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/// Send a message on a socket.
///
/// Allow sending ancillary data.
///
/// # Examples
///
/// ```
/// use std::ffi::c_void;
/// use std::ptr;
/// use std::thread;
///
/// const READ_SIDE: usize = 0;
/// const WRITE_SIDE: usize = 1;
///
/// fn main() {
/// let mut fds = [-1_i32; 2];
///
/// let ret = unsafe { nc::socketpair(nc::AF_UNIX, nc::SOCK_STREAM, 0, &mut fds) };
/// assert!(ret.is_ok());
/// println!("socket pairs: {}, {}", fds[0], fds[1]);
///
/// // Start worker thread
/// thread::spawn(move || {
/// println!("worker thread started");
/// let msg = "Hello, Rust";
/// println!("[worker] Will send msg: {msg}");
/// let mut iov = [nc::iovec_t {
/// iov_base: msg.as_ptr() as *const c_void,
/// iov_len: msg.len(),
/// }];
/// let msg_hdr = nc::msghdr_t {
/// msg_name: ptr::null(),
/// msg_namelen: 0,
/// msg_iov: iov.as_mut_ptr(),
/// msg_iovlen: iov.len(),
/// msg_control: ptr::null(),
/// msg_controllen: 0,
/// msg_flags: 0,
/// };
/// let ret = unsafe { nc::sendmsg(fds[WRITE_SIDE], &msg_hdr, 0) };
/// assert!(ret.is_ok());
/// assert_eq!(ret, Ok(msg.len() as nc::ssize_t));
/// });
///
/// let mut buf = [[0_u8; 64]; 4];
/// let mut iov = Vec::with_capacity(buf.len());
/// for ref mut item in (&mut buf).iter() {
/// iov.push(nc::iovec_t {
/// iov_base: item.as_ptr() as *const c_void,
/// iov_len: item.len(),
/// });
/// }
/// let mut msg_hdr = nc::msghdr_t {
/// msg_name: ptr::null(),
/// msg_namelen: 0,
/// msg_iov: iov.as_mut_ptr(),
/// msg_iovlen: iov.len(),
/// msg_control: ptr::null(),
/// msg_controllen: 0,
/// msg_flags: 0,
/// };
/// let ret = unsafe { nc::recvmsg(fds[READ_SIDE], &mut msg_hdr, 0) };
/// assert!(ret.is_ok());
/// let nread = ret.unwrap();
/// // We only read the first buffer block.
/// let msg = std::str::from_utf8(&buf[0][..nread as usize]).unwrap();
/// println!("[main] recv msg: {msg}");
///
/// unsafe {
/// let _ = nc::close(fds[0]);
/// let _ = nc::close(fds[1]);
/// }
/// }
/// ```
pub unsafe fn sendmsg(sockfd: i32, msg: &msghdr_t, flags: i32) -> Result<ssize_t, Errno> {
let sockfd = sockfd as usize;
let msg_ptr = msg as *const msghdr_t as usize;
let flags = flags as usize;
syscall3(SYS_SENDMSG, sockfd, msg_ptr, flags).map(|ret| ret as ssize_t)
}