Closures in C have a nasty syntax. you can short circuit typedef to create a fake type (in this case iTOi) that can be used as a parameter type.

closures.c

typedef int (*iTOi) (int a);


int incr (int n) { return n+1; }
int decr (int n) { return n-1; }

void start(iTOi cc) {
  printf("%d\n", cc(0));
}

int main ( int argc, char *argv[] ) {
  start(&incr); // returns 1
  start(&decr); // returns -1
  return 0;
}