;
frac add_frac(frac, frac);

int main(void) {
  frac f1, f2, f3;
  f1.n = 1; f1.d = 3;
  f2.n = 2; f2.d = 4;
  f3 = add_frac(f1, f2);
  printf("%d/%d + %d/%d is %d/%d\n", f1.n, f1.d, f2.n, f2.d,
         f3.n, f3.d);
  return 0;
frac add_frac(frac g1, frac g2) {
  frac g3;
  g3.n = g1.n*g2.d + g2.n*g1.d;
  g3.d = g1.d*g2.d;
  return g3;
Structures can also be used within other structures.