getgrouplist(3) Library Functions Manual getgrouplist(3)
NAME
getgrouplist - get list of groups to which a user belongs
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <grp.h>
int getgrouplist(const char *user, gid_t group,
gid_t *groups, int *ngroups);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getgrouplist():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_BSD_SOURCE
DESCRIPTION
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 getgrou-
plist(); 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.
RETURN VALUE
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 getgrou-
plist() returns -1. In this case, the value returned in *ngroups can
be used to resize the buffer passed to a further call to getgrou-
plist().
VERSIONS
This function is present since glibc 2.2.4.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌─────────────────────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼────────────────┤
│getgrouplist() │ Thread safety │ MT-Safe locale │
└─────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
This function is nonstandard; it appears on most BSDs.
BUGS
Before glibc 2.3.3, the implementation of this function contains a buf-
fer-overrun bug: it returns the complete list of groups for user in the
array groups, even when the number of groups exceeds *ngroups.
EXAMPLES
The program below displays the group list for the user named in its
first command-line argument. The second command-line argument speci-
fies 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)
Program source
#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);
}
SEE ALSO
getgroups(2), setgroups(2), getgrent(3), group_member(3), group(5),
passwd(5)
Linux man-pages 6.03 2023-02-05 getgrouplist(3)
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