new file mode 100644
--- /dev/null
+++ b/lib/idcache.c
@@ -0,0 +1,206 @@
+/* idcache.c -- map user and group IDs, cached for speed
+   Copyright (C) 1985, 1988, 1989, 1990 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 2, 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, write to the Free Software
+   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <grp.h>
+
+#if defined(USG) || defined(STDC_HEADERS)
+#include <string.h>
+#else
+#include <strings.h>
+#endif
+
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#ifndef _POSIX_VERSION
+struct passwd *getpwuid ();
+struct passwd *getpwnam ();
+struct group *getgrgid ();
+struct group *getgrnam ();
+#endif
+
+char *xmalloc ();
+char *xstrdup ();
+
+struct userid
+{
+  union
+    {
+      uid_t u;
+      gid_t g;
+    } id;
+  char *name;
+  struct userid *next;
+};
+
+static struct userid *user_alist;
+
+/* The members of this list have names not in the local passwd file.  */
+static struct userid *nouser_alist;
+
+/* Translate UID to a login name or a stringified number,
+   with cache.  */
+
+char *
+getuser (uid)
+     uid_t uid;
+{
+  register struct userid *tail;
+  struct passwd *pwent;
+  char usernum_string[20];
+
+  for (tail = user_alist; tail; tail = tail->next)
+    if (tail->id.u == uid)
+      return tail->name;
+
+  pwent = getpwuid (uid);
+  tail = (struct userid *) xmalloc (sizeof (struct userid));
+  tail->id.u = uid;
+  if (pwent == 0)
+    {
+      sprintf (usernum_string, "%u", (unsigned) uid);
+      tail->name = xstrdup (usernum_string);
+    }
+  else
+    tail->name = xstrdup (pwent->pw_name);
+
+  /* Add to the head of the list, so most recently used is first.  */
+  tail->next = user_alist;
+  user_alist = tail;
+  return tail->name;
+}
+
+/* Translate USER to a UID, with cache.
+   Return NULL if there is no such user.
+   (We also cache which user names have no passwd entry,
+   so we don't keep looking them up.)  */
+
+uid_t *
+getuidbyname (user)
+     char *user;
+{
+  register struct userid *tail;
+  struct passwd *pwent;
+
+  for (tail = user_alist; tail; tail = tail->next)
+    /* Avoid a function call for the most common case.  */
+    if (*tail->name == *user && !strcmp (tail->name, user))
+      return &tail->id.u;
+
+  for (tail = nouser_alist; tail; tail = tail->next)
+    /* Avoid a function call for the most common case.  */
+    if (*tail->name == *user && !strcmp (tail->name, user))
+      return 0;
+
+  pwent = getpwnam (user);
+
+  tail = (struct userid *) xmalloc (sizeof (struct userid));
+  tail->name = xstrdup (user);
+
+  /* Add to the head of the list, so most recently used is first.  */
+  if (pwent)
+    {
+      tail->id.u = pwent->pw_uid;
+      tail->next = user_alist;
+      user_alist = tail;
+      return &tail->id.u;
+    }
+
+  tail->next = nouser_alist;
+  nouser_alist = tail;
+  return 0;
+}
+
+/* Use the same struct as for userids.  */
+static struct userid *group_alist;
+static struct userid *nogroup_alist;
+
+/* Translate GID to a group name or a stringified number,
+   with cache.  */
+
+char *
+getgroup (gid)
+     gid_t gid;
+{
+  register struct userid *tail;
+  struct group *grent;
+  char groupnum_string[20];
+
+  for (tail = group_alist; tail; tail = tail->next)
+    if (tail->id.g == gid)
+      return tail->name;
+
+  grent = getgrgid (gid);
+  tail = (struct userid *) xmalloc (sizeof (struct userid));
+  tail->id.g = gid;
+  if (grent == 0)
+    {
+      sprintf (groupnum_string, "%u", (unsigned int) gid);
+      tail->name = xstrdup (groupnum_string);
+    }
+  else
+    tail->name = xstrdup (grent->gr_name);
+
+  /* Add to the head of the list, so most recently used is first.  */
+  tail->next = group_alist;
+  group_alist = tail;
+  return tail->name;
+}
+
+/* Translate GROUP to a UID, with cache.
+   Return NULL if there is no such group.
+   (We also cache which group names have no group entry,
+   so we don't keep looking them up.)  */
+
+gid_t *
+getgidbyname (group)
+     char *group;
+{
+  register struct userid *tail;
+  struct group *grent;
+
+  for (tail = group_alist; tail; tail = tail->next)
+    /* Avoid a function call for the most common case.  */
+    if (*tail->name == *group && !strcmp (tail->name, group))
+      return &tail->id.g;
+
+  for (tail = nogroup_alist; tail; tail = tail->next)
+    /* Avoid a function call for the most common case.  */
+    if (*tail->name == *group && !strcmp (tail->name, group))
+      return 0;
+
+  grent = getgrnam (group);
+
+  tail = (struct userid *) xmalloc (sizeof (struct userid));
+  tail->name = xstrdup (group);
+
+  /* Add to the head of the list, so most recently used is first.  */
+  if (grent)
+    {
+      tail->id.g = grent->gr_gid;
+      tail->next = group_alist;
+      group_alist = tail;
+      return &tail->id.g;
+    }
+  
+  tail->next = nogroup_alist;
+  nogroup_alist = tail;
+  return 0;
+}