> says to allocate from the lowest fd but that calls which may return multiple fds do not need to guarantee they are adjacent.
If the program has fd 0-3 and 5 open, socketpair should return 4 and 6, which are not adjacent. If socketpair is called again, while close(N) (N < 7) is being called in a separate thread, you could get {7, 8}, {N, 7}, or {7, N}, depending on kernel and timing details. All of those returns fit the requirement that the fds be allocated lowest first, but may or may not be adjacent or in absolute order.
If the program has fd 0-3 and 5 open, socketpair should return 4 and 6, which are not adjacent. If socketpair is called again, while close(N) (N < 7) is being called in a separate thread, you could get {7, 8}, {N, 7}, or {7, N}, depending on kernel and timing details. All of those returns fit the requirement that the fds be allocated lowest first, but may or may not be adjacent or in absolute order.