1GETGROUPLIST(3) Linux Programmer's Manual GETGROUPLIST(3)
2
6 getgrouplist - get list of groups to which a user belongs
7
9 #include <grp.h>
10 int getgrouplist(const char *user, gid_t group,
12 gid_t *groups, int *ngroups);
13 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
15 getgrouplist():
17 Since glibc 2.19:
18 _DEFAULT_SOURCE
19 Glibc 2.19 and earlier:
20 _BSD_SOURCE
21
23 The getgrouplist() function scans the group database (see group(5)) to
24 obtain the list of groups that user belongs to. Up to *ngroups of
25 these groups are returned in the array groups.
26 If it was not among the groups defined for user in the group database,
28 then group is included in the list of groups returned by getgrou‐
29 plist(); typically this argument is specified as the group ID from the
30 password record for user.
31 The ngroups argument is a value-result argument: on return it always
33 contains the number of groups found for user, including group; this
34 value may be greater than the number of groups stored in groups.
35
37 If the number of groups of which user is a member is less than or equal
38 to *ngroups, then the value *ngroups is returned.
39 If the user is a member of more than *ngroups groups, then getgrou‐
41 plist() returns -1. In this case, the value returned in *ngroups can
42 be used to resize the buffer passed to a further call getgrouplist().
43
45 This function is present since glibc 2.2.4.
46
48 For an explanation of the terms used in this section, see at‐
49 tributes(7).
50 ┌─────────────────────────────────────┬───────────────┬────────────────┐
52 │Interface │ Attribute │ Value │
53 ├─────────────────────────────────────┼───────────────┼────────────────┤
54 │getgrouplist() │ Thread safety │ MT-Safe locale │
55 └─────────────────────────────────────┴───────────────┴────────────────┘
56
58 This function is nonstandard; it appears on most BSDs.
59
61 In glibc versions before 2.3.3, the implementation of this function
62 contains a buffer-overrun bug: it returns the complete list of groups
63 for user in the array groups, even when the number of groups exceeds
64 *ngroups.
65
67 The program below displays the group list for the user named in its
68 first command-line argument. The second command-line argument speci‐
69 fies the ngroups value to be supplied to getgrouplist(). The following
70 shell session shows examples of the use of this program:
71 $ ./a.out cecilia 0
73 getgrouplist() returned -1; ngroups = 3
74 $ ./a.out cecilia 3
75 ngroups = 3
76 16 (dialout)
77 33 (video)
78 100 (users)
79 Program source
81 #include <stdio.h>
83 #include <stdlib.h>
84 #include <grp.h>
85 #include <pwd.h>
86 int
88 main(int argc, char *argv[])
89 {
90 int ngroups;
91 struct passwd *pw;
92 struct group *gr;
93 if (argc != 3) {
95 fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
96 exit(EXIT_FAILURE);
97 }
98 ngroups = atoi(argv[2]);
100 gid_t *groups = malloc(sizeof(*groups) * ngroups);
102 if (groups == NULL) {
103 perror("malloc");
104 exit(EXIT_FAILURE);
105 }
106 /* Fetch passwd structure (contains first group ID for user). */
108 pw = getpwnam(argv[1]);
110 if (pw == NULL) {
111 perror("getpwnam");
112 exit(EXIT_SUCCESS);
113 }
114 /* Retrieve group list. */
116 if (getgrouplist(argv[1], pw->pw_gid, groups, &ngroups) == -1) {
118 fprintf(stderr, "getgrouplist() returned -1; ngroups = %d\n",
119 ngroups);
120 exit(EXIT_FAILURE);
121 }
122 /* Display list of retrieved groups, along with group names. */
124 fprintf(stderr, "ngroups = %d\n", ngroups);
126 for (int j = 0; j < ngroups; j++) {
127 printf("%d", groups[j]);
128 gr = getgrgid(groups[j]);
129 if (gr != NULL)
130 printf(" (%s)", gr->gr_name);
131 printf("\n");
132 }
133 exit(EXIT_SUCCESS);
135 }
136
138 getgroups(2), setgroups(2), getgrent(3), group_member(3), group(5),
139 passwd(5)
140
142 This page is part of release 5.12 of the Linux man-pages project. A
143 description of the project, information about reporting bugs, and the
144 latest version of this page, can be found at
145 https://www.kernel.org/doc/man-pages/.
146GNU 2021-03-22 GETGROUPLIST(3)