int getgrouplist(const char *user, gid_t group,
gid_t *groups, int *ngroups);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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 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 getgrouplist().
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 <stdio.h>
#include <stdlib.h>
#include <grp.h>
#include <pwd.h>
int
main(int argc, char *argv[])
{
int j, ngroups;
gid_t *groups;
struct passwd *pw;
struct group *gr;
if (argc != 3) {
fprintf(stderr, "Usage: %s <user> <ngroups>\n", argv[0]);
exit(EXIT_FAILURE);
}
ngroups = atoi(argv[2]);
groups = malloc(ngroups * sizeof (gid_t));
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 (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);
}