process.c:1095
static VALUE
proc_getpgrp()
{
    int pgrp;

#if defined(HAVE_GETPGRP) && defined(GETPGRP_VOID)
    pgrp = getpgrp();
    if (pgrp < 0) rb_sys_fail(0);
    return INT2FIX(pgrp);
#else
# ifdef HAVE_GETPGID
    pgrp = getpgid(0);
    if (pgrp < 0) rb_sys_fail(0);
    return INT2FIX(pgrp);
# else
    rb_notimplement();
# endif
#endif
}
