- regular file or a contiguous file. This is AT_FDCWD if it is the
- working directory, which is possible only for a dummy parent node
- just above the top level. It may be -1 if the file could not be
- opened. Zero represents an otherwise-uninitialized value;
- standard input is never used here. */
+ regular file or a contiguous file.
+
+ It is zero if no file descriptor is available, either because it
+ was never needed or because it was open and then closed to
+ conserve on file descriptors. (Standard input is never used
+ here, so zero cannot be a valid file descriptor.)
+
+ It is negative if it could not be reopened after it was closed.
+ Negate it to find out what errno was when the reopen failed. */