Function splice_write

src/yes.c:101–187

   report any error).  Return FALSE if splice could not be used.  */

static bool
splice_write (MAYBE_UNUSED char const *buf, MAYBE_UNUSED idx_t copysize)
{
  bool output_started = false;
#if HAVE_SPLICE
  idx_t page_size = getpagesize ();

  bool stdout_is_pipe = isapipe (STDOUT_FILENO) > 0;

  /* Determine buffer size: enlarge the target pipe,
     then use 1/4 of actual capacity as the transfer size.  */
  int pipefd[2] = { -1, -1 };
  idx_t splice_bufsize;
  char *splice_buf = NULL;

  if (stdout_is_pipe)
    splice_bufsize = pipe_splice_size (STDOUT_FILENO, copysize);
  else
    {
      if (pipe2 (pipefd, 0) < 0)
        return false;
      splice_bufsize = pipe_splice_size (pipefd[0], copysize);
    }

  if (splice_bufsize == 0)
    goto done;

  /* Allocate page-aligned buffer for vmsplice.
     Needed with SPLICE_F_GIFT, but generally good for performance.  */
  if (! (splice_buf = alignalloc (page_size, splice_bufsize)))
    goto done;

  repeat_pattern (splice_buf, buf, copysize, splice_bufsize);

  /* For the pipe case, vmsplice directly to stdout.
     For the non-pipe case, vmsplice into the intermediate pipe
     and then splice from it to stdout.  */
  int vmsplice_fd = stdout_is_pipe ? STDOUT_FILENO : pipefd[1];

  for (;;)
    {
      struct iovec iov = { .iov_base = splice_buf,
                           .iov_len = splice_bufsize };

      while (iov.iov_len > 0)
        {
          /* Use SPLICE_F_{GIFT,MOVE} to allow the kernel to take references
             to the pages.  I.e., we're indicating we won't make changes.
             SPLICE_F_GIFT is only appropriate for full pages.  */
          unsigned int flags = iov.iov_len % page_size ? 0 : SPLICE_F_GIFT;
          ssize_t n = vmsplice (vmsplice_fd, &iov, 1, flags);
          if (n <= 0)
            goto done;

          if (stdout_is_pipe)
            output_started = true;
          else
            {