grpowned?

/*
 * call-seq:
 *    File.grpowned?(file_name)   => true or false
 *
 * Returns <code>true</code> if the named file exists and the
 * effective group id of the calling process is the owner of
 * the file. Returns <code>false</code> on Windows.
 */

static VALUE
test_grpowned(obj, fname)
    VALUE obj, fname;
{
#ifndef _WIN32
    struct stat st;

    if (rb_stat(fname, &st) < 0) return Qfalse;
    if (group_member(st.st_gid)) return Qtrue;
#endif
    return Qfalse;
}