Function select

src/backend/libc/event/syscalls.rs:216–289

(
    nfds: i32,
    readfds: Option<&mut [FdSetElement]>,
    writefds: Option<&mut [FdSetElement]>,
    exceptfds: Option<&mut [FdSetElement]>,
    timeout: Option<&Timespec>,
)

#[cfg(any(bsd, linux_kernel))]
pub(crate) unsafe fn select(
    nfds: i32,
    readfds: Option<&mut [FdSetElement]>,
    writefds: Option<&mut [FdSetElement]>,
    exceptfds: Option<&mut [FdSetElement]>,
    timeout: Option<&Timespec>,
) -> io::Result<i32> {
    let len = crate::event::fd_set_num_elements_for_bitvector(nfds);

    let readfds = match readfds {
        Some(readfds) => {
            assert!(readfds.len() >= len);
            readfds.as_mut_ptr()
        }
        None => null_mut(),
    };
    let writefds = match writefds {
        Some(writefds) => {
            assert!(writefds.len() >= len);
            writefds.as_mut_ptr()
        }
        None => null_mut(),
    };
    let exceptfds = match exceptfds {
        Some(exceptfds) => {
            assert!(exceptfds.len() >= len);
            exceptfds.as_mut_ptr()
        }
        None => null_mut(),
    };

    let timeout_data;
    let timeout_ptr = match timeout {
        Some(timeout) => {
            // Convert from `Timespec` to `c::timeval`.
            timeout_data = c::timeval {
                tv_sec: timeout.tv_sec.try_into().map_err(|_| io::Errno::INVAL)?,
                tv_usec: ((timeout.tv_nsec + 999) / 1000) as _,
            };
            &timeout_data
        }
        None => null(),
    };

    // On Apple platforms, use the specially mangled `select` which doesn't
    // have an `FD_SETSIZE` limitation.
    #[cfg(apple)]
    {
        extern "C" {
            #[link_name = "select$DARWIN_EXTSN$NOCANCEL"]
            fn select(
                nfds: c::c_int,
                readfds: *mut FdSetElement,
                writefds: *mut FdSetElement,
                errorfds: *mut FdSetElement,
                timeout: *const c::timeval,
            ) -> c::c_int;
        }