--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2009-11-13  Eric Blake  <ebb9@byu.net>
 
+	mgetgroups: new module, taken from coreutils
+	* modules/mgetgroups: New file.
+	* lib/mgetgroups.h: Likewise.
+	* lib/mgetgroups.c (mgetgroups): Likewise.
+	* m4/mgetgroups.m4 (gl_MGETGROUPS): Likewise.
+	* MODULES.html.sh (Users and groups): Mention it.
+
 	getgroups: don't expose GETGROUPS_T to user
 	* lib/getgroups.c (rpl_getgroups): Change signature.  Copy array
 	an element at a time if GETGROUPS_T is wrong size.

--- a/MODULES.html.sh
+++ b/MODULES.html.sh
@@ -2580,6 +2580,7 @@
   func_module getugroups
   func_module group-member
   func_module idcache
+  func_module mgetgroups
   func_module userspec
   func_end_table
 

new file mode 100644
--- /dev/null
+++ b/lib/mgetgroups.c
@@ -0,0 +1,149 @@
+/* mgetgroups.c -- return a list of the groups a user or current process is in
+
+   Copyright (C) 2007-2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Extracted from coreutils' src/id.c. */
+
+#include <config.h>
+
+#include "mgetgroups.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdint.h>
+#include <string.h>
+#include <errno.h>
+#if HAVE_GETGROUPLIST
+# include <grp.h>
+#endif
+
+#include "getugroups.h"
+#include "xalloc.h"
+
+static gid_t *
+realloc_groupbuf (gid_t *g, size_t num)
+{
+  if (xalloc_oversized (num, sizeof *g))
+    {
+      errno = ENOMEM;
+      return NULL;
+    }
+
+  return realloc (g, num * sizeof *g);
+}
+
+/* Like getugroups, but store the result in malloc'd storage.
+   Set *GROUPS to the malloc'd list of all group IDs of which USERNAME
+   is a member.  If GID is not -1, store it first.  GID should be the
+   group ID (pw_gid) obtained from getpwuid, in case USERNAME is not
+   listed in the groups database (e.g., /etc/groups).  If USERNAME is
+   NULL, store the supplementary groups of the current process, and GID
+   should be -1 or the effective group ID (getegid).  Upon failure,
+   don't modify *GROUPS, set errno, and return -1.  Otherwise, return
+   the number of groups.  */
+
+int
+mgetgroups (char const *username, gid_t gid, gid_t **groups)
+{
+  int max_n_groups;
+  int ng;
+  gid_t *g;
+
+#if HAVE_GETGROUPLIST
+  /* We prefer to use getgrouplist if available, because it has better
+     performance characteristics.
+
+     In glibc 2.3.2, getgrouplist is buggy.  If you pass a zero as the
+     length of the output buffer, getgrouplist will still write to the
+     buffer.  Contrary to what some versions of the getgrouplist
+     manpage say, this doesn't happen with nonzero buffer sizes.
+     Therefore our usage here just avoids a zero sized buffer.  */
+  if (username)
+    {
+      enum { N_GROUPS_INIT = 10 };
+      max_n_groups = N_GROUPS_INIT;
+
+      g = realloc_groupbuf (NULL, max_n_groups);
+      if (g == NULL)
+        return -1;
+
+      while (1)
+        {
+          gid_t *h;
+          int last_n_groups = max_n_groups;
+
+          /* getgrouplist updates max_n_groups to num required.  */
+          ng = getgrouplist (username, gid, g, &max_n_groups);
+
+          /* Some systems (like Darwin) have a bug where they
+             never increase max_n_groups.  */
+          if (ng < 0 && last_n_groups == max_n_groups)
+            max_n_groups *= 2;
+
+          if ((h = realloc_groupbuf (g, max_n_groups)) == NULL)
+            {
+              int saved_errno = errno;
+              free (g);
+              errno = saved_errno;
+              return -1;
+            }
+          g = h;
+
+          if (0 <= ng)
+            {
+              *groups = g;
+              /* On success some systems just return 0 from getgrouplist,
+                 so return max_n_groups rather than ng.  */
+              return max_n_groups;
+            }
+        }
+    }
+  /* else no username, so fall through and use getgroups. */
+#endif
+
+  max_n_groups = (username
+                  ? getugroups (0, NULL, username, gid)
+                  : getgroups (0, NULL) + (gid != -1));
+
+  /* If we failed to count groups with NULL for a buffer,
+     try again with a non-NULL one, just in case.  */
+  if (max_n_groups < 0)
+      max_n_groups = 5;
+
+  g = realloc_groupbuf (NULL, max_n_groups);
+  if (g == NULL)
+    return -1;
+
+  ng = (username
+        ? getugroups (max_n_groups, g, username, gid)
+        : getgroups (max_n_groups, g + (gid != -1)));
+
+  if (ng < 0)
+    {
+      int saved_errno = errno;
+      free (g);
+      errno = saved_errno;
+      return -1;
+    }
+
+  if (!username && gid != -1)
+    {
+      *g = gid;
+      ng++;
+    }
+  *groups = g;
+  return ng;
+}

new file mode 100644
--- /dev/null
+++ b/lib/mgetgroups.h
@@ -0,0 +1,19 @@
+/* Get a list of all group IDs associated with a specified user ID.
+   Copyright (C) 2007, 2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <sys/types.h>
+
+int mgetgroups (const char *username, gid_t gid, gid_t **groups);

new file mode 100644
--- /dev/null
+++ b/m4/mgetgroups.m4
@@ -0,0 +1,11 @@
+#serial 4
+dnl Copyright (C) 2007-2009 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_MGETGROUPS],
+[
+  AC_CHECK_FUNCS_ONCE([getgrouplist])
+  AC_LIBOBJ([mgetgroups])
+])

new file mode 100644
--- /dev/null
+++ b/modules/mgetgroups
@@ -0,0 +1,26 @@
+Description:
+Return the group IDs of a user or current process in malloc'd storage.
+
+Files:
+lib/mgetgroups.c
+lib/mgetgroups.h
+m4/mgetgroups.m4
+
+Depends-on:
+getgroups
+getugroups
+xalloc
+
+configure.ac:
+gl_MGETGROUPS
+
+Makefile.am:
+
+Include:
+"mgetgroups.h"
+
+License:
+GPL
+
+Maintainer:
+Jim Meyering, Eric Blake