source: azure_iot_hub/trunk/musl-1.1.18/src/passwd/getgrouplist.c@ 388

#define _GNU_SOURCE
#include "pwf.h"
#include <grp.h>
#include <string.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <byteswap.h>
#include <errno.h>
#include "nscd.h"

int getgrouplist(const char *user, gid_t gid, gid_t *groups, int *ngroups)
{
        int rv, nlim, ret = -1;
        ssize_t i, n = 1;
        struct group gr;
        struct group *res;
        FILE *f;
        int swap = 0;
        int32_t resp[INITGR_LEN];
        uint32_t *nscdbuf = 0;
        char *buf = 0;
        char **mem = 0;
        size_t nmem = 0;
        size_t size;
        nlim = *ngroups;
        if (nlim >= 1) *groups++ = gid;

        f = __nscd_query(GETINITGR, user, resp, sizeof resp, &swap);
        if (!f) goto cleanup;
        if (resp[INITGRFOUND]) {
                nscdbuf = calloc(resp[INITGRNGRPS], sizeof(uint32_t));
                if (!nscdbuf) goto cleanup;
                if (!fread(nscdbuf, sizeof(*nscdbuf)*resp[INITGRNGRPS], 1, f)) {
                        if (!ferror(f)) errno = EIO;
                        goto cleanup;
                }
                if (swap) {
                        for (i = 0; i < resp[INITGRNGRPS]; i++)
                                nscdbuf[i] = bswap_32(nscdbuf[i]);
                }
        }
        fclose(f);

        f = fopen("/etc/group", "rbe");
        if (!f && errno != ENOENT && errno != ENOTDIR)
                goto cleanup;

        if (f) {
                while (!(rv = __getgrent_a(f, &gr, &buf, &size, &mem, &nmem, &res)) && res) {
                        if (nscdbuf)
                                for (i=0; i < resp[INITGRNGRPS]; i++) {
                                        if (nscdbuf[i] == gr.gr_gid) nscdbuf[i] = gid;
                                }
                        for (i=0; gr.gr_mem[i] && strcmp(user, gr.gr_mem[i]); i++);
                        if (!gr.gr_mem[i]) continue;
                        if (++n <= nlim) *groups++ = gr.gr_gid;
                }
                if (rv) {
                        errno = rv;
                        goto cleanup;
                }
        }
        if (nscdbuf) {
                for(i=0; i < resp[INITGRNGRPS]; i++) {
                        if (nscdbuf[i] != gid)
                                if(++n <= nlim) *groups++ = nscdbuf[i];
                }
        }

        ret = n > nlim ? -1 : n;
        *ngroups = n;

cleanup:
        if (f) fclose(f);
        free(nscdbuf);
        free(buf);
        free(mem);
        return ret;
}