Define a Scheme binding to ‘fchownat’ when it exists.

* configure.ac: Detect whether ‘fchownat’ is available.
* libguile/filesys.c (scm_chownat): Define a Scheme binding to
  ‘fchownat’ when available.
* libguile/filesys.h (scm_chownat): Make it part of the API.
* doc/ref/posix.texi (File System): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

configure.ac

@@ -509,7 +509,7 @@ AC_CHECK_HEADERS([crt_externs.h])
 #   isblank - available as a GNU extension or in C99
 #   _NSGetEnviron - Darwin specific
 #   strcoll_l, newlocale, uselocale, utimensat, futimens, fchmodat,
-#   unlinkat - POSIX.1-2008
+#   unlinkat, fchownat - POSIX.1-2008
 #   strtol_l - non-POSIX, found in glibc
 #   fork - unavailable on Windows
 #   sched_getaffinity, sched_setaffinity - GNU extensions (glibc)
@@ -517,7 +517,7 @@ AC_CHECK_HEADERS([crt_externs.h])
 #   pipe2 - non-POSIX, found in glibc (GNU/Linux and GNU/Hurd)
 #
 AC_CHECK_FUNCS([DINFINITY DQNAN cexp chsize clog clog10 ctermid         \
-  fesetround ftime ftruncate fchown fchmod fchdir readlinkat		\
+  fesetround ftime ftruncate fchown fchownat fchmod fchdir readlinkat	\
   fchmodat symlinkat mkdirat renameat unlinkat getcwd geteuid getsid	\
   gettimeofday getuid getgid gmtime_r ioctl lstat mkdir mkdtemp mknod   \
   nice readlink rmdir setegid seteuid                            \

doc/ref/posix.texi

@@ -802,6 +802,17 @@ unsupported at present).  If @var{owner} or @var{group} is specified
 as @code{-1}, then that ID is not changed.
 @end deffn
 
+@findex fchownat
+@deffn {Scheme Procedure} chownat dir name owner group [flags]
+@deffnx {C Function} scm_chownat (dir, name, owner, group, flags)
+Like @code{chown}, but modify the owner and/or group of
+the file named @var{name} in the directory referred to
+by the file port @var{dir} instead.  The optional argument
+@var{flags} is a bitmask.  If @code{AT_SYMLINK_NOFOLLOW} is
+present, then @var{name} will not be dereferenced if it is a
+symbolic link.
+@end deffn
+
 @findex fchmod
 @deffn {Scheme Procedure} chmod object mode
 @deffnx {C Function} scm_chmod (object, mode)

libguile/filesys.c

@@ -180,6 +180,41 @@ SCM_DEFINE (scm_chown, "chown", 3, 0, 0,
 #undef FUNC_NAME
 #endif /* HAVE_CHOWN */
 
+#ifdef HAVE_FCHOWNAT
+SCM_DEFINE (scm_chownat, "chown-at", 4, 1, 0,
+            (SCM dir, SCM name, SCM owner, SCM group, SCM flags),
+            "Like @code{chown}, but modify the owner and/or group of\n"
+            "the file named @var{name} in the directory referred to\n"
+            "by the file port @var{dir} instead.  The optional argument\n"
+            "@var{flags} is a bitmask.  If @code{AT_SYMLINK_NOFOLLOW} is\n"
+            "present, then @var{name} will not be dereferenced if it is a\n"
+            "symbolic link.")
+#define FUNC_NAME s_scm_chownat
+{
+  int rv;
+  int dir_fdes;
+  int c_flags;
+
+  if (SCM_UNBNDP (flags))
+    c_flags = 0;
+  else
+    c_flags = scm_to_int (flags);
+
+  SCM_VALIDATE_OPFPORT (SCM_ARG1, dir);
+  dir_fdes = SCM_FPORT_FDES (dir);
+
+  STRING_SYSCALL (name, c_name,
+                  rv = fchownat (dir_fdes, c_name,
+                                 scm_to_int (owner), scm_to_int (group),
+                                 c_flags));
+  scm_remember_upto_here_1 (dir);
+  if (rv == -1)
+    SCM_SYSERROR;
+  return SCM_UNSPECIFIED;
+}
+#undef FUNC_NAME
+#endif /* HAVE_FCHOWNAT */
+
 
 
 SCM_DEFINE (scm_open_fdes, "open-fdes", 2, 1, 0, 

libguile/filesys.h

@@ -39,6 +39,7 @@ SCM_API scm_t_bits scm_tc16_dir;
 
 
 SCM_API SCM scm_chown (SCM object, SCM owner, SCM group);
+SCM_API SCM scm_chownat (SCM dir, SCM object, SCM owner, SCM group, SCM flags);
 SCM_API SCM scm_chmod (SCM object, SCM mode);
 SCM_API SCM scm_chmodat (SCM dir, SCM pathname, SCM mode, SCM flags);
 SCM_API SCM scm_umask (SCM mode);