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

int putgrent(const struct group* gr, FILE* f) {
  int r;
  size_t i;
  flockfile(f);
  if ((r = fprintf(f, "%s:%s:%d:", gr->gr_name, gr->gr_passwd, gr->gr_gid)) < 0)
    goto done;
  if (gr->gr_mem)
    for (i = 0; gr->gr_mem[i]; i++)
      if ((r = fprintf(f, "%s%s", i ? "," : "", gr->gr_mem[i])) < 0)
        goto done;
  r = fputc('\n', f);
done:
  funlockfile(f);
  return r < 0 ? -1 : 0;
}