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98 lines
4.3 KiB
Markdown
98 lines
4.3 KiB
Markdown
## Name
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mount - mount a filesystem
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## Synopsis
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```**c++
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#include <unistd.h>
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int mount(int source_fd, const char* target, const char* fs_type, int flags);
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```
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## Description
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`mount()` mounts a filesystem stored at `source_fd` by overlaying its contents
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over `target`.
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`fs_type` must be one of the following supported filesystems:
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* `Ext2FS` (or `ext2`): The ext2 filesystem.
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* `ProcFS` (or `proc`): The process pseudo-filesystem (normally mounted at `/proc`).
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* `DevPtsFS` (or `devpts`): The pseudoterminal pseudo-filesystem (normally mounted at `/dev/pts`).
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* `TmpFS` (or `tmp`): A non-persistent filesystem that stores all its data in RAM. An instance of this filesystem is normally mounted at `/tmp`.
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* `Plan9FS` (or `9p`): A remote filesystem served over the 9P protocol.
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For Ext2FS, `source_fd` must refer to an open file descriptor to a file
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containing the filesystem image. This may be a device file or any other seekable
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file. For Plan9FS, `source_fd` must refer to a socket or a device connected to a
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9P server. All the other filesystems ignore the `source_fd` - you can even pass
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an invalid file descriptor such as -1.
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The following `flags` are supported:
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* `MS_NODEV`: Disallow opening any devices from this file system.
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* `MS_NOEXEC`: Disallow executing any executables from this file system.
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* `MS_NOSUID`: Ignore set-user-id bits on executables from this file system.
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* `MS_BIND`: Perform a bind-mount (see below).
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* `MS_RDONLY`: Mount the filesystem read-only.
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* `MS_REMOUNT`: Remount an already mounted filesystem (see below).
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These flags can be used as a security measure to limit the possible abuses of the newly
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mounted file system.
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### Bind mounts
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If `MS_BIND` is specified in `flags`, `fs_type` is ignored and a bind mount is
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performed instead. In this case, the file or directory specified by `source_fd`
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is overlaid over `target` — the target appears to be replaced by a copy of the
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source. This can be used as an alternative to symlinks or hardlinks.
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Each bind mount has its own set of flags, independent of the others or the
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original file system. It is possible to bind-mount a file or directory over
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itself, which may be useful for changing mount flags for a part of a filesystem.
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### Remounting
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If `MS_REMOUNT` is specified in `flags`, `source_fd` and `fs_type` are ignored,
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and a remount is performed instead. `target` must point to an existing mount
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point. The mount flags for that mount point are reset to `flags` (except the
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`MS_REMOUNT` flag itself, which is stripped from the value).
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Note that remounting a file system will only affect future operations with the
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file system, not any already opened files. For example, if you open a directory
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on a filesystem that's mounted with `MS_NODEV`, then remount the filesystem to
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allow opening devices, attempts to open a devices relative to the directory file
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descriptor (such as by using `openat()`) will still fail.
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In particular, current working directory and root directory of any already
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running processes behave the same way, and don't automatically "pick up" changes
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in mount flags of the underlying file system. To "refresh" the working directory
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to use the new mount flags after remounting a filesystem, a process can call
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`chdir()` with the path to the same directory.
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Similarly, to change the mount flags used by the root directory, a process can
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remount the root filesystem using `MS_REMOUNT`.
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However, it only have a noticeable effect if
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the kernel was to launch more userspace processes directly, the way it does
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launch the initial userspace process.
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## Errors
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* `EFAULT`: The `fs_type` or `target` are invalid strings.
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* `EPERM`: The current process does not have superuser privileges.
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* `ENODEV`: The `fs_type` is unrecognized, or the file descriptor to source is
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not found, or the source doesn't contain a valid filesystem image. Also, this
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error occurs if `fs_type` is valid and required to be seekable, but the file
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descriptor from `source_fd` is not seekable.
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* `EBADF`: If the `source_fd` is not valid, and either `fs_type` specifies a
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file-backed filesystem (and not a pseudo filesystem), or `MS_BIND` is
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specified in flags.
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* `ENOTBLK`: If the `source_fd` is not a block device, but one is required (i.e.
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when `fs_type` is `Ext2FS`)
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All of the usual path resolution errors may also occur.
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## See also
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* [`mount`(8)](help://man/8/mount)
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