<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>

4 files changed, 58 insertions, 28 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 */
diff --git a/sysdeps/mach/hurd/arch-fd_to_filename.h b/sysdeps/mach/hurd/arch-fd_to_filename.h
new file mode 100644
index 0000000000..b45cd8d836
--- /dev/null
+++ b/sysdeps/mach/hurd/arch-fd_to_filename.h
@@ -0,0 +1,19 @@
+/* Query filename corresponding to an open FD.  Hurd version.
+   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/>.  */
+
+#define FD_TO_FILENAME_PREFIX "/dev/fd/"
diff --git a/sysdeps/unix/sysv/linux/fd_to_filename.h b/sysdeps/unix/sysv/linux/arch-fd_to_filename.h
index 92a5e02976..b6017214c7 100644
--- a/sysdeps/unix/sysv/linux/fd_to_filename.h
+++ b/sysdeps/unix/sysv/linux/arch-fd_to_filename.h
@@ -1,5 +1,5 @@
 /* Query filename corresponding to an open FD.  Linux version.
-   Copyright (C) 2001-2020 Free Software Foundation, Inc.
+   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
@@ -16,22 +16,4 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <sys/stat.h>
-#include <string.h>
-#include <_itoa.h>
-
-#define FD_TO_FILENAME_SIZE ((sizeof ("/proc/self/fd/") - 1) \
-			     + (sizeof ("4294967295") - 1) + 1)
-
-static inline const char *
-fd_to_filename (unsigned int fd, char *buf)
-{
-  *_fitoa_word (fd, __stpcpy (buf, "/proc/self/fd/"), 10, 0) = '\0';
-
-  /* We must make sure the file exists.  */
-  struct stat64 st;
-  if (__lxstat64 (_STAT_VER, buf, &st) < 0)
-    /* /proc is not mounted or something else happened.  */
-    return NULL;
-  return buf;
-}
+#define FD_TO_FILENAME_PREFIX "/proc/self/fd/"