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+/* group-member.c -- determine whether group id is in calling user's group list
+
+ Copyright (C) 1994, 1997-1998, 2003, 2005-2006, 2009-2020 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 <https://www.gnu.org/licenses/>. */
+
+#include <config.h>
+
+/* Specification. */
+#include <unistd.h>
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <stdlib.h>
+
+#include "xalloc-oversized.h"
+
+/* Most processes have no more than this many groups, and for these
+ processes we can avoid using malloc. */
+enum { GROUPBUF_SIZE = 100 };
+
+struct group_info
+ {
+ gid_t *group;
+ gid_t groupbuf[GROUPBUF_SIZE];
+ };
+
+static void
+free_group_info (struct group_info const *g)
+{
+ if (g->group != g->groupbuf)
+ free (g->group);
+}
+
+static int
+get_group_info (struct group_info *gi)
+{
+ int n_groups = getgroups (GROUPBUF_SIZE, gi->groupbuf);
+ gi->group = gi->groupbuf;
+
+ if (n_groups < 0)
+ {
+ int n_group_slots = getgroups (0, NULL);
+ if (0 <= n_group_slots
+ && ! xalloc_oversized (n_group_slots, sizeof *gi->group))
+ {
+ gi->group = malloc (n_group_slots * sizeof *gi->group);
+ if (gi->group)
+ n_groups = getgroups (n_group_slots, gi->group);
+ }
+ }
+
+ /* In case of error, the user loses. */
+ return n_groups;
+}
+
+/* Return non-zero if GID is one that we have in our groups list.
+ Note that the groups list is not guaranteed to contain the current
+ or effective group ID, so they should generally be checked
+ separately. */
+
+int
+group_member (gid_t gid)
+{
+ int i;
+ int found;
+ struct group_info gi;
+ int n_groups = get_group_info (&gi);
+
+ /* Search through the list looking for GID. */
+ found = 0;
+ for (i = 0; i < n_groups; i++)
+ {
+ if (gid == gi.group[i])
+ {
+ found = 1;
+ break;
+ }
+ }
+
+ free_group_info (&gi);
+
+ return found;
+}
+
+#ifdef TEST
+
+int
+main (int argc, char **argv)
+{
+ int i;
+
+ for (i = 1; i < argc; i++)
+ {
+ gid_t gid;
+
+ gid = atoi (argv[i]);
+ printf ("%d: %s\n", gid, group_member (gid) ? "yes" : "no");
+ }
+ exit (0);
+}
+
+#endif /* TEST */