mirror of https://github.com/mkerrisk/man-pages
A near complete rewrite, including additional information and
a new example program.
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.\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
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.\" Distributed under GPL
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.\" Thanks to glibc info pages
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.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk
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.\" <mtk.manpages@gmail.com>
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.\"
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.\" Modified 2003-11-18, aeb: glibc is broken
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.TH GETGROUPLIST 3 2007-07-26 "GNU" "Linux Programmer's Manual"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" A few pieces remain from an earlier version written in
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.\" 2002 by Walter Harms (walter.harms@informatik.uni-oldenburg.de)
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.\"
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.TH GETGROUPLIST 3 2008-04-15 "GNU" "Linux Programmer's Manual"
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.SH NAME
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getgrouplist \- list of groups a user belongs to
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getgrouplist \- get list of groups to which a user belongs
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.SH SYNOPSIS
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.B #include <grp.h>
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.sp
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@ -23,65 +44,147 @@ _BSD_SOURCE
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.SH DESCRIPTION
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The
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.BR getgrouplist ()
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function scans the group database for all the groups
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function scans the group database (see
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.BR group (5))
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to obtain the list of groups that
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.I user
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belongs to.
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Up to
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.I *ngroups
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group IDs corresponding to these groups are stored in the array
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.IR groups ;
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the return value from the function is the number of group IDs
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actually stored.
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The group
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of these groups are returned in the array
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.IR groups .
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If it was not among the groups defined for
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.I user
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in the group database, then
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.I group
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is automatically included in the list of groups returned by
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.BR getgrouplist ().
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is included in the list of groups returned by
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.BR getgrouplist ();
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typically this argument is specified as the group from
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the password record for
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.IR user .
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The
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.I ngroups
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argument is a value-result argument:
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on return it always contains the number of groups found for
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.IR user ,
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including
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.IR group ;
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this value may be greater than the number of groups stored in
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.IR groups .
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.SH "RETURN VALUE"
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If
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If the number of groups of which
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.I user
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is a member is less than or equal to
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.IR *ngroups ,
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then the value
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.I *ngroups
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is smaller than the total number of groups found, then
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is returned.
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If the user is a member of more than
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.I *ngroups
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groups, then
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.BR getgrouplist ()
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returns \-1.
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In all cases the actual number of groups is stored in
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.IR *ngroups .
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In this case the value returned in
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.IR *ngroups
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can be used to resize the buffer passed to a further call
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.BR getgrouplist ().
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.SH "VERSIONS"
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This function is present since glibc 2.2.4.
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.SH "CONFORMING TO"
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This function is non-standard; it appears on most BSDs.
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.SH BUGS
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The glibc 2.3.2 implementation of this function is broken:
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it overwrites memory when the actual number of groups is larger than
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In glibc versions before 2.3.3,
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the implementation of this function contains a buffer-overrun bug:
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it returns the complete list of groups for
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.IR user
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in the array
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.IR groups ,
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even when the number of groups exceeds
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.IR *ngroups .
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.SH EXAMPLE
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.PP
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The program below displays the group list for the user named in its
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first command-line argument.
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The second command-line argument specifies the
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.I ngroups
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value to be supplied to
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.BT getgrouplist ().
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The following shell session shows examples of the use of this program:
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.in +4n
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.nf
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$ ./a.out cecilia 0
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getgrouplist() returned -1; ngroups = 3
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$ ./a.out cecilia 3
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ngroups = 3
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16 (dialout)
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33 (video)
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100 (users)
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.fi
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.sp
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.in
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.nf
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/* This crashes with glibc 2.3.2 */
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#include <stdio.h>
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#include <stdlib.h>
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#include <grp.h>
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#include <pwd.h>
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int
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main(void)
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main(int argc, char *argv[])
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{
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int i, ng = 0;
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char *user = "who"; /* username here */
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gid_t *groups = NULL;
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struct passwd *pw = getpwnam(user);
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int j, ngroups;
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gid_t *groups;
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struct passwd *pw;
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struct group *gr;
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if (pw == NULL)
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exit(EXIT_SUCCESS);
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if (getgrouplist(user, pw\->pw_gid, NULL, &ng) < 0) {
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groups = (gid_t *) malloc(ng * sizeof (gid_t));
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getgrouplist(user, pw\->pw_gid, groups, &ng);
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if (argc != 3) {
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fprintf(stderr, "Usage: %s <user> <ngroups>\\n", argv[0]);
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exit(EXIT_FAILURE);
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}
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for (i = 0; i < ng; i++)
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printf("%d\en", groups[i]);
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ngroups = atoi(argv[2]);
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groups = malloc(ngroups * sizeof (gid_t));
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if (groups == NULL) {
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perror("malloc");
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exit(EXIT_FAILURE);
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}
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/* Fetch passwd structure (contains first group ID for user) */
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pw = getpwnam(argv[1]);
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if (pw == NULL) {
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perror("getpwnam");
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exit(EXIT_SUCCESS);
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}
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/* Retrieve group list */
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if (getgrouplist(argv[1], pw\->pw_gid, groups, &ngroups) == \-1) {
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fprintf(stderr, "getgrouplist() returned \-1; ngroups = %d\\n",
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ngroups);
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exit(EXIT_FAILURE);
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}
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/* Display list of retrieved groups, along with group names */
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fprintf(stderr, "ngroups = %d\\n", ngroups);
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for (j = 0; j < ngroups; j++) {
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printf("%d", groups[j]);
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gr = getgrgid(groups[j]);
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if (gr != NULL)
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printf(" (%s)", gr\->gr_name);
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printf("\\n");
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}
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exit(EXIT_SUCCESS);
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}
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.fi
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.SH "SEE ALSO"
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.BR getgroups (2),
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.BR setgroups (2)
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.BR setgroups (2),
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.BR getgrent (3),
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.BR group (5)
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