getgroups function

[merkava]

Hi guys.

I have wrote a simple program to test getgroups functionality:
Here is my group configuration:

group1:501:test_user
group2:502:test_user
group3:503:test_user
group4:504:test_user
group5:505:test_user

test_user is member of all groups. and here is the program:

int main(void)
{
//I get number of supplementary groups
int number_of_groups = getgroups(0, NULL);
printf("%d\n", number_of_groups);
return EXIT_SUCCESS;
}

This program is running with user id of test_user. But the problem is this:

'number_of_groups' is always '1'

[giulix]

If you access the info documentation from the command line for getgroups, you'll get a usage example, from which the following

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

typedef struct s S_STR;
#define S_STR struct s
struct s {
    int counter;
    gid_t *g;
};

S_STR *read_all_groups();

int main(int argc, char *argv[]) {

    int i = 0;
    S_STR *p = NULL;

    if((p = (S_STR *)read_all_groups()) == NULL) {
        perror("Could not determine process group(s)");
        exit(1);
    }

    for(i = 0; i < p->counter; p->g++, i++)
        printf("Group: %d\n", *p->g);

    return 0;
}

S_STR *read_all_groups(void) {

    S_STR *p = NULL;
    int ngroups = getgroups(0, NULL);

    if((p = (S_STR *)malloc(sizeof(S_STR))) == NULL) {
        perror("Error allocating memory for S_STR struct");
        exit(1);
    }
    else
        p->g = NULL;

    if((p->g = (gid_t *)malloc(ngroups * sizeof(gid_t))) == NULL) {
        perror("Error allocating memory for groups array");
        exit(1);
    }
    p->counter = getgroups(ngroups, p->g);
    if(p->counter < 0) {
        if(p->g != NULL) {
            free(p->g);
            p->g = NULL;
        }
        return NULL;
    }

    return p;
}