确定文件系统是否安装的最佳POSIX方法是只读的 [英] Best POSIX way to determine if a filesystem is mounted read only

问题描述

如果我有一个像Linux或者Mac OS X这样的POSIX系统，那么确定一个路径是否在只读文件系统上的最好和最便携的方法是什么？我可以想到4种方法：我的头顶上：


$ c> open（2） code>一个包含 O_WRONLY 的文件 - 您需要创建一个唯一的文件名并传入 O_CREAT 和 O_EXCL 。如果失败，并且你有一个 EROFS 的errno，那么你知道这是一个只读的文件系统。这会产生实际上创建一个你不关心的文件的烦人的副作用，但是你可以在创建它之后马上 unlink（2）。

statvfs（3） - 返回的 struct statvfs 是 f_flag ，其中一个标志是 ST_RDONLY 。但是， statvfs（3）的规范清楚地表明，应用程序不能依赖任何包含有效信息的字段。看起来像 ST_RDONLY 可能没有为只读文件系统设置。

access（2） - 如果知道挂载点，可以使用 access（2）和 W_OK 标记，只要您以具有写入访问权限的用户身份运行挂载点即可。也就是说，你是root用户，或者是以你的UID作为挂载参数安装的。你会得到一个-1的返回值和一个errno EROFS 。

解析 / etc / mtab 或 / proc / mounts - 似乎不便携。例如，Mac OS X似乎没有这些。即使系统确实有 / etc / mtab 我不确定这些字段在操作系统之间是否一致，或者是只读的挂载选项（

有没有其他的方法，我错过了？如果您需要知道文件系统是否以只读方式挂载，那么您将如何执行它？ 解决方案

  utime（path，NULL）;如果你有写perms，那么这将给你ROFS或 - 如果允许的话 - 只需更新目录上的mtime，基本上是无害的。
 
If I have a POSIX system like Linux or Mac OS X, what's the best and most portable way to determine if a path is on a read-only filesystem?  I can think of 4 ways off the top of my head:


open(2) a file with O_WRONLY - You would need to come up with a unique filename and also pass in O_CREAT and O_EXCL.  If it fails and you have an errno of EROFS then you know it's a read-only filesystem.  This would have the annoying side effect of actually creating a file you didn't care about, but you could unlink(2) it immediately after creating it.
statvfs(3) - One of the fields of the returned struct statvfs is f_flag, and one of the flags is ST_RDONLY for a read-only filesystem.  However, the spec for statvfs(3) makes it clear that applications cannot depend on any of the fields containing valid information.  It would seem there's a decent possibility ST_RDONLY might not be set for a read-only filesystem.
access(2) - If you know the mount point, you can use access(2) with the W_OK flag as long as you are running as a user who would have write access to the mountpoint.  Ie, either you are root or it was mounted with your UID as a mount parameter.  You would get a return value of -1 and an errno of EROFS.
Parsing /etc/mtab or /proc/mounts - Doesn't seem portable.  Mac OS X seems to have neither of these, for example.  Even if the system did have /etc/mtab I'm not sure the fields are consistent between OSes or if the mount options for read-only (ro on Linux) are portable.


Are there other ways I'm missing?  If you needed to know if a filesystem was mounted read-only, how would you do it?
解决方案
utime(path, NULL);
If you have write perms, then that will give you ROFS or -- if permitted -- simply update the mtime on the directory, which is basically harmless.

                        
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