int from_backend(void *frontend_handle, int is_stderr, char *data, int len)
int from_backend(void *frontend_handle, int is_stderr, const char *data, int len)
int from_backend(void *frontend_handle, int is_stderr, char *data, int len){    int osize, esize;    assert(len > 0);    if (is_stderr) {	bufchain_add(&stderr_data, data, len);	try_output(1);    } else {	bufchain_add(&stdout_data, data, len);	try_output(0);    }    osize = bufchain_size(&stdout_data);    esize = bufchain_size(&stderr_data);    return osize + esize;}