getgrouplist - get list of groups to which a user belongs
Standard C library (
libc,
-lc)
#include <grp.h>
int getgrouplist(const char *user, gid_t group,
gid_t *groups, int *ngroups);
getgrouplist():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_BSD_SOURCE
The
getgrouplist() function scans the group database (see
group(5)) to obtain the list of groups that
user belongs to. Up
to
*ngroups of these groups are returned in the array
groups.
If it was not among the groups defined for
user in the group database,
then
group is included in the list of groups returned by
getgrouplist(); typically this argument is specified as the group ID
from the password record for
user.
The
ngroups argument is a value-result argument: on return it always
contains the number of groups found for
user, including
group;
this value may be greater than the number of groups stored in
groups.
If the number of groups of which
user is a member is less than or equal
to
*ngroups, then the value
*ngroups is returned.
If the user is a member of more than
*ngroups groups, then
getgrouplist() returns -1. In this case, the value returned in
*ngroups can be used to resize the buffer passed to a further call to
getgrouplist().
This function is present since glibc 2.2.4.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
getgrouplist () |
Thread safety |
MT-Safe locale |
This function is nonstandard; it appears on most BSDs.
Before glibc 2.3.3, the implementation of this function contains a
buffer-overrun bug: it returns the complete list of groups for
user in
the array
groups, even when the number of groups exceeds
*ngroups.
The program below displays the group list for the user named in its first
command-line argument. The second command-line argument specifies the
ngroups value to be supplied to
getgrouplist(). The following
shell session shows examples of the use of this program:
$ ./a.out cecilia 0
getgrouplist() returned -1; ngroups = 3
$ ./a.out cecilia 3
ngroups = 3
16 (dialout)
33 (video)
100 (users)
#include <grp.h>
#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
int ngroups;
struct passwd *pw;
struct group *gr;
gid_t *groups;
if (argc != 3) {
fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
exit(EXIT_FAILURE);
}
ngroups = atoi(argv[2]);
groups = malloc(sizeof(*groups) * ngroups);
if (groups == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
/* Fetch passwd structure (contains first group ID for user). */
pw = getpwnam(argv[1]);
if (pw == NULL) {
perror("getpwnam");
exit(EXIT_SUCCESS);
}
/* Retrieve group list. */
if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",
ngroups);
exit(EXIT_FAILURE);
}
/* Display list of retrieved groups, along with group names. */
fprintf(stderr, "ngroups = %d\n", ngroups);
for (size_t j = 0; j < ngroups; j++) {
printf("%d", groups[j]);
gr = getgrgid(groups[j]);
if (gr != NULL)
printf(" (%s)", gr->gr_name);
printf("\n");
}
exit(EXIT_SUCCESS);
}
getgroups(2),
setgroups(2),
getgrent(3),
group_member(3),
group(5),
passwd(5)