Function pipe_build_write_buffer

freebsd/kern/sys_pipe.c:870–910

* Map the sending processes' buffer into kernel space and wire it. * This is similar to a physical write operation. */

 * This is similar to a physical write operation.
 */
static int
pipe_build_write_buffer(struct pipe *wpipe, struct uio *uio)
{
	u_int size;
	int i;

	PIPE_LOCK_ASSERT(wpipe, MA_OWNED);
	KASSERT((wpipe->pipe_state & PIPE_DIRECTW) == 0,
	    ("%s: PIPE_DIRECTW set on %p", __func__, wpipe));
	KASSERT(wpipe->pipe_pages.cnt == 0,
	    ("%s: pipe map for %p contains residual data", __func__, wpipe));

	if (uio->uio_iov->iov_len > wpipe->pipe_buffer.size)
                size = wpipe->pipe_buffer.size;
	else
                size = uio->uio_iov->iov_len;

	wpipe->pipe_state |= PIPE_DIRECTW;
	PIPE_UNLOCK(wpipe);
	i = vm_fault_quick_hold_pages(&curproc->p_vmspace->vm_map,
	    (vm_offset_t)uio->uio_iov->iov_base, size, VM_PROT_READ,
	    wpipe->pipe_pages.ms, PIPENPAGES);
	PIPE_LOCK(wpipe);
	if (i < 0) {
		wpipe->pipe_state &= ~PIPE_DIRECTW;
		return (EFAULT);
	}

	wpipe->pipe_pages.npages = i;
	wpipe->pipe_pages.pos =
	    ((vm_offset_t) uio->uio_iov->iov_base) & PAGE_MASK;
	wpipe->pipe_pages.cnt = size;

	uio->uio_iov->iov_len -= size;
	uio->uio_iov->iov_base = (char *)uio->uio_iov->iov_base + size;
	if (uio->uio_iov->iov_len == 0)
		uio->uio_iov++;
	uio->uio_resid -= size;
	uio->uio_offset += size;
	return (0);
}

/*
 * Unwire the process buffer.