<fd_to_filename.h>: Add type safety and port to Hurd

The new type struct fd_to_filename makes the allocation of the
backing storage explicit.

Hurd uses /dev/fd, not /proc/self/fd.

Co-Authored-By: Paul Eggert <eggert@cs.ucla.edu>

2 files changed, 37 insertions, 8 deletions

diff --git a/sysdeps/generic/arch-fd_to_filename.h b/sysdeps/generic/arch-fd_to_filename.h
new file mode 100644
index 0000000000..ecaaa14dba
--- /dev/null
+++ b/sysdeps/generic/arch-fd_to_filename.h
@@ -0,0 +1,19 @@
+/* Query filename corresponding to an open FD.  Generic stub.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#error "<arch-fd_to_filename.h> must be ported to this architecture"
diff --git a/sysdeps/generic/fd_to_filename.h b/sysdeps/generic/fd_to_filename.h
index eff6ca211b..5ca22f02bc 100644
--- a/sysdeps/generic/fd_to_filename.h
+++ b/sysdeps/generic/fd_to_filename.h
@@ -1,4 +1,4 @@
-/* Query filename corresponding to an open FD.  Generic version.
+/* Query filename corresponding to an open FD.
    Copyright (C) 2001-2020 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
@@ -16,12 +16,22 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#define FD_TO_FILENAME_SIZE 0
+#ifndef _FD_TO_FILENAME_H
+#define _FD_TO_FILENAME_H
 
-/* In general there is no generic way to query filename for an open
-   file descriptor.  */
-static inline const char *
-fd_to_filename (int fd, char *buf)
+#include <arch-fd_to_filename.h>
+#include <intprops.h>
+
+struct fd_to_filename
 {
-  return NULL;
-}
+  /* A positive int value has at most 10 decimal digits.  */
+  char buffer[sizeof (FD_TO_FILENAME_PREFIX) + INT_STRLEN_BOUND (int)];
+};
+
+/* Writes a /proc/self/fd-style path for DESCRIPTOR to *STORAGE and
+   returns a pointer to the start of the string.  DESCRIPTOR must be
+   non-negative.  */
+char *__fd_to_filename (int descriptor, struct fd_to_filename *storage)
+  attribute_hidden;
+
+#endif /* _FD_TO_FILENAME_H */