macOS SO_REUSEPORT of setsockopt() - macos

Question

SO_REUSEPORT is deleted to make the example work on macOS.
Socket programming - setsockopt: Protocol not available?
But man setsockopt on macOS clearly document the option SO_REUSEPORT.
SO_REUSEPORT enables duplicate address and port bindings
Why SO_REUSEPORT has to be removed to make the example? Is there a bug in setsockopt() on macOS? Where is the source code of setsockopt() on macOS?

Answer 1

SO_REUSEPORT is an option for secsockopt, so the source for it is in the kernel proper. That'd be xnu's sources, specifically the sys call handler checks validity (bsd/kern/uipc_socket.c) and the IPv4/v6 stacks
(bsd/netinet/in_pcb.c and bsd/netinet6/in6_pcb.c, respectively) implement it. From it, you can see two things:
that SO_REUSEPORT and SO_REUSEADDR actually work interchangeably in many cases (e.g. multicast).
it would work in the sample you referred to when put in two setsockopt(2) calls, as well: The mistake was that the options are not bitmasks, so | and + won't work with them - even though
#define SO_REUSEADDR 0x0004 /* allow local address reuse */
#define SO_REUSEPORT 0x0200 /* allow local address & port reuse */
are used in the kernel as bit masks later on (|'ed) , the code in kernel to handle getsockopt and check for validity uses a switch statement, so it ends up that neither option is honored, because it doesn't fall in those cases. Specifically, it's bsd/kern/uipc_socket.c:
int sosetoptlock(struct socket *so, struct sockopt *sopt, int dolock)
{
. ...
switch (sopt->sopt_name) {
. ..
case SO_REUSEADDR:
case SO_REUSEPORT:
case SO_OOBINLINE:
case SO_TIMESTAMP:
case SO_TIMESTAMP_MONOTONIC:
case SO_TIMESTAMP_CONTINUOUS:
case SO_DONTTRUNC:
case SO_WANTMORE:
case SO_WANTOOBFLAG:
case SO_NOWAKEFROMSLEEP:
case SO_NOAPNFALLBK:
error = sooptcopyin(sopt, &optval, sizeof(optval),
...