求助：makefile错误 ： commands commence before first target. Stop

代码:

while(1){
        testfds=readfds;
        timeout.tv_sec=2;
        timeout.tv_usec=500000;     //超时必须在while内

        printf("server waiting\n");
        result=select(FD_SETSIZE, &testfds, (fd_set *)NULL, (fd_set *)NULL, &timeout);
        switch(result){
            case 0:
                printf("timeout\n");
                break;