c - 来自 OS161 的用于查找冒号的代码如何工作？

Question

从这里：

/*
 * Set bootfs_vnode.
 *
 * Bootfs_vnode is the vnode used for beginning path translation of
 * pathnames starting with /.
 *
 * It is also incidentally the system's first current directory.
 */
int
vfs_setbootfs(const char *fsname)
{
    char tmp[NAME_MAX+1];
    char *s;
    int result;
    struct vnode *newguy;

    vfs_biglock_acquire();

    snprintf(tmp, sizeof(tmp)-1, "%s", fsname);
    s = strchr(tmp, ':');
    if (s) {
        /* If there's a colon, it must be at the end */
        if (strlen(s)>0) {
            vfs_biglock_release();
            return EINVAL;
        }
    }
    else {
        strcat(tmp, ":");
    }

    result = vfs_chdir(tmp);
    if (result) {
        vfs_biglock_release();
        return result;
    }

    result = vfs_getcurdir(&newguy);
    if (result) {
        vfs_biglock_release();
        return result;
    }

    change_bootfs(newguy);

    vfs_biglock_release();
    return 0;
}

为什么应该strlen(s)等于零？换句话说，为什么如果它大于零然后返回错误？由于冒号应该在末尾（如果存在）（如评论所述），它的大小不应该是 1 吗？