Chapter 3. Using Cygwin

Table of Contents

Mapping path names
Introduction
The Cygwin Mount Table
UNC paths
The cygdrive path prefix
Symbolic links
Using native Win32 paths
Using the Win32 file API in Cygwin applications
Additional Path-related Information
Text and Binary modes
The Issue
The default Cygwin behavior
Binary or text?
Programming
File permissions
Special filenames
Special files in /etc
Invalid filenames
Forbidden characters in filenames
Filenames with unusual (foreign) characters
Case sensitive filenames
POSIX devices
The .exe extension
The /proc filesystem
The /proc/registry filesystem
The @pathnames
The CYGWIN environment variable
Implemented options
Obsolete options
Using Windows security in Cygwin
Overview
File permissions
Special values of user and group ids
The POSIX permission mapping leak
Switching the user context
Switching the user context with password authentication
Switching the user context without password, Method 1: Create a token from scratch
Switching the user context without password, Method 2: LSA authentication package
Switching the user context without password, Method 3: With password
Switching the user context, how does it all fit together?
Cygserver
What is Cygserver?
Cygserver command line options
How to start Cygserver
The Cygserver configuration file
Cygwin Utilities
cygcheck
cygpath
dumper
getconf
getfacl
kill
ldd
locale
minidumper
mkgroup
mkpasswd
mount
passwd
pldd
ps
regtool
setfacl
setmetamode
ssp
strace
tzset
umount
Using Cygwin effectively with Windows
Pathnames
Cygwin and Windows Networking
The cygutils package
Creating shortcuts with cygutils
Printing with cygutils

This chapter explains some key differences between the Cygwin environment and traditional UNIX systems. It assumes a working knowledge of standard UNIX commands.

Mapping path names

Introduction

Cygwin supports both POSIX- and Win32-style paths. Directory delimiters may be either forward slashes or backslashes. Paths using backslashes or starting with a drive letter are always handled as Win32 paths. POSIX paths must only use forward slashes as delimiter, otherwise they are treated as Win32 paths and file access might fail in surprising ways.

Note

The usage of Win32 paths, though possible, is deprecated, since it circumvents important internal path handling mechanisms. See the section called “Using native Win32 paths” and the section called “Using the Win32 file API in Cygwin applications” for more information.

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 /disk2 directory).

Because many programs written to run on UNIX systems assume the existence 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.

The Cygwin Mount Table

The /etc/fstab file is used to map Win32 drives and network shares into Cygwin's internal POSIX directory tree. This is a similar concept to the typical UNIX fstab file. The mount points stored in /etc/fstab are globally set for all users. Sometimes there's a requirement to have user specific mount points. The Cygwin DLL supports user specific fstab files. These are stored in the directory /etc/fstab.d and the name of the file is the Cygwin username of the user, as it's stored in the /etc/passwd file. The structure of the user specific file is identical to the system-wide fstab file.

The file fstab contains descriptive information about the various file systems. fstab is only read by programs, and not written; it is the duty of the system administrator to properly create and maintain this file. Each filesystem is described on a separate line; fields on each line are separated by tabs or spaces. Lines starting with '#' are comments.

The first field describes the block special device or remote filesystem to be mounted. On Cygwin, this is the native Windows path which the mount point links in. As path separator you MUST use a slash. Usage of a backslash might lead to unexpected results. UNC paths (using slashes, not backslashes) are allowed. If the path contains spaces these can be escaped as '\040'.

The second field describes the mount point for the filesystem. If the name of the mount point contains spaces these can be escaped as '\040'.

The third field describes the type of the filesystem. Cygwin supports any string here, since the file system type is usually not evaluated. So it doesn't matter if you write FAT into this field even if the filesystem is NTFS. Cygwin figures out the filesystem type and its capabilities by itself.

The only exception is the file system type cygdrive. This type is used to set the cygdrive prefix. For a description of the cygdrive prefix see the section called “The cygdrive path prefix”

The fourth field describes the mount options associated with the filesystem. It is formatted as a comma separated list of options. It contains at least the type of mount (binary or text) plus any additional options appropriate to the filesystem type. Recognized options are binary, text, nouser, user, exec, notexec, cygexec, nosuid, posix=[0|1]. The meaning of the options is as follows.

  acl       - Cygwin uses the filesystem's access control lists (ACLs) to
              implement real POSIX permissions (default).  This flag only
	      affects filesystems supporting ACLs (NTFS, for instance) and
	      is ignored otherwise.
  auto      - Ignored.
  binary    - Files default to binary mode (default).
  bind      - Allows to remount part of the file hierarchy somewhere else.
              In contrast to other entries, the first field in the fstab
	      line specifies an absolute POSIX path.  This path is remounted
	      to the POSIX path specified as the second path.  The conversion
	      to a Win32 path is done on the fly.  Only the root path and
	      paths preceding the bind entry in the fstab file are used to
	      convert the POSIX path in the first field to an absolute Win32
	      path.  Note that symlinks are ignored while performing this path
	      conversion.
  cygexec   - Treat all files below mount point as cygwin executables.
  dos       - Always convert leading spaces and trailing dots and spaces to
	      characters in the UNICODE private use area.  This allows to use
	      broken filesystems which only allow DOS filenames, even if they
	      are not recognized as such by Cygwin.
  exec      - Treat all files below mount point as executable.
  ihash     - Always fake inode numbers rather than using the ones returned
	      by the filesystem.  This allows to use broken filesystems which
	      don't return unambiguous inode numbers, even if they are not
	      recognized as such by Cygwin.
  noacl     - Cygwin ignores filesystem ACLs and only fakes a subset of
	      permission bits based on the DOS readonly attribute.  This
	      behaviour is the default on FAT and FAT32.  The flag is
	      ignored on NFS filesystems.
  nosuid    - No suid files are allowed (currently unimplemented).
  notexec   - Treat all files below mount point as not executable.
  nouser    - Mount is a system-wide mount.
  override  - Force the override of an immutable mount point (currently "/").
  posix=0   - Switch off case sensitivity for paths under this mount point
	      (default for the cygdrive prefix).
  posix=1   - Switch on case sensitivity for paths under this mount point
	      (default for all other mount points).
  sparse    - Switch on support for sparse files.  This option only makes
              sense on NTFS and then only if you really need sparse files.
	      Cygwin does not try to create sparse files by default for
	      performance reasons.
  text      - Files default to CRLF text mode line endings.
  user      - Mount is a user mount.

While normally the execute permission bits are used to evaluate executability, this is not possible on filesystems which don't support permissions at all (like FAT/FAT32), or if ACLs are ignored on filesystems supporting them (see the aforementioned acl mount option). In these cases, the following heuristic is used to evaluate if a file is executable: Files ending in certain extensions (.exe, .com, .bat, .btm, .cmd) are assumed to be executable. Files whose first two characters begin with '#!' are also considered to be executable. The exec option is used to instruct Cygwin that the mounted file is "executable". If the exec option is used with a directory then all files in the directory are executable. This option allows other files to be marked as executable and avoids the overhead of opening each file to check for a '#!'. The cygexec option is very similar to exec, but also prevents Cygwin from setting up commands and environment variables for a normal Windows program, adding another small performance gain. The opposite of these options is the notexec option, which means that no files should be marked as executable under that mount point.

A correct root directory is quite essential to the operation of Cygwin. A default root directory is evaluated at startup so a fstab entry for the root directory is not necessary. If it's wrong, nothing will work as expected. Therefore, the root directory evaluated by Cygwin itself is treated as an immutable mount point and can't be overridden in /etc/fstab... unless you think you really know what you're doing. In this case, use the override flag in the options field in the /etc/fstab file. Since this is a dangerous thing to do, do so at your own risk.

/usr/bin and /usr/lib are by default also automatic mount points generated by the Cygwin DLL similar to the way the root directory is evaluated. /usr/bin points to the directory the Cygwin DLL is installed in, /usr/lib is supposed to point to the /lib directory. This choice is safe and usually shouldn't be changed. An fstab entry for them is not required.

nouser mount points are not overridable by a later call to mount. Mount points given in /etc/fstab are by default nouser mount points, unless you specify the option user. This allows the administrator to set certain paths so that they are not overridable by users. In contrast, all mount points in the user specific fstab file are user mount points.

The fifth and sixth field are ignored. They are so far only specified to keep a Linux-like fstab file layout.

Note that you don't have to specify an fstab entry for the root dir, unless you want to have the root dir pointing to somewhere entirely different (hopefully you know what you're doing), or if you want to mount the root dir with special options (for instance, as text mount).

Example entries:

  • Just a normal mount point:

      c:/foo /bar fat32 binary 0 0
  • A mount point for a textmode mount with case sensitivity switched off:

      C:/foo /bar/baz ntfs text,posix=0 0 0
  • A mount point for a Windows directory with spaces in it:

      C:/Documents\040and\040Settings /docs ext3 binary 0 0
  • A mount point for a remote directory, don't store POSIX permissions in ACLs:

      //server/share/subdir /srv/subdir smbfs binary,noacl 0 0
  • This is just a comment:

      # This is just a comment
  • Set the cygdrive prefix to /mnt:

      none /mnt cygdrive binary 0 0
  • Remount /var to /usr/var:

      /var /usr/var none bind

    Assuming /var points to C:/cygwin/var, /usr/var now also points to C:/cygwin/var. This is equivalent to the Linux bind option available since Linux 2.4.0.

Whenever Cygwin generates a Win32 path from a POSIX one, it uses the longest matching prefix in the mount table. Thus, if C: is mounted as /c and also as /, then Cygwin would translate C:/foo/bar to /c/foo/bar. 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.

If you want to see the current set of mount points valid in your session, you can invoke the Cygwin tool mount without arguments:

Example 3.1. Displaying the current set of mount points

  bash$ mount
  f:/cygwin/bin on /usr/bin type ntfs (binary,auto)
  f:/cygwin/lib on /usr/lib type ntfs (binary,auto)
  f:/cygwin on / type ntfs (binary,auto)
  e:/src on /usr/src type vfat (binary)
  c: on /cygdrive/c type ntfs (binary,posix=0,user,noumount,auto)
  e: on /cygdrive/e type vfat (binary,posix=0,user,noumount,auto)

You can also use the mount command to add new mount points, and the umount to delete them. However, since they are only stored in memory, these mount points will disappear as soon as your last Cygwin process ends. See the section called “mount” and the section called “umount” for more information.

Note

When you upgrade an existing older Cygwin installation to Cygwin 1.7, your old system mount points (stored in the HKEY_LOCAL_MACHINE branch of your registry) are read by a script and the /etc/fstab file is generated from these entries. Note that entries for /, /usr/bin, and /usr/lib are never generated.

The old user mount points in your HKEY_CURRENT_USER branch of the registry are not used to generate /etc/fstab. If you want to create a user specific /etc/fstab.d/${USER} file from your old entries, there's a script available which does exactly that for you, /bin/copy-user-registry-fstab. Just start the script and it will create your user specific fstab file. Stop all your Cygwin processes and restart them, and you can simply use your old user mount points as before.

UNC paths

Apart from the unified POSIX tree starting at the / directory, UNC pathnames starting with two slashes and a server name (//machine/share/...) are supported as well. They are handled as POSIX paths if only containing forward slashes. There's also a virtual directory // which allows to enumerate the fileservers known to the local machine with ls. Same goes for the UNC paths of the type //machine, which allow to enumerate the shares provided by the server machine. For often used UNC paths it makes sense to add them to the mount table (see the section called “The Cygwin Mount Table” so they are included in the unified POSIX path tree.

The cygdrive path prefix

As already outlined in the section called “File Access”, you can access arbitary drives on your system by using the cygdrive path prefix. The default value for this prefix is /cygdrive, and a path to any drive can be constructed by using the cygdrive prefix and appending the drive letter as subdirectory, like this:

  bash$ ls -l /cygdrive/f/somedir

This lists the content of the directory F:\somedir.

The cygdrive prefix is a virtual directory under which all drives on a system are subsumed. The mount options of the cygdrive prefix is used for all file access through the cygdrive prefixed drives. For instance, assuming the cygdrive mount options are binary,posix=0, then any file /cygdrive/x/file will be opened in binary mode by default (mount option binary), and the case of the filename doesn't matter (mount option posix=0).

The cygdrive prefix flags are also used for all UNC paths starting with two slashes, unless they are accessed through a mount point. For instance, consider these /etc/fstab entries:

  //server/share /mysrv    ntfs     posix=1,acl   0 0
  none           /cygdrive cygdrive posix=0,noacl 0 0

Assume there's a file \\server\share\foo on the share. When accessing it as /mysrv/foo, then the flags posix=1,acl of the /mysrv mount point are used. When accessing it as //server/share/foo, then the flags for the cygdrive prefix, posix=0,noacl are used.

Note

This only applies to UNC paths using forward slashes. When using backslashes the flags for native paths are used. See the section called “Using native Win32 paths”.

The cygdrive prefix may be changed in the fstab file as outlined above. Please note that you must not use the cygdrive prefix for any other mount point. For instance this:

  none /cygdrive cygdrive binary 0 0
  D:   /cygdrive/d somefs text 0 0

will not make file access using the /mnt/d path prefix suddenly using textmode. If you want to mount any drive explicitly in another mode than the cygdrive prefix, use a distinct path prefix:

  none /cygdrive cygdrive binary 0 0
  D:   /mnt/d somefs text 0 0

Symbolic links

Symbolic links are not present and supported on Windows until Windows Vista/Server 2008, and then only on some filesystems. Since POSIX applications are rightfully expecting to use symlinks and the symlink(2) system call, Cygwin had to find a workaround for this Windows flaw.

Cygwin creates symbolic links potentially in multiple different ways:

  • The default symlinks are plain files containing a magic cookie followed by the path to which the link points. They are marked with the DOS SYSTEM attribute so that only files with that attribute have to be read to determine whether or not the file is a symbolic link.

    Note

    Starting with Cygwin 1.7, symbolic links are using UTF-16 to encode the filename of the target file, to better support internationalization. Symlinks created by older Cygwin releases can be read just fine. However, you could run into problems with them if you're now using another character set than the one you used when creating these symlinks (see the section called “Potential Problems when using Locales”). Please note that this new UTF-16 style of symlinks is not compatible with older Cygwin release, which can't read the target filename correctly.

  • The shortcut style symlinks are Windows .lnk shortcut files with a special header and the DOS READONLY attribute set. This symlink type is created if the environment variable CYGWIN (see the section called “The CYGWIN environment variable”) is set to contain the string winsymlinks or winsymlinks:lnk. On the MVFS filesystem, which does not support the DOS SYSTEM attribute, this is the one and only supported symlink type, independently from the winsymlinks setting.

  • Native Windows symlinks are only created on Windows Vista/2008 and later, and only on filesystems supporting reparse points. Due to to their weird restrictions and behaviour, they are only created if the user explicitely requests creating them. This is done by setting the environment variable CYGWIN to contain the string winsymlinks:native or winsymlinks:nativestrict. For the difference between these two settings, see the section called “The CYGWIN environment variable”. On AFS, native symlinks are the only supported type of symlink due to AFS lacking support for DOS attributes. This is independent from the winsymlinks setting.

  • On the NFS filesystem, Cygwin always creates real NFS symlinks.

All of the above four symlink types are recognized and used as symlinks under all circumstances. However, if the default plain file symlink type is lacking its DOS SYSTEM bit, or if the shortcut file is lacking the DOS READONLY attribute, they are not recognized as symlink.

Apart from these four types, there's also a fifth type, which is recognized as symlink but never generated by Cygwin, directory junctions. This is an older reparse point type, supported by Windows since Windows 2000. Filesystem junctions on the other hand are not handled as symlinks, since otherwise they would not be recognized as filesystem borders by commands like find -xdev.

Using native Win32 paths

Using native Win32 paths in Cygwin, while possible, is generally inadvisable. Those paths circumvent all internal integrity checking and bypass the information given in the Cygwin mount table.

The following paths are treated as native Win32 paths in Cygwin:

  • All paths starting with a drive specifier

      C:\foo
      C:/foo
    
  • All paths containing at least one backslash as path component

      C:/foo/bar\baz/...
    
  • UNC paths using backslashes

      \\server\share\...
    

When accessing files using native Win32 paths as above, Cygwin uses a default setting for the mount flags. All paths using DOS notation will be treated as case insensitive, and permissions are just faked as if the underlying drive is a FAT drive. This also applies to NTFS and other filesystems which usually are capable of case sensitivity and storing permissions.

Using the Win32 file API in Cygwin applications

Special care must be taken if your application uses Win32 file API functions like CreateFile to access files using relative pathnames, or if your application uses functions like CreateProcess or ShellExecute to start other applications.

When a Cygwin application is started, the Windows idea of the current working directory (CWD) is not necessarily the same as the Cygwin CWD. There are a couple of restrictions in the Win32 API, which disallow certain directories as Win32 CWD:

  • The Windows subsystem only supports CWD paths of up to 258 chars. This restriction doesn't apply for Cygwin processes, at least not as long as they use the POSIX API (chdir, getcwd). This means, if a Cygwin process has a CWD using an absolute path longer than 258 characters, the Cygwin CWD and the Windows CWD differ.

  • The Win32 API call to set the current directory, SetCurrentDirectory, fails for directories for which the user has no permissions, even if the user is an administrator. This restriction doesn't apply for Cygwin processes, if they are running under an administrator account.

  • SetCurrentDirectory does not support case-sensitive filenames.

  • Last, but not least, SetCurrentDirectory can't work on virtual Cygwin paths like /proc or /cygdrive. These paths only exists in the Cygwin realm so they have no meaning to a native Win32 process.

As long as the Cygwin CWD is usable as Windows CWD, the Cygwin and Windows CWDs are in sync within a process. However, if the Cygwin process changes its working directory into one of the directories which are unusable as Windows CWD, we're in trouble. If the process uses the Win32 API to access a file using a relative pathname, the resulting absolute path would not match the expectations of the process. In the worst case, the wrong files are deleted.

To workaround this problem, Cygwin sets the Windows CWD to a special directory in this case. This special directory points to a virtual filesystem within the native NT namespace (\??\PIPE\). Since it's not a real filesystem, the deliberate effect is that a call to, for instance, CreateFile ("foo", ...); will fail, as long as the processes CWD doesn't work as Windows CWD.

So, in general, don't use the Win32 file API in Cygwin applications. If you really need to access files using the Win32 API, or if you really have to use CreateProcess to start applications, rather than the POSIX exec(3) family of functions, you have to make sure that the Cygwin CWD is set to some directory which is valid as Win32 CWD.

Additional Path-related Information

The cygpath program provides the ability to translate between Win32 and POSIX pathnames in shell scripts. See the section called “cygpath” for the details.

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

Symbolic links can also be used to map Win32 pathnames to POSIX. For example, the command ln -s //pollux/home/joe/data /data would have about the same effect as creating a mount point from //pollux/home/joe/data to /data using mount, 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).