Eventfd's does not support seeking. This is also invalid code:

int main() {
    int64_t b = 42;
    int efd = eventfd(0, 0);
    if (efd < 0) err(EXIT_FAILURE, "creating eventfd failed");
    ssize_t res = pwrite(efd, &b, sizeof(b), 0);
    if (res != sizeof(b)) err(EXIT_FAILURE, "pwrite to evfd failed");
}