Some platforms (but not Linux) let you define FD_SETSIZE before including C headers to override the default. The fd_set structure will be made correspondingly

Some platforms (but not Linux) let you define FD_SETSIZE before including C headers to override the default. The fd_set structure will be made correspondingly larger. On these platforms it should work fine, though there may still be an undocumented kernel limit. Some really old platforms with select() don't even define fd_set or FD_SETSIZE, and you're on your own :-) Linux added the poll() syscall in 2.1.23, and Linux select() had a kernel-side limit of 256 fds up to 2.1.26, when it raised the kernel-side limit to 5120/10752 depending on architecture. It was 2.5 years until the kernel-side limit was removed, by which point everyone would have had poll() in Glibc. So if you are going to the effort of writing portable select() except using the trick of defining your own, larger arrays, and using poll() when it's available, on Linux don't bother. You'll hit a kernel limit anyway on those very old systems which don't have poll().