fusl/src/misc/initgroups.c

#define _GNU_SOURCE
#include <grp.h>
#include <limits.h>

int initgroups(const char* user, gid_t gid) {
  gid_t groups[NGROUPS_MAX];
  int count = NGROUPS_MAX;
  if (getgrouplist(user, gid, groups, &count) < 0)
    return -1;
  return setgroups(count, groups);
}