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| |
| <h4 class="subsection">30.3.2 Mount, Unmount, Remount</h4> |
| |
| <p>This section describes the functions for mounting, unmounting, and |
| remounting filesystems. |
| |
| <p>Only the superuser can mount, unmount, or remount a filesystem. |
| |
| <p>These functions do not access the <samp><span class="file">fstab</span></samp> and <samp><span class="file">mtab</span></samp> files. You |
| should maintain and use these separately. See <a href="Mount-Information.html#Mount-Information">Mount Information</a>. |
| |
| <p>The symbols in this section are declared in <samp><span class="file">sys/mount.h</span></samp>. |
| |
| <!-- sys/mount.h --> |
| <!-- SVID, BSD --> |
| <div class="defun"> |
| — Function: int <b>mount</b> (<var>const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data</var>)<var><a name="index-mount-3491"></a></var><br> |
| <blockquote> |
| <p><code>mount</code> mounts or remounts a filesystem. The two operations are |
| quite different and are merged rather unnaturally into this one function. |
| The <code>MS_REMOUNT</code> option, explained below, determines whether |
| <code>mount</code> mounts or remounts. |
| |
| <p>For a mount, the filesystem on the block device represented by the |
| device special file named <var>special_file</var> gets mounted over the mount |
| point <var>dir</var>. This means that the directory <var>dir</var> (along with any |
| files in it) is no longer visible; in its place (and still with the name |
| <var>dir</var>) is the root directory of the filesystem on the device. |
| |
| <p>As an exception, if the filesystem type (see below) is one which is not |
| based on a device (e.g. “proc”), <code>mount</code> instantiates a |
| filesystem and mounts it over <var>dir</var> and ignores <var>special_file</var>. |
| |
| <p>For a remount, <var>dir</var> specifies the mount point where the filesystem |
| to be remounted is (and remains) mounted and <var>special_file</var> is |
| ignored. Remounting a filesystem means changing the options that control |
| operations on the filesystem while it is mounted. It does not mean |
| unmounting and mounting again. |
| |
| <p>For a mount, you must identify the type of the filesystem as |
| <var>fstype</var>. This type tells the kernel how to access the filesystem |
| and can be thought of as the name of a filesystem driver. The |
| acceptable values are system dependent. On a system with a Linux kernel |
| and the <code>proc</code> filesystem, the list of possible values is in the |
| file <samp><span class="file">filesystems</span></samp> in the <code>proc</code> filesystem (e.g. type |
| <kbd>cat /proc/filesystems</kbd> to see the list). With a Linux kernel, the |
| types of filesystems that <code>mount</code> can mount, and their type names, |
| depends on what filesystem drivers are configured into the kernel or |
| loaded as loadable kernel modules. An example of a common value for |
| <var>fstype</var> is <code>ext2</code>. |
| |
| <p>For a remount, <code>mount</code> ignores <var>fstype</var>. |
| |
| <!-- This is traditionally called "rwflag" for historical reasons. --> |
| <!-- No point in confusing people today, though. --> |
| <p><var>options</var> specifies a variety of options that apply until the |
| filesystem is unmounted or remounted. The precise meaning of an option |
| depends on the filesystem and with some filesystems, an option may have |
| no effect at all. Furthermore, for some filesystems, some of these |
| options (but never <code>MS_RDONLY</code>) can be overridden for individual |
| file accesses via <code>ioctl</code>. |
| |
| <p><var>options</var> is a bit string with bit fields defined using the |
| following mask and masked value macros: |
| |
| <dl> |
| <dt><code>MS_MGC_MASK</code><dd>This multibit field contains a magic number. If it does not have the value |
| <code>MS_MGC_VAL</code>, <code>mount</code> assumes all the following bits are zero and |
| the <var>data</var> argument is a null string, regardless of their actual values. |
| |
| <br><dt><code>MS_REMOUNT</code><dd>This bit on means to remount the filesystem. Off means to mount it. |
| <!-- There is a mask MS_RMT_MASK in mount.h that says only two of the options --> |
| <!-- can be reset by remount. But the Linux kernel has its own version of --> |
| <!-- MS_RMT_MASK that says they all can be reset. As far as I can tell, --> |
| <!-- libc just passes the arguments straight through to the kernel. --> |
| |
| <br><dt><code>MS_RDONLY</code><dd>This bit on specifies that no writing to the filesystem shall be allowed |
| while it is mounted. This cannot be overridden by <code>ioctl</code>. This |
| option is available on nearly all filesystems. |
| |
| <br><dt><code>S_IMMUTABLE</code><dd>This bit on specifies that no writing to the files in the filesystem |
| shall be allowed while it is mounted. This can be overridden for a |
| particular file access by a properly privileged call to <code>ioctl</code>. |
| This option is a relatively new invention and is not available on many |
| filesystems. |
| |
| <br><dt><code>S_APPEND</code><dd>This bit on specifies that the only file writing that shall be allowed |
| while the filesystem is mounted is appending. Some filesystems allow |
| this to be overridden for a particular process by a properly privileged |
| call to <code>ioctl</code>. This is a relatively new invention and is not |
| available on many filesystems. |
| |
| <br><dt><code>MS_NOSUID</code><dd>This bit on specifies that Setuid and Setgid permissions on files in the |
| filesystem shall be ignored while it is mounted. |
| |
| <br><dt><code>MS_NOEXEC</code><dd>This bit on specifies that no files in the filesystem shall be executed |
| while the filesystem is mounted. |
| |
| <br><dt><code>MS_NODEV</code><dd>This bit on specifies that no device special files in the filesystem |
| shall be accessible while the filesystem is mounted. |
| |
| <br><dt><code>MS_SYNCHRONOUS</code><dd>This bit on specifies that all writes to the filesystem while it is |
| mounted shall be synchronous; i.e., data shall be synced before each |
| write completes rather than held in the buffer cache. |
| |
| <br><dt><code>MS_MANDLOCK</code><dd>This bit on specifies that mandatory locks on files shall be permitted while |
| the filesystem is mounted. |
| |
| <br><dt><code>MS_NOATIME</code><dd>This bit on specifies that access times of files shall not be updated when |
| the files are accessed while the filesystem is mounted. |
| |
| <br><dt><code>MS_NODIRATIME</code><dd>This bit on specifies that access times of directories shall not be updated |
| when the directories are accessed while the filesystem in mounted. |
| |
| <!-- there is also S_QUOTA Linux fs.h (mount.h still uses its former name --> |
| <!-- S_WRITE), but I can't see what it does. Turns on quotas, I guess. --> |
| </dl> |
| |
| <p>Any bits not covered by the above masks should be set off; otherwise, |
| results are undefined. |
| |
| <p>The meaning of <var>data</var> depends on the filesystem type and is controlled |
| entirely by the filesystem driver in the kernel. |
| |
| <p>Example: |
| |
| <pre class="smallexample"> #include <sys/mount.h> |
| |
| mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, ""); |
| |
| mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, ""); |
| </pre> |
| <p>Appropriate arguments for <code>mount</code> are conventionally recorded in |
| the <samp><span class="file">fstab</span></samp> table. See <a href="Mount-Information.html#Mount-Information">Mount Information</a>. |
| |
| <p>The return value is zero if the mount or remount is successful. Otherwise, |
| it is <code>-1</code> and <code>errno</code> is set appropriately. The values of |
| <code>errno</code> are filesystem dependent, but here is a general list: |
| |
| <dl> |
| <dt><code>EPERM</code><dd>The process is not superuser. |
| <br><dt><code>ENODEV</code><dd>The file system type <var>fstype</var> is not known to the kernel. |
| <br><dt><code>ENOTBLK</code><dd>The file <var>dev</var> is not a block device special file. |
| <br><dt><code>EBUSY</code><dd> |
| <ul> |
| <li>The device is already mounted. |
| |
| <li>The mount point is busy. (E.g. it is some process' working directory or |
| has a filesystem mounted on it already). |
| |
| <li>The request is to remount read-only, but there are files open for write. |
| </ul> |
| |
| <br><dt><code>EINVAL</code><dd> |
| <ul> |
| <li>A remount was attempted, but there is no filesystem mounted over the |
| specified mount point. |
| |
| <li>The supposed filesystem has an invalid superblock. |
| |
| </ul> |
| |
| <br><dt><code>EACCES</code><dd> |
| <ul> |
| <li>The filesystem is inherently read-only (possibly due to a switch on the |
| device) and the process attempted to mount it read/write (by setting the |
| <code>MS_RDONLY</code> bit off). |
| |
| <li><var>special_file</var> or <var>dir</var> is not accessible due to file permissions. |
| |
| <li><var>special_file</var> is not accessible because it is in a filesystem that is |
| mounted with the <code>MS_NODEV</code> option. |
| |
| </ul> |
| |
| <br><dt><code>EM_FILE</code><dd>The table of dummy devices is full. <code>mount</code> needs to create a |
| dummy device (aka “unnamed” device) if the filesystem being mounted is |
| not one that uses a device. |
| |
| </dl> |
| |
| </blockquote></div> |
| |
| <!-- sys/mount.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>umount2</b> (<var>const char *file, int flags</var>)<var><a name="index-umount2-3492"></a></var><br> |
| <blockquote> |
| <p><code>umount2</code> unmounts a filesystem. |
| |
| <p>You can identify the filesystem to unmount either by the device special |
| file that contains the filesystem or by the mount point. The effect is |
| the same. Specify either as the string <var>file</var>. |
| |
| <p><var>flags</var> contains the one-bit field identified by the following |
| mask macro: |
| |
| <dl> |
| <dt><code>MNT_FORCE</code><dd>This bit on means to force the unmounting even if the filesystem is |
| busy, by making it unbusy first. If the bit is off and the filesystem is |
| busy, <code>umount2</code> fails with <code>errno</code> = <code>EBUSY</code>. Depending |
| on the filesystem, this may override all, some, or no busy conditions. |
| |
| </dl> |
| |
| <p>All other bits in <var>flags</var> should be set to zero; otherwise, the result |
| is undefined. |
| |
| <p>Example: |
| |
| <pre class="smallexample"> #include <sys/mount.h> |
| |
| umount2("/mnt", MNT_FORCE); |
| |
| umount2("/dev/hdd1", 0); |
| </pre> |
| <p>After the filesystem is unmounted, the directory that was the mount point |
| is visible, as are any files in it. |
| |
| <p>As part of unmounting, <code>umount2</code> syncs the filesystem. |
| |
| <p>If the unmounting is successful, the return value is zero. Otherwise, it |
| is <code>-1</code> and <code>errno</code> is set accordingly: |
| |
| <dl> |
| <dt><code>EPERM</code><dd>The process is not superuser. |
| <br><dt><code>EBUSY</code><dd>The filesystem cannot be unmounted because it is busy. E.g. it contains |
| a directory that is some process's working directory or a file that some |
| process has open. With some filesystems in some cases, you can avoid |
| this failure with the <code>MNT_FORCE</code> option. |
| |
| <br><dt><code>EINVAL</code><dd><var>file</var> validly refers to a file, but that file is neither a mount |
| point nor a device special file of a currently mounted filesystem. |
| |
| </dl> |
| |
| <p>This function is not available on all systems. |
| </p></blockquote></div> |
| |
| <!-- sys/mount.h --> |
| <!-- SVID, GNU --> |
| <div class="defun"> |
| — Function: int <b>umount</b> (<var>const char *file</var>)<var><a name="index-umount-3493"></a></var><br> |
| <blockquote> |
| <p><code>umount</code> does the same thing as <code>umount2</code> with <var>flags</var> set |
| to zeroes. It is more widely available than <code>umount2</code> but since it |
| lacks the possibility to forcefully unmount a filesystem is deprecated |
| when <code>umount2</code> is also available. |
| </p></blockquote></div> |
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