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mount_fdesc(8) [osx man page]

MOUNT_FDESC(8)						    BSD System Manager's Manual 					    MOUNT_FDESC(8)

mount_fdesc -- mount the file-descriptor file system SYNOPSIS
mount_fdesc [-o options] fdesc mount_point DESCRIPTION
The mount_fdesc command attaches an instance of the per-process file descriptor namespace to the global filesystem namespace. The conven- tional mount point is /dev and the filesystem should be union mounted in order to augment, rather than replace, the existing entries in /dev. This command is normally executed by mount(8) at boot time. The options are as follows: -o Options are specified with a -o flag followed by a comma separated string of options. See the mount(8) man page for possible options and their meanings. The contents of the mount point are fd, stderr, stdin, stdout and tty. fd is a directory whose contents appear as a list of numbered files which correspond to the open files of the process reading the directory. The files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be accessed through the file system. If the file descriptor is open and the mode the file is being opened with is a subset of the mode of the existing descriptor, the call: fd = open("/dev/fd/0", mode); and the call: fd = fcntl(0, F_DUPFD, 0); are equivalent. The files /dev/stdin, /dev/stdout and /dev/stderr appear as symlinks to the relevant entry in the /dev/fd sub-directory. Opening them is equivalent to the following calls: fd = fcntl(STDIN_FILENO, F_DUPFD, 0); fd = fcntl(STDOUT_FILENO, F_DUPFD, 0); fd = fcntl(STDERR_FILENO, F_DUPFD, 0); Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. The /dev/tty entry is an indirect reference to the current process's controlling terminal. It appears as a named pipe (FIFO) but behaves in exactly the same way as the real controlling terminal device. FILES
/dev/fd/# /dev/stdin /dev/stdout /dev/stderr /dev/tty SEE ALSO
mount(2), unmount(2), tty(4), fstab(5), mount(8) CAVEATS
No ~. and .. entries appear when listing the contents of the /dev/fd directory. This makes sense in the context of this filesystem, but is inconsistent with usual filesystem conventions. However, it is still possible to refer to both ~. and .. in a pathname. This filesystem may not be NFS-exported. HISTORY
The mount_fdesc utility first appeared in 4.4BSD. 4.4BSD March 27, 1994 4.4BSD

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MOUNT_UNION(8)						    BSD System Manager's Manual 					    MOUNT_UNION(8)

mount_union -- mount union filesystems SYNOPSIS
mount_union [-b] [-o options] directory uniondir DESCRIPTION
The mount_union command attaches directory above uniondir in such a way that the contents of both directory trees remain visible. By default, directory becomes the upper layer and uniondir becomes the lower layer. Both directory and uniondir are converted to absolute paths before use. The options are as follows: -b Invert the default position, so that directory becomes the lower layer and uniondir becomes the upper layer. However, uniondir remains the mount point. -o Options are specified with a -o flag followed by a comma separated string of options. See the mount(8) man page for possible options and their meanings. Filenames are looked up in the upper layer and then in the lower layer. If a directory is found in the lower layer, and there is no entry in the upper layer, then a shadow directory will be created in the upper layer. It will be owned by the user who originally did the union mount, with mode "rwxrwxrwx" (0777) modified by the umask in effect at that time. If a file exists in the upper layer then there is no way to access a file with the same name in the lower layer. If necessary, a combination of loopback and union mounts can be made which will still allow the lower files to be accessed by a different pathname. Except in the case of a directory, access to an object is granted via the normal filesystem access checks. For directories, the current user must have access to both the upper and lower directories (should they both exist). Requests to create or modify objects in uniondir are passed to the upper layer with the exception of a few special cases. An attempt to open for writing a file which exists in the lower layer causes a copy of the entire file to be made to the upper layer, and then for the upper layer copy to be opened. Similarly, an attempt to truncate a lower layer file to zero length causes an empty file to be created in the upper layer. Any other operation which would ultimately require modification to the lower layer fails with EROFS. The union filesystem manipulates the namespace, rather than individual filesystems. The union operation applies recursively down the direc- tory tree now rooted at uniondir. Thus any filesystems which are mounted under uniondir will take part in the union operation. This differs from the union option to mount(8) which only applies the union operation to the mount point itself, and then only for lookups. EXAMPLES
The commands mount -t cd9660 -o ro /dev/cd0a /usr/src mount -t union /var/obj /usr/src mount the CD-ROM drive /dev/cd0a on /usr/src and then attaches /var/obj on top. For most purposes the effect of this is to make the source tree appear writable even though it is stored on a CD-ROM. The command mount -t union -o -b /sys $HOME/sys attaches the system source tree below the sys directory in the user's home directory. This allows individual users to make private changes to the source, and build new kernels, without those changes becoming visible to other users. Note that the files in the lower layer remain accessible via /sys. SEE ALSO
intro(2), mount(2), unmount(2), fstab(5), fsck_ffs(8), mount(8), mount_null(8), sysctl(8) HISTORY
The mount_union command first appeared in 4.4BSD. BUGS
Without whiteout support from the filesystem backing the upper layer, there is no way that delete and rename operations on lower layer objects can be done. An attempt to mount a union directory under one which does not have whiteout support will return EOPNOTSUPP ("Operation not supported"). Whiteout support can be added to an existing FFS filesystem by using the -c option of fsck_ffs(8). Running find(1) over a union tree has the side-effect of creating a tree of shadow directories in the upper layer. BSD
February 5, 2008 BSD
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