length actually written, persisting an errno from write()
*/
ssize_t
-write_to_fd_buf(int fd, uschar *buf, size_t length)
+write_to_fd_buf(int fd, const uschar *buf, size_t length)
{
ssize_t wrote;
size_t total_written = 0;
-uschar *p = buf;
+const uschar *p = buf;
size_t left = length;
while (1)