newlib/winsup/doc/pathnames.sgml

<sect1 id="using-pathnames"><title>Mapping path names</title>

<sect2><title>Introduction</title>

<para>Cygwin supports both Win32- and POSIX-style paths, where
directory delimiters may be either forward or back slashes.  UNC
pathnames (starting with two slashes and a network name) are also
supported.</para>

<para>POSIX operating systems (such as Linux) do not have the concept
of drive letters.  Instead, all absolute paths begin with a
slash (instead of a drive letter such as "c:") and all file systems
appear as subdirectories (for example, you might buy a new disk and
make it be the <filename>/disk2</filename> directory).</para>

<para>Because many programs written to run on UNIX systems assume
the existance of a single unified POSIX file system structure, Cygwin
maintains a special internal POSIX view of the Win32 file system
that allows these programs to successfully run under Windows.  Cygwin
uses this mapping to translate from POSIX to Win32 paths as
necessary.</para>

</sect2>

<sect2 id="mount-table"><title>The Cygwin Mount Table</title>

<para>The <command>mount</command> utility program is used to
to map Win32 drives and network shares into Cygwin's internal POSIX
directory tree.  This is a similar concept to the typical UNIX mount
program.  For those people coming from a Windows background, the
<command>mount</command> utility is very similar to the old DOS
<command>join</command>, in that it makes your drive letters appear as
subdirectories somewhere else.</para>

<para>The mapping is stored in the current user's Cygwin
<firstterm>mount table</firstterm> in the Windows registry so that the
information will be retrieved next time the user logs in.  Because it
is sometimes desirable to have system-wide as well as user-specific
mounts, there is also a system-wide mount table that all Cygwin users
inherit.  The system-wide table may only be modified by a user with
the appropriate privileges (Administrator privileges in Windows
NT).</para>

<para>The current user's table is located under
"HKEY_CURRENT_USER/Software/Cygnus Solutions/Cygwin/mounts
v&lt;version&gt;"
where &lt;version&gt; is the latest registry version associated with
the Cygwin library (this version is not the same as the release
number).  The system-wide table is located under the same subkeys
under HKEY_LOCAL_SYSTEM.  The user mount table takes precedence over 
the system-wide table if a path is mounted in both.  This includes the
setting of the cygdrive prefix.</para>

<para>Since Windows uses drive letters instead of a single filesystem
root, the POSIX root <filename>/</filename> must be set to a directory
in the Windows file system using the <command>mount</command> command.
Without a <filename>/</filename> mount, Cygwin processes cannot
distinguish between the Windows CurrentDrive and SystemDrive.
</para>

<para>Whenever Cygwin generates a Win32 path from a POSIX one, it uses
the longest matching prefix in the mount table.  Thus, if
<filename>C:</filename> is mounted as <filename>/c</filename> and also
as <filename>/</filename>, then Cygwin would translate
<filename>C:/foo/bar</filename> to <filename>/c/foo/bar</filename>.
This translation is normally only used when trying to derive the
POSIX equivalent current directory.  Otherwise, the handling of MS-DOS
filenames bypasses the mount table.
</para>

<para>Invoking <command>mount</command> without any arguments displays
Cygwin's current set of mount points.
In the following example, the C
drive is the POSIX root and D drive is mapped to
<filename>/d</filename>.  Note that in this case, the root mount is a
system-wide mount point that is visible to all users running Cygwin
programs, whereas the <filename>/d</filename> mount is only visible
to the current user.</para>

<example>
<title>Displaying the current set of mount points</title>
<screen>
<prompt>c:\&gt;</prompt> <userinput>mount</userinput>
f:\cygwin\bin on /usr/bin type system (binmode)
f:\cygwin\lib on /usr/lib type system (binmode)
f:\cygwin on / type system (binmode)
e:\src on /usr/src type system (binmode)
c: on /cygdrive/c type user (binmode,noumount)
e: on /cygdrive/e type user (binmode,noumount)
</screen>
</example>

<para>You can also use the <command>mount</command> command to add
new mount points, and the <command>umount</command> to delete
them.  See <xref linkend="mount"></xref> and <xref linkend="umount"></xref> for more
information on how to use these utilities to set up your Cygwin POSIX
file system.</para>

<para>Whenever Cygwin cannot use any of the existing mounts to convert
from a particular Win32 path to a POSIX one, Cygwin will
automatically default to an imaginary mount point under the default POSIX
path <filename>/cygdrive</filename>.  For example, if Cygwin accesses
<filename>Z:\foo</filename> and the Z drive is not currently in the
mount table, then <filename>Z:\</filename> would be automatically
converted to <filename>/cygdrive/Z</filename>.  The default
prefix of <filename>/cygdrive</filename> may be changed (see the
<xref linkend="mount"></xref> for more information).</para>

<para>It is possible to assign some special attributes to each mount
point.  Automatically mounted partitions are displayed as "auto"
mounts.  Mounts can also be marked as either "textmode" or "binmode"
-- whether text files are read in the same manner as binary files by
default or not (see <xref linkend="using-textbinary"></xref> for more
information on text and binary modes.</para>

</sect2>

<sect2><title>Additional Path-related Information</title>

<para>The <command>cygpath</command> program provides the ability to
translate between Win32 and POSIX pathnames in shell scripts. See
<xref linkend="cygpath"></xref> for the details.</para>

<para>The <envar>HOME</envar>, <envar>PATH</envar>, and
<envar>LD_LIBRARY_PATH</envar> environment variables are automatically
converted from Win32 format to POSIX format (e.g.  from
<filename>c:\cygwin\bin</filename> to <filename>/bin</filename>, if
there was a mount from that Win32 path to that POSIX path) when a Cygwin
process first starts.</para>

<para>Symbolic links can also be used to map Win32 pathnames to POSIX.
For example, the command
<command>ln -s //pollux/home/joe/data /data</command> would have about
the same effect as creating a mount point from
<filename>//pollux/home/joe/data</filename> to <filename>/data</filename>
using <command>mount</command>, except that symbolic links cannot set
the default file access mode.  Other differences are that the mapping is
distributed throughout the file system and proceeds by iteratively
walking the directory tree instead of matching the longest prefix in a
kernel table.  Note that symbolic links will only work on network
drives that are properly configured to support the "system" file
attribute.  Many do not do so by default (the Unix Samba server does
not by default, for example).</para>

</sect2>

</sect1>

<sect1 id="using-specialnames"><title>Special filenames</title>

<sect2> <title>DOS devices</title>

<para>Windows filenames invalid under Windows are also invalid under
Cygwin.  This means that base filenames such as 
<filename>AUX</filename>, <filename>COM1</filename>,
<filename>LPT1</filename> or <filename>PRN</filename> (to name a few)
cannot be used in a regular Cygwin Windows or POSIX path, even with an
extension (<filename>prn.txt</filename>). However the special names can be
used as filename extensions (<filename>file.aux</filename>). You can use
the special names as you would under DOS, for example you can print on your
default printer with the command <command>cat filename > PRN</command>
(make sure to end with a Form Feed).
</para>

</sect2>

<sect2> <title>POSIX devices</title>
<para>There is no need to create a POSIX <filename>/dev</filename> 
directory as Cygwin automatically simulates it internally. 
These devices cannot be seen with the command <command>ls /dev/</command>
although commands such as <command>ls /dev/tty</command> work fine.
If you want to be able to see all devices in
<filename>/dev/</filename>, you can use Igor Pechtchanski's
<ulink
url="http://cygwin.com/ml/cygwin/2004-03/txt00028.txt">create_devices.sh</ulink>
script.
</para>
<para>
Cygwin supports the following devices commonly found on POSIX systems:
<filename>/dev/dsp</filename>, <filename>/dev/null</filename>, 
<filename>/dev/zero</filename>, <filename>/dev/console</filename>,
<filename>/dev/tty</filename>, <filename>/dev/ttym</filename>, 
<filename>/dev/ttyX</filename>, <filename>/dev/ttySX</filename>, 
<filename>/dev/pipe</filename>, <filename>/dev/port</filename>, 
<filename>/dev/ptmx</filename>, <filename>/dev/mem</filename>,
<filename>/dev/random</filename>, and <filename>/dev/urandom</filename>.
Some other POSIX devices, such as 
<filename>/dev/kmem</filename>, are planned for development.
Cygwin also has several Windows-specific devices:
<filename>/dev/comX</filename> (the serial ports, starting with 
<filename>COM1</filename> which is the same as <filename>ttyS0</filename>),
<filename>/dev/conin</filename> (Windows <filename>CONIN$</filename>),
<filename>/dev/conout</filename> (Windows <filename>CONOUT$</filename>),
<filename>/dev/clipboard</filename> (the Windows clipboard, currently
text only), and <filename>/dev/windows</filename> (the Windows message
queue).
</para>

<para>Windows NT/W2K/XP additionally support raw devices like floppies,
disks, partitions and tapes.  These are accessed from Cygwin applications
using POSIX device names which are supported in two different ways.
</para>

<para>Up to Cygwin 1.3.3 the only way to access those devices was
to mount the Win32 device names to a POSIX device name but this usage
is discouraged since Cygwin 1.3.4 and only kept for backward compatibility.
</para>

<para>
Beginning with Cygwin 1.3.4, raw devices are accessible by Cygwin processes
using fixed POSIX device names.  These fixed POSIX device names are generated
using a direct conversion from the POSIX namespace to the internal NT namespace.
E.g. the first harddisk is the NT internal device \device\harddisk0\partition0
or the first partition on the third harddisk is \device\harddisk2\partition1.
The first floppy in the system is \device\floppy0, the first CD-ROM is
\device\cdrom0 and the first tape drive is \device\tape0.
</para>

<para>The new fixed POSIX names are mapped to NT internal devices as
follows:</para>

<screen>
/dev/st0	\device\tape0, rewind
/dev/nst0	\device\tape0, no-rewind
/dev/st1	\device\tape1
...

/dev/fd0	\device\floppy0
/dev/fd1	\device\floppy1
...

/dev/scd0	\device\cdrom0
/dev/scd1	\device\cdrom1
...

/dev/sr0	\device\cdrom0
/dev/sr1	\device\cdrom1
...

/dev/sda	\device\harddisk0\partition0	(whole disk)
/dev/sda1	\device\harddisk0\partition1	(first partition)
...
/dev/sda15	\device\harddisk0\partition15	(fifteenth partition)

/dev/sdb	\device\harddisk1\partition0
/dev/sdb1	\device\harddisk1\partition1

[up to]

/dev/sdl	\device\harddisk11\partition0
/dev/sdl1	\device\harddisk11\partition1
...
/dev/sdl15	\device\harddisk11\partition15
</screen>

<para>
if you don't like these device names, feel free to create symbolic
links as they are created on Linux systems for convenience:
</para>

<screen>
ln -s /dev/scd0 /dev/cdrom
ln -s /dev/nst0  /dev/tape
...
</screen>

<warning>
<para>
Note that you can't use the mount table to map from fixed device name
to your own device name or to map from internal NT device name to
your own device name.  Also using symbolic links to map from the internal
NT device name to your own device name will not do what you want.
The following three examples will not work as expected:
</para>

<screen>
mount -f -b /dev/nst0 /dev/tape     # DOES NOT WORK
mount -f -b /device/tape0 /dev/tape # DOES NOT WORK
ln -s /device/tape0 /dev/tape       # DOES NOT WORK
</screen>
</warning>

</sect2>

<sect2><title>The .exe extension</title>

<para> Executable program filenames end with <filename>.exe</filename>
but the <filename>.exe</filename> need not be included in the command,
so that traditional UNIX names can be used.  However, for programs that
end in <filename>.bat</filename> and <filename>.com</filename>, you
cannot omit the extension.  </para>

<para>As a side effect, the <command> ls filename</command> gives
information about <filename>filename.exe</filename> if
<filename>filename.exe</filename> exists and <filename>filename</filename>
does not.  In the same situation the function call
<function>stat("filename",..)</function> gives information about
<filename>filename.exe</filename>.  The two files can be distinguished
by examining their inodes, as demonstrated below.
<screen>
<prompt>C:\&gt;</prompt> <userinput>ls * </userinput>
a      a.exe     b.exe
<prompt>C:\&gt;</prompt> <userinput>ls -i a a.exe</userinput>
445885548 a       435996602 a.exe
<prompt>C:\&gt;</prompt> <userinput>ls -i b b.exe</userinput>
432961010 b       432961010 b.exe
</screen>
If a shell script <filename>myprog</filename> and a program
<filename>myprog.exe</filename> coexist in a directory, the program
has precedence and is selected for execution of
<command>myprog</command>.</para>

<para>The <command>gcc</command> compiler produces an executable named
<filename>filename.exe</filename> when asked to produce
<filename>filename</filename>. This allows many makefiles written
for UNIX systems to work well under Cygwin.</para>

<para>Unfortunately, the <command>install</command> and
<command>strip</command> commands do distinguish between
<filename>filename</filename> and <filename>filename.exe</filename>. They
fail when working on a non-existing <filename>filename</filename> even if
<filename>filename.exe</filename> exists, thus breaking some makefiles.
This problem can be solved by writing <command>install</command> and
<command>strip</command> shell scripts to provide the extension ".exe"
when needed.
</para>
</sect2>

<sect2><title>The /proc filesystem</title> 
<para>
Cygwin, like Linux and other similar operating systems, supports the
<filename>/proc</filename> virtual filesystem. The files in this
directory are representations of various aspects of your system,
for example the command <userinput>cat /proc/cpuinfo</userinput> 
displays information such as what model and speed processor you have.
</para>
<para>
One unique aspect of the Cygwin <filename>/proc</filename> filesystem
is <filename>/proc/registry</filename>, which displays the Windows
registry with each <literal>KEY</literal> as a directory and each
<literal>VALUE</literal> as a file. As anytime you deal with the
Windows registry, use caution since changes may result in an unstable
or broken system.
</para>
<para>
The Cygwin <filename>/proc</filename> is not as complete as the
one in Linux, but it provides significant capabilities. The
<systemitem>procps</systemitem> package contains several utilities
that use it.
</para>
</sect2>

<sect2><title>The @pathnames</title> 
<para>To circumvent the limitations on shell line length in the native
Windows command shells, Cygwin programs expand their arguments
starting with "@" in a special way.  If a file
<filename>pathname</filename> exists, the argument
<filename>@pathname</filename> expands recursively to the content of
<filename>pathname</filename>. Double quotes can be used inside the
file to delimit strings containing blank space. 
Embedded double quotes must be repeated.
In the following example compare the behaviors of the bash built-in
<command>echo</command> and of the program <command>/bin/echo</command>.</para>

<example><title> Using @pathname</title>
<screen>
<prompt>bash$</prompt> <userinput>echo  'This   is   "a     long"  line' > mylist</userinput>
<prompt>bash$</prompt> <userinput>echo @mylist</userinput>
@mylist
<prompt>c:\&gt;</prompt> <userinput>c:\cygwin\bin\echo @mylist</userinput>
This is a     long line
</screen>
</example>
</sect2> 
</sect1>