<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>

1 files changed, 38 insertions, 0 deletions

diff --git a/misc/fd_to_filename.c b/misc/fd_to_filename.c
new file mode 100644
index 0000000000..03d19194c1
--- /dev/null
+++ b/misc/fd_to_filename.c
@@ -0,0 +1,38 @@
+/* Construct a pathname under /proc/self/fd (or /dev/fd for Hurd).
+   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/>.  */
+
+#include <fd_to_filename.h>
+
+#include <assert.h>
+#include <string.h>
+
+char *
+__fd_to_filename (int descriptor, struct fd_to_filename *storage)
+{
+  assert (descriptor >= 0);
+
+  char *p = mempcpy (storage->buffer, FD_TO_FILENAME_PREFIX,
+                     strlen (FD_TO_FILENAME_PREFIX));
+
+  for (int d = descriptor; p++, (d /= 10) != 0; )
+    continue;
+  *p = '\0';
+  for (int d = descriptor; *--p = '0' + d % 10, (d /= 10) != 0; )
+    continue;
+  return storage->buffer;
+}