+ * \param rfds An optional fd set pointer.
+ * \param num_bytes Result pointer. Contains the number of bytes read from \a fd.
+ *
+ * If \a rfds is not \p NULL and the (non-blocking) file descriptor \a fd is
+ * not set in \a rfds, this function returns early without doing anything.
+ * Otherwise The function tries to read up to \a sz bytes from \a fd. As for
+ * write_nonblock(), EAGAIN is not considered an error condition. However, EOF
+ * is.
+ *
+ * \return Zero or a negative error code. If the underlying call to readv(2)
+ * returned zero (indicating an end of file condition) or failed for some
+ * reason other than \p EAGAIN, a negative return value is returned.
+ *
+ * In any case, \a num_bytes contains the number of bytes that have been
+ * successfully read from \a fd (zero if the first readv() call failed with
+ * EAGAIN). Note that even if the function returns negative, some data might
+ * have been read before the error occurred. In this case \a num_bytes is
+ * positive.
+ *
+ * \sa \ref write_nonblock(), read(2), readv(2).
+ */
+int readv_nonblock(int fd, struct iovec *iov, int iovcnt, fd_set *rfds,
+ size_t *num_bytes)
+{
+ int ret, i, j;
+
+ *num_bytes = 0;
+ /*
+ * Avoid a shortcoming of select(): Reads from a non-blocking fd might
+ * return EAGAIN even if FD_ISSET() returns true. However, FD_ISSET()
+ * returning false definitely means that no data can currently be read.
+ * This is the common case, so it is worth to avoid the overhead of the
+ * read() system call in this case.
+ */
+ if (rfds && !FD_ISSET(fd, rfds))
+ return 0;
+
+ for (i = 0, j = 0; i < iovcnt;) {
+
+ /* fix up the first iov */
+ assert(j < iov[i].iov_len);
+ iov[i].iov_base += j;
+ iov[i].iov_len -= j;
+ ret = readv(fd, iov + i, iovcnt - i);
+ iov[i].iov_base -= j;
+ iov[i].iov_len += j;
+
+ if (ret == 0)
+ return -E_EOF;
+ if (ret < 0) {
+ if (errno == EAGAIN)
+ return 0;
+ return -ERRNO_TO_PARA_ERROR(errno);
+ }
+ *num_bytes += ret;
+ while (ret > 0) {
+ if (ret < iov[i].iov_len - j) {
+ j += ret;
+ break;
+ }
+ ret -= iov[i].iov_len - j;
+ j = 0;
+ if (++i >= iovcnt)
+ break;
+ }
+ }
+ return 0;
+}
+
+/**
+ * Read from a non-blocking file descriptor into a single buffer.