rem: return EINVAL;
add: return FS_EINVAL;
context: do_write_test(int argc, char **argv){    int    i, err;    char   *buff;    size_t len = 256;        if (cur_fd < 0) {        printf("no file open! (open or create one with open or make)\n");        return EINVAL;    }    if (argv[1] && isdigit(argv[1][0]))        len = strtoul(&argv[1][0], NULL, 0);    buff = (char*)malloc(len);    if (buff == NULL) {        printf("no memory for write buffer of %lu bytes\n", len);        return ENOMEM;    }    for (i = 0; (size_t)i < len; i++)        buff[i] = i;    err = sys_write(1, cur_fd, buff, len);    free(buff);        printf("write wrote %lu bytes and returned %d\n", len, err);	return (err < 0 ? err : 0);}