libmodbus 3.1.12 does not connect in windows

[hdabis1234]

Calling modbus_connect() in windows returns -1.

Reason is the following in modbus-tcp.c -> if (ctx->s >= FD_SETSIZE) {

In windows socket() returns an arbitrary number 1100, 1200 etc, however FD_SETSIZE is defined as 64 hence the failure.

What should I do about this?

Thanks

Homam

[chrullrich]

Revert 7b13341, or stay on 3.1.11 for now.

fd_set works entirely differently on Windows compared to POSIX; it is an array of handles, not a bit field indexed by their values. FD_SETSIZE still controls how many sockets you can select() on at once, but by limiting the size of the array, not the values of its elements.

If the array is full, the Winsock default implementation of FD_SET() does nothing. This breaks the application, but it won't overrun the array. The FD_SETSIZE check added in 3.1.12 assumes an implementation detail (the bit field) that does not apply on Windows, and is therefore wrong.

Further reading: https://devblogs.microsoft.com/oldnewthing/20221102-00/?p=107343