SDLNet_TCP_Accept non-blocking contract for cross-platform servers

SDLNet_TCP_Accept() non-blocking contract across platforms remains the central tension for SDL_net 2.0 developers. The documented contract states that when a listening socket is set to non-blocking mode via SDLNet_TCP_SetNonblocking(), the function should return NULL and set errno to EWOULDBLOCK if no pending connections exist. On POSIX systems—Linux and macOS—the underlying accept() syscall respects the socket flag and immediately returns EWOULDBLOCK when the queue is empty, making the behavior predictable for event‑driven loops that use SDLNet_SocketSet and SDLNet_CheckEvent. Windows, however, introduces a divergence: the SDL_net wrapper does not always translate the non‑blocking flag to the OS accept call, so SDLNet_TCP_Accept() may block until a connection arrives even when the flag is set. This means an application that assumes universal non‑blocking semantics can experience unexpected stalls on Windows, forcing a choice between defensive polling loops or platform‑specific readiness checks. The question of whether this divergence is an intentional design trade‑off or an implementation gap is unresolved; SDL_net maintainers have not issued a binding contract guaranteeing uniform non‑blocking suppression across all supported OSes. Consequently, developers must either poll SDLNet_TCP_Accept() in a tight loop until a connection appears, or transition to the SDLNet_SocketSet + SDLNet_CheckEvent pattern, which abstracts readiness detection without busy‑waiting.

1. Create an SDL_ListenSocket with SDLNet_TCP_Open() on a selected port.
2. Call SDLNet_TCP_SetNonblocking(listen_socket, true) before any acceptance logic.
3. Invoke SDLNet_TCP_Accept(listen_socket) and examine the return value and global errno.

If the return is NULL and errno is EWOULDBLOCK, the socket is truly non‑blocking on that platform. If Accept returns a valid socket or blocks without EWOULDBLOCK, the platform‑specific wrapper is in effect. Running this sequence on Linux, Windows (MinGW or MSVC), and macOS with the same SDL_net 2.0 revision will reveal whether the contract holds uniformly or requires fallback logic. Verification requires compiling the test program with the target SDL_net headers and linking against the library. The only missing diagnostic detail that would change the recommendation is whether the build in use ships a patched SDL_net 2.0.1 or later revision that may have addressed the Windows wrapper. Confirming the exact SDL_net version and compiler/toolchain for the target platform would allow a precise recommendation: if the version is 2.0.0 or earlier, assume platform‑specific blocking and adopt the SocketSet pattern; if 2.0.1 or newer, re‑run the test to confirm any bug‑fix changes. For now, the safest cross‑platform approach remains to treat SDLNet_TCP_Accept() on non‑blocking sockets as potentially blocking on Windows and to structure the event loop around SDLNet_SocketSet readiness rather than relying on immediate NULL/EWOULDBLOCK returns.