glibc/grp/initgroups.c

/* Copyright (C) 1989, 1991, 1993, 1996 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 Library General Public License as
published by the Free Software Foundation; either version 2 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
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include <alloca.h>
#include <unistd.h>
#include <string.h>
#include <grp.h>
#include <limits.h>
#include <sys/types.h>


/* Initialize the group set for the current user
   by reading the group database and using all groups
   of which USER is a member.  Also include GROUP.  */
int
initgroups (user, group)
     const char *user;
     gid_t group;
{
#if defined (NGROUPS_MAX) && NGROUPS_MAX == 0

  /* No extra groups allowed.  */
  return 0;

#else

  struct group *g;
  register size_t n;
  size_t ngroups;
  gid_t *groups;
#ifdef NGROUPS_MAX
# define limit NGROUPS_MAX

  ngroups = limit;
#else
  long int limit = sysconf (_SC_NGROUPS_MAX);

  if (limit > 0)
    ngroups = limit;
  else
    /* No fixed limit on groups.  Pick a starting buffer size.  */
    ngroups = 16;
#endif

  groups = __alloca (ngroups * sizeof *groups);

  setgrent ();

  n = 0;
  groups[n++] = group;

  while ((g = getgrent ()) != NULL)
    if (g->gr_gid != group)
      {
	register char **m;

	for (m = g->gr_mem; *m != NULL; ++m)
	  if (!strcmp (*m, user))
	    break;

	if (*m == NULL)
	  {
	    /* Matched the user.  Insert this group.  */
	    if (n == ngroups && limit <= 0)
	      {
		/* Need a bigger buffer.  */
		groups = memcpy (__alloca (ngroups * 2 * sizeof *groups),
				 groups, ngroups * sizeof *groups);
		ngroups *= 2;
	      }

	    groups[n++] = g->gr_gid;

	    if (n == limit)
	      /* Can't take any more groups; stop searching.  */
	      break;
	  }
      }

  endgrent ();

  return setgroups (n, groups);
#endif
}