1. About Cygwin

1.1. What is it?

Cygwin is a distribution of popular GNU and other Open Source tools running on Microsoft Windows. The core part is the Cygwin library which provides the POSIX system calls and environment these programs expect. The Cygwin distribution contains thousands of packages from the Open Source world including most GNU tools, many BSD tools, an X server and a full set of X applications. If you're a developer you will find tools, headers and libraries allowing to write Windows console or GUI applications that make use of significant parts of the POSIX API. Cygwin allows easy porting of many Unix programs without the need for extensive changes to the source code. This includes configuring and building most of the available GNU or BSD software, including the packages included with the Cygwin distribution themselves. They can be used from one of the provided Unix shells like bash, tcsh or zsh.

1.2. What versions of Windows are supported?

Cygwin can be expected to run on all modern, released versions of Windows. State January 2016 this includes Windows Vista, Windows Server 2008 and all later versions of Windows up to Windows 10 and Windows Server 2016. The 32 bit version of Cygwin also runs in the WOW64 32 bit environment on released 64 bit versions of Windows, the 64 bit version of course only on 64 bit Windows. Keep in mind that Cygwin can only do as much as the underlying OS supports. Because of this, Cygwin will behave differently, and exhibit different limitations, on the various versions of Windows.

1.3. Where can I get it?

The home page for the Cygwin project is https://cygwin.com/. There you should find everything you need for Cygwin, including links for download and setup, a current list of mirror sites, a User's Guide, an API Reference, mailing lists and archives. You can find documentation for the individual GNU tools typically as man pages or info pages as part of the Cygwin net distribution. Additionally you can get the latest docs at http://www.gnu.org/manual.

1.4. Is it free software?

Yes. Parts are GNU software (gcc, gas, ld, etc...), parts are covered by the standard X11 license, some of it is public domain, some of it was written by Red Hat (or the former Cygnus Solutions) and placed under the GPL. None of it is shareware. You don't have to pay anyone to use it but you should be sure to read the copyright section of the FAQ for more information on how the GNU General Public License may affect your use of these tools. Note that when we say "free" we mean freedom, not price. The goal of such freedom is that the people who use a given piece of software should be able to change it to fit their needs, learn from it, share it with their friends, etc. The GPL or LGPL licenses allows you those freedoms, so it is free software.

1.5. What version of Cygwin is this, anyway?

To find the version of the Cygwin DLL installed, you can use uname as on Linux or cygcheck . Refer to each command's --help output and the Cygwin User's Guide for more information. If you are looking for the version number for the whole Cygwin release, there is none. Each package in the Cygwin release has its own version. The packages in Cygwin are continually improving, thanks to the efforts of net volunteers who maintain the Cygwin binary ports. Each package has its own version numbers and its own release process. So, how do you get the most up-to-date version of Cygwin? Easy. Just download the Cygwin Setup program by following the instructions here. The setup program will handle the task of updating the packages on your system to the latest version. For more information about using Cygwin's setup.exe , see Setting Up Cygwin in the Cygwin User's Guide.

1.6. Who's behind the project?

(Please note that if you have cygwin-specific questions, all of these people will appreciate it if you use the cygwin mailing lists rather than sending personal email.) Corinna Vinschen is the current project lead. Corinna is a senior Red Hat engineer. Corinna is responsible for the Cygwin library and maintains a couple of packages, for instance OpenSSH, OpenSSL, and a lot more. Yaakov Selkowitz is another Red Hat engineer working on the Cygwin project. He's the guy behind the current build and packaging system and maintains by far the most packages in the Cygwin distribution. Jon Turney is developer and maintainer of the Cygwin X server and a couple of related packages. The packages in the Net release are maintained by a large group of people; a complete list can be found here. Please note that all of us working on Cygwin try to be as responsive as possible and deal with patches and questions as we get them, but realistically we don't have time to answer all of the email that is sent to the main mailing list. Making Net releases of the Win32 tools and helping people on the Net out is not our primary job function, so some email will have to go unanswered. Many thanks to everyone using the tools for their many contributions in the form of advice, bug reports, and code fixes. Keep them coming!

2. Setting up Cygwin

2.1. What is the recommended installation procedure?

There is only one recommended way to install Cygwin, which is to use the GUI installer setup-*.exe. It is flexible and easy to use. You can pick and choose the packages you wish to install, and update them individually. Full source code is available for all packages and tools. More information on using Cygwin Setup may be found at https://cygwin.com/cygwin-ug-net/setup-net.html. If you do it any other way, you're on your own! If something doesn't work right for you, and it's not covered here or in the latest development snapshot at https://cygwin.com/snapshots/, then by all means report it to the mailing list. For a searchable list of packages that can be installed with Cygwin, see https://cygwin.com/packages/.

2.2. What about an automated Cygwin installation?

The Cygwin Setup program is designed to be interactive, but there are a few different ways to automate it. If you are deploying to multiple systems, the best way is to run through a full installation once, saving the entire downloaded package tree. Then, on target systems, run Cygwin Setup as a "Local Install" pointed at your downloaded package tree. You could do this non-interactively with the command line options -q -L -l x:\cygwin-local\ , where your downloaded package tree is in x:\cygwin-local\ (see the next FAQ for an explanation of those options.) For other options, search the mailing lists with terms such as cygwin automated setup or automated cygwin install.

2.3. Does Setup accept command-line arguments?

Yes, run setup-x86.exe --help or setup-x86_64.exe --help for a list. --allow-unsupported-windows Allow old, unsupported Windows versions -a --arch architecture to install (x86_64 or x86) -C --categories Specify entire categories to install -o --delete-orphans remove orphaned packages -A --disable-buggy-antivirus Disable known or suspected buggy anti virus software packages during execution. -D --download Download from internet -f --force-current select the current version for all packages -h --help print help -I --include-source Automatically include source download -i --ini-basename Use a different basename, e.g. "foo", instead of "setup" -U --keep-untrusted-keys Use untrusted keys and retain all -L --local-install Install from local directory -l --local-package-dir Local package directory -m --mirror-mode Skip availability check when installing from local directory (requires local directory to be clean mirror!) -B --no-admin Do not check for and enforce running as Administrator -d --no-desktop Disable creation of desktop shortcut -r --no-replaceonreboot Disable replacing in-use files on next reboot. -n --no-shortcuts Disable creation of desktop and start menu shortcuts -N --no-startmenu Disable creation of start menu shortcut -X --no-verify Don't verify setup.ini signatures -O --only-site Ignore all sites except for -s -M --package-manager Semi-attended chooser-only mode -P --packages Specify packages to install -p --proxy HTTP/FTP proxy (host:port) -Y --prune-install prune the installation to only the requested packages -K --pubkey URL of extra public key file (gpg format) -q --quiet-mode Unattended setup mode -c --remove-categories Specify categories to uninstall -x --remove-packages Specify packages to uninstall -R --root Root installation directory -S --sexpr-pubkey Extra public key in s-expr format -s --site Download site -u --untrusted-keys Use untrusted keys from last-extrakeys -g --upgrade-also also upgrade installed packages --user-agent User agent string for HTTP requests -v --verbose Verbose output -W --wait When elevating, wait for elevated child process

2.4. Can I install Cygwin without administrator rights?

Yes. The default installation requests administrator rights because this allows to set up the Cygwin environment so that all users can start a Cygwin shell out of the box. However, if you don't have administrator rights for your machine, and the admins don't want to install it for you, you can install Cygwin just for yourself by downloading setup-x86.exe (for a 32 bit install) or setup-x86_64.exe (for a 64 bit install) and then start it from the command line or via the "Run..." dialog from the start menu using the --no-admin option, for instance: setup-x86.exe --no-admin

2.5. Why not install in C:\?

The Cygwin Setup program will prompt you for a "root" directory. The default is C:\cygwin , but you can change it. You are urged not to choose something like C:\ (the root directory on the system drive) for your Cygwin root. If you do, then critical Cygwin system directories like etc , lib and bin could easily be corrupted by other (non-Cygwin) applications or packages that use \etc , \lib or \bin . Perhaps there is no conflict now, but who knows what you might install in the future? It's also just good common sense to segregate your Cygwin "filesystems" from the rest of your Windows system disk. (In the past, there had been genuine bugs that would cause problems for people who installed in C:\ , but we believe those are gone now.)

2.6. Can I use Cygwin Setup to get old versions of packages (like gcc-2.95)?

Cygwin Setup can be used to install any packages that are on a Cygwin mirror, which usually includes one version previous to the current one. The complete list may be searched at https://cygwin.com/packages/. There is no complete archive of older packages. If you have a problem with the current version of a Cygwin package, please report it to the mailing list using the guidelines at https://cygwin.com/problems.html. That said, if you really need an older package, you may be able to find an outdated or archival mirror by searching the web for an old package version (for example, gcc2-2.95.3-10-src.tar.bz2 ), but keep in mind that this older version will not be supported by the mailing list and that installing the older version will not help improve Cygwin.

2.7. How does Cygwin secure the installation and update process?

Here is how Cygwin secures the installation and update process to counter man-in-the-middle (MITM) attacks: The Cygwin website provides the setup program ( setup-x86.exe or setup-x86_64.exe ) using HTTPS (SSL/TLS). This authenticates that the setup program came from the Cygwin website (users simply use their web browsers to download the setup program). You can use tools like Qualsys' SSL Server Test, https://www.ssllabs.com/ssltest/, to check the HTTPS configuration of Cygwin. The cygwin.com site supports HTTP Strict Transport Security (HSTS), which forces the browser to keep using HTTPS once the browser has seen it before (this counters many downgrade attacks). The setup program has the Cygwin public key embedded in it. The Cygwin public key is protected from attacker subversion during transmission by the previous step, and this public key is then used to protect all later steps. You can confirm that the key is in setup by looking at the setup project (http://sourceware.org/cygwin-apps/setup.html) source code file cyg-pubkey.h (the key is automatically generated from file cygwin.pub ). The setup program downloads the package list setup.ini from a mirror and checks its digital signature. The package list is in the file setup.bz2 (compressed) or setup.ini (uncompressed) on the selected mirror. The package list includes for every official Cygwin package the package name, cryptographic hash, and length (in bytes). The setup program also gets the relevant .sig (signature) file for that package list, and checks that the package list is properly signed with the Cygwin public key embedded in the setup program. A mirror could corrupt the package list and/or signature, but this would be detected by setup program's signature detection (unless you use the -X option to disable signature checking). The setup program also checks the package list timestamp/version and reports to the user if the file goes backwards in time; that process detects downgrade attacks (e.g., where an attacker subverts a mirror to send a signed package list that is older than the currently-downloaded version). The packages to be installed (which may be updates) are downloaded and both their lengths and cryptographic hashes (from the signed setup.{bz2,ini} file) are checked. Non-matching packages are rejected, countering any attacker's attempt to subvert the files on a mirror. Cygwin currently uses the cryptographic hash function SHA-512 for the setup.ini files. Cygwin uses the cryptographic hash algorithm SHA-512 as of 2015-03-23. The earlier 2015-02-06 update of the setup program added support for SHA-512 (Cygwin previously used MD5). There are no known practical exploits of SHA-512 (SHA-512 is part of the widely-used SHA-2 suite of cryptographic hashes).

2.8. What else can I do to ensure that my installation and updates are secure?

To best secure your installation and update process, download the setup program setup-x86.exe (32-bit) or setup-x86_64.exe (64-bit), and then check its signature (using a signature-checking tool you trust) using the Cygwin public key (https://cygwin.com/key/pubring.asc). This was noted on the front page for installing and updating. If you use the actual Cygwin public key, and have an existing secure signature-checking process, you will counter many other attacks such as subversion of the Cygwin website and malicious certificates issued by untrustworthy certificate authorities (CAs). One challenge, of course, is ensuring that you have the actual Cygwin public key. You can increase confidence in the Cygwin public key by checking older copies of the Cygwin public key (to see if it's been the same over time). Another challenge is having a secure signature-checking process. You can use GnuPG to check signatures; if you have a trusted Cygwin installation you can install GnuPG. Otherwise, to check the signature you must use an existing trusted tool or install a signature-checking tool you can trust. Not everyone will go through this additional effort, but we make it possible for those who want that extra confidence. We also provide automatic mechanisms (such as our use of HTTPS) for those with limited time and do not want to perform the signature checking on the setup program itself. Once the correct setup program is running, it will counter other attacks as described in https://cygwin.com/faq/faq.html#faq.setup.install-security.

2.9. Is Cygwin Setup, or one of the packages, infected with a virus?

Unlikely. Unless you can confirm it, please don't report it to the mailing list. Anti-virus products have been known to detect false positives when extracting compressed tar archives. If this causes problems for you, consider disabling your anti-virus software when running setup . Read the next entry for a fairly safe way to do this.

2.10. My computer hangs when I run Cygwin Setup!

Both Network Associates (formerly McAfee) and Norton anti-virus products have been reported to "hang" when extracting Cygwin tar archives. If this happens to you, consider disabling your anti-virus software when running Cygwin Setup. The following procedure should be a fairly safe way to do that: Download setup-x86.exe or setup-x86_64.exe and scan it explicitly. Turn off the anti-virus software. Run setup to download and extract all the tar files. Re-activate your anti-virus software and scan everything in C:\cygwin (or wherever you chose to install), or your entire hard disk if you are paranoid. This should be safe, but only if Cygwin Setup is not substituted by something malicious. See also https://cygwin.com/faq/faq.html#faq.setup.install-security for a description of how the Cygwin project counters man-in-the-middle (MITM) attacks. See also https://cygwin.com/faq/faq.html#faq.using.bloda for a list of applications that have been known, at one time or another, to interfere with the normal functioning of Cygwin.

2.11. What packages should I download? Where are 'make', 'gcc', 'vi', etc?

When using Cygwin Setup for the first time, the default is to install a minimal subset of all available packages. If you want anything beyond that, you will have to select it explicitly. See https://cygwin.com/packages/ for a searchable list of available packages, or use cygcheck -p as described in the Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/cygcheck.html. If you want to build programs, of course you'll need gcc , binutils , make and probably other packages from the ``Devel'' category. Text editors can be found under ``Editors''.

2.12. How do I just get everything?

Long ago, the default was to install everything, much to the irritation of most users. Now the default is to install only a basic core of packages. Cygwin Setup is designed to make it easy to browse categories and select what you want to install or omit from those categories. It's also easy to install everything: At the ``Select Packages'' screen, in ``Categories'' view, at the line marked ``All'', click on the word ``default'' so that it changes to ``install''. (Be patient, there is some computing to do at this step. It may take a second or two to register the change.) This tells Setup to install everything, not just what it thinks you should have by default. Now click on the ``View'' button (twice) until you get to the ``Pending'' view. This shows exactly which packages are about to be downloaded and installed. This procedure only works for packages that are currently available. There is no way to tell Cygwin Setup to install all packages by default from now on. As new packages become available that would not be installed by default, you have to repeat the above procedure to get them. In general, a better method (in my opinion), is to: First download & install all packages that would normally be installed by default. This includes fundamental packages and any updates to what you have already installed. Then... Run Cygwin Setup again, and apply the above technique to get all new packages that would not be installed by default. You can check the list in the ``Pending'' view before proceeding, in case there's something you really don't want. In the latest version of Cygwin Setup, if you click the ``View'' button (twice) more, it shows packages not currently installed. You ought to check whether you really want to install everything!

2.13. How much disk space does Cygwin require?

That depends, obviously, on what you've chosen to download and install. A full installation today is probably larger than 1 GB installed, not including the package archives themselves nor the source code. After installation, the package archives remain in your ``Local Package Directory''. By default the location of setup-x86{_64}.exe . You may conserve disk space by deleting the subdirectories there. These directories will have very weird looking names, being encoded with their URLs (named ftp%3a%2f... ). Of course, you can keep them around in case you want to reinstall a package. If you want to clean out only the outdated packages, Michael Chase has written a script called clean_setup.pl , available at unsupported/clean_setup.pl in a Cygwin mirror.

2.14. How do I know which version I upgraded from?

Detailed logs of the most recent Cygwin Setup session can be found in /var/log/setup.log.full and less verbose information about prior actions is in /var/log/setup.log .

2.15. What if setup fails?

First, make sure that you are using the latest version of Cygwin Setup. The latest version is always available from the Cygwin Home Page at https://cygwin.com/. If you are downloading from the Internet, setup will fail if it cannot download the list of mirrors at https://cygwin.com/mirrors.html. It could be that the network is too busy. Something similar could be the cause of a download site not working. Try another mirror, or try again later. If setup refuses to download a package that you know needs to be upgraded, try deleting that package's entry from /etc/setup. If you are reacting quickly to an announcement on the mailing list, it could be that the mirror you are using doesn't have the latest copy yet. Try another mirror, or try again tomorrow. If setup has otherwise behaved strangely, check the files setup.log and setup.log.full in /var/log ( C:\cygwin\var\log by default). It may provide some clues as to what went wrong and why. If you're still baffled, search the Cygwin mailing list for clues. Others may have the same problem, and a solution may be posted there. If that search proves fruitless, send a query to the Cygwin mailing list. You must provide complete details in your query: version of setup, options you selected, contents of setup.log and setup.log.full, what happened that wasn't supposed to happen, etc.

2.16. My Windows logon name has a space in it, will this cause problems?

Most definitely yes! UNIX shells (and thus Cygwin) use the space character as a word delimiter. Under certain circumstances, it is possible to get around this with various shell quoting mechanisms, but you are much better off if you can avoid the problem entirely. You have two choices: You can rename the user in the Windows User Manager GUI. If that's not possible, you can create an /etc/passwd file using the mkpasswd command. Then you can simply edit your Cygwin user name (first field). It's also a good idea to avoid spaces in the home directory.

2.17. My HOME environment variable is not what I want.

When starting Cygwin from Windows, HOME is determined as follows: If HOME is set in the Windows environment, translated to POSIX form. Otherwise, use the pw_home field from the passwd entry as returned by getent passwd. If you want to learn how this field is set by Cygwin and how you can change it, this is explained in great detail in the Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/ntsec.html. When using Cygwin from a network login (via ssh for instance), HOME is always taken from the passwd entry. If your HOME is set to a value such as /cygdrive/c, it is likely that it was set in Windows. Start a DOS Command Window and type "set HOME" to verify if this is the case. Access to shared drives is often restricted when starting from the network, thus Domain users may wish to have a different HOME in the Windows environment (on shared drive) than in Cygwin (on local drive). Note that ssh only considers the account information as retrieved by getpwnam(3), disregarding HOME .

2.18. How do I uninstall individual packages?

Run Cygwin Setup as you would to install packages. In the list of packages to install, browse the relevant category or click on the ``View'' button to get a full listing. Click on the cycle glyph until the action reads ``Uninstall''. Proceed by clicking ``Next''.

2.19. How do I uninstall a Cygwin service?

List all services you have installed with cygrunsrv -L . If you do not have cygrunsrv installed, skip this FAQ. Before removing the service, you should stop it with cygrunsrv --stop service_name . If you have inetd configured to run as a standalone service, it will not show up in the list, but cygrunsrv --stop inetd will work to stop it as well. Lastly, remove the service with cygrunsrv --remove service_name .

2.20. How do I uninstall all of Cygwin?

Setup has no automatic uninstall facility. The recommended method to remove all of Cygwin is as follows: If you have any Cygwin services running, remove by repeating the instructions in https://cygwin.com/faq/faq.html#faq.setup.uninstall-service for all services that you installed. Common services that might have been installed are sshd , cron , cygserver , inetd , apache , postgresql , and so on. Stop the X11 server if it is running, and terminate any Cygwin programs that might be running in the background. Exit the command prompt and ensure that no Cygwin processes remain. Note: If you want to save your mount points for a later reinstall, first save the output of mount -m as described at https://cygwin.com/cygwin-ug-net/mount.html. If you installed cyglsa.dll by running the shell script /usr/bin/cyglsa-config as described in https://cygwin.com/cygwin-ug-net/ntsec.html, then you need to configure Windows to stop using the LSA authentication package. You do so by editing the registry and restoring /HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/Lsa/Authentication Packages back to it's original value of msv1_0 , and then rebooting. Delete the Cygwin root folder and all subfolders. If you get an error that an object is in use, then ensure that you've stopped all services and closed all Cygwin programs. If you get a 'Permission Denied' error then you will need to modify the permissions and/or ownership of the files or folders that are causing the error. For example, sometimes files used by system services end up owned by the SYSTEM account and not writable by regular users. The quickest way to delete the entire tree if you run into this problem is to take ownership of all files and folders to your account. To do this in Windows Explorer, right click on the root Cygwin folder, choose Properties, then the Security tab. If you are using Simple File Sharing, you will need to boot into Safe Mode to access the Security tab. Select Advanced, then go to the Owner tab and make sure your account is listed as the owner. Select the 'Replace owner on subcontainers and objects' checkbox and press Ok. After Explorer applies the changes you should be able to delete the entire tree in one operation. Note that you can also achieve by using other tools such as icacls.exe or directly from Cygwin by using chown . Please note that you shouldn't use the recursive form of chown on directories that have other file systems mounted under them (specifically you must avoid /proc ) since you'd change ownership of the files under those mount points as well. Delete the Cygwin shortcuts on the Desktop and Start Menu, and anything left by setup-x86{_64}.exe in the download directory. However, if you plan to reinstall Cygwin it's a good idea to keep your setup-x86{_64}.exe download directory since you can reinstall the packages left in its cache without redownloading them. If you added Cygwin to your system path, you should remove it unless you plan to reinstall Cygwin to the same location. Similarly, if you set your CYGWIN environment variable system-wide and don't plan to reinstall, you should remove it. Finally, if you want to be thorough you can delete the registry tree Software\Cygwin under HKEY_LOCAL_MACHINE and/or HKEY_CURRENT_USER . However, if you followed the directions above you will have already removed everything important. Typically only the installation directory has been stored in the registry at all.

2.21. How do I install snapshots?

First, are you sure you want to do this? Snapshots are risky. They have not been tested. Use them only if there is a feature or bugfix that you need to try, and you are willing to deal with any problems, or at the request of a Cygwin developer. You cannot use Cygwin Setup to install a snapshot. First, you will need to download the snapshot from the snapshots page at https://cygwin.com/snapshots/. Note the directory where you saved the snapshot tarball. Before installing a snapshot, you must first Close all Cygwin applications, including shells and services (e.g., inetd , sshd ). You will not be able to replace cygwin1.dll if any Cygwin process is running. You may have to restart Windows to clear the DLL from memory (beware of automatic service startup). Most of the downloaded snapshot can be installed using tar . Cygwin tar won't be able to update /usr/bin/cygwin1.dll (because it's used by tar itself), but it should succeed with everything else. If you are only installing the DLL snapshot, skip the first tar command. Open a bash shell (it should be the only running Cygwin process) and issue the following commands: /bin/tar -C / -xvf /posix/path/to/cygwin-inst-YYYYMMDD.tar.* --exclude=usr/bin/cygwin1.dll /bin/tar -C /tmp -xvf /posix/path/to/cygwin-inst-YYYYMMDD.tar.* usr/bin/cygwin1.dll Exit the bash shell, and use Explorer or the Windows command shell to first rename C:\cygwin\bin\cygwin1.dll to C:\cygwin\bin\cygwin1-prev.dll and then move C:\cygwin\tmp\usr\bin\cygwin1.dll to C:\cygwin\bin\cygwin1.dll (assuming you installed Cygwin in C:\cygwin ). The operative word in trying the snapshots is "trying". If you notice a problem with the snapshot that was not present in the release DLL (what we call a "regression"), please report it to the Cygwin mailing list (see https://cygwin.com/problems.html for problem reporting guidelines). If you wish to go back to the older version of the DLL, again, close all Cygwin processes, delete C:\cygwin\bin\cygwin1.dll , and rename C:\cygwin\bin\cygwin1-prev.dll back to C:\cygwin\bin\cygwin1.dll (again assuming that your " / " is C:\cygwin ). To restore the rest of the snapshot files, reinstall the " cygwin " package using Setup.

2.22. Can Cygwin Setup maintain a ``mirror''?

NO. Cygwin Setup cannot do this for you. Use a tool designed for this purpose. See http://rsync.samba.org/, http://www.gnu.org/software/wget/ for utilities that can do this for you. For more information on setting up a custom Cygwin package server, see the Cygwin Setup homepage at https://sourceware.org/cygwin-apps/setup.html.

2.23. How can I make my own portable Cygwin on CD?

While some users have successfully done this, for example Indiana University's XLiveCD http://racinfo.indiana.edu/research/xlivecd.php, there is no easy way to do it. Full instructions for constructing a portable Cygwin on CD by hand can be found on the mailing list at https://www.cygwin.com/ml/cygwin/2003-07/msg01117.html (Thanks to fergus at bonhard dot uklinux dot net for these instructions.) Please note that these instructions are very old and are referring to the somewhat different setup of a Cygwin 1.5.x release. As soon as somebody set this up for recent Cygwin releases, we might add this information here.

2.24. How do I save, restore, delete, or modify the Cygwin information stored in the registry?

Cygwin doesn't store anything important in the registry anymore for quite some time. There's no reason to save, restore or delete it.

3. Further Resources

3.1. Where's the documentation?

If you have installed Cygwin, you can find lots of documentation in /usr/share/doc/ . Some packages have Cygwin specific instructions in a file /usr/share/doc/Cygwin/ package_name .README . In addition, many packages ship with standard documentation, which you can find in /usr/share/doc/ package_name or by using the man or info tools. (Hint: use cygcheck -l package_name to list what man pages the package includes.) Some older packages still keep their documentation in /usr/doc/ instead of /usr/share/doc/ . There are links to quite a lot of documentation on the main Cygwin project web page, https://cygwin.com/, including this FAQ. Be sure to at least read any 'Release Notes' or 'Readme' or 'read this' links on the main web page, if there are any. There is a comprehensive Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/cygwin-ug-net.html and an API Reference at https://cygwin.com/cygwin-api/cygwin-api.html. You can find documentation for the individual GNU tools at http://www.gnu.org/manual/.

3.2. What Cygwin mailing lists can I join?

Comprehensive information about the Cygwin mailing lists can be found at https://cygwin.com/lists.html.

3.3. What if I have a problem? (Or: Why won't you/the mailing list answer my questions?)

Comprehensive information about reporting problems with Cygwin can be found at https://cygwin.com/problems.html.

4. Using Cygwin

4.1. Why can't my application locate cygncurses-8.dll? or cygintl-3.dll? or cygreadline6.dll? or ...?

Well, something has gone wrong somehow... To repair the damage, you must run Cygwin Setup again, and re-install the package which provides the missing DLL package. If you already installed the package at one point, Cygwin Setup won't show the option to install the package by default. In the ``Select packages to install'' dialog, click on the Full/Part button. This lists all packages, even those that are already installed. Scroll down to locate the missing package, for instance libncurses8 . Click on the ``cycle'' glyph until it says ``Reinstall''. Continue with the installation. For a detailed explanation of the general problem, and how to extend it to other missing DLLs and identify their containing packages, see https://cygwin.com/ml/cygwin/2002-01/msg01619.html.

4.2. Starting a new terminal window is slow. What's going on?

There are many possible causes for this. If your terminal windows suddenly began starting slowly after a Cygwin upgrade, it may indicate issues in the authentication setup. For almost all its lifetime, Cygwin has used Unix-like /etc/passwd and /etc/group files to mirror the contents of the Windows SAM and AD databases. Although these files can still be used, since Cygwin 1.7.34, new installations now use the SAM/AD databases directly. To switch to the new method, move these two files out of the way and restart the Cygwin terminal. That runs Cygwin in its new default mode. If you are on a system that isn't using AD domain logins, this makes Cygwin use the native Windows SAM database directly, which may be faster than the old method involving /etc/passwd and such. At worst, it will only be a bit slower. (The speed difference you see depends on which benchmark you run.) For the AD case, it can be slower than the old method, since it is trading a local file read for a network request. Version 1.7.35 will reduce the number of AD server requests the DLL makes relative to 1.7.34, with the consequence that you will now have to alter /etc/nsswitch.conf in order to change your Cygwin home directory, instead of being able to change it from the AD configuration. If you are still experiencing very slow shell startups, there are a number of other things you can look into: One common cause of slow Cygwin Terminal starts is a bad DNS setup. This particularly affects AD clients, but there may be other things in your Cygwin startup that depend on getting fast answers back from a network server. Keep in mind that this may affect Cygwin even when the domain controller is on the same machine as Cygwin, or is on a nearby server. A bad DNS server IP can cause long delays while the local TCP/IP stack times out on a connection to a server that simply isn't there, for example. Another cause for AD client system is slow DC replies, commonly observed in configurations with remote DC access. The Cygwin DLL queries information about every group you're in to populate the local cache on startup. You may speed up this process a little by caching your own information in local files. Run these commands in a Cygwin terminal with write access to /etc : getent passwd $(id -u) > /etc/passwd getent group $(id -G) > /etc/group Also, set /etc/nsswitch.conf as follows: passwd: files db group: files db This will limit the need for Cygwin to contact the AD domain controller (DC) while still allowing for additional information to be retrieved from DC, such as when listing remote directories. Either in addition to the previous item or instead of it, you can run cygserver as a local caching service to speed up DC requests. Cygwin programs will check with cygserver before trying to query the DC directly. A less preferable option is to create a static read-only cache of the authentication data. This is the old-fashioned method of making Cygwin integrate with AD, the only method available in releases before 1.7.34. To do this, run mkpasswd and mkgroup, then put the following into /etc/nsswitch.conf to make Cygwin treat these files as the only sources of user and group information: passwd: files group: files By leaving out the db option, we are telling the Cygwin DLL not to even try to do AD lookups. If your AD servers are slow, this local cache will speed things up. The downside is that you open yourself up to the stale cache problem: any time the AD databases change, your local cache will go out of date until you update the files manually. If none of the above helps, the best troubleshooting method is to run your startup scripts in debug mode. Right-click your Cygwin Terminal icon, go to Properties, and edit the command. It should be something like C:\cygwin\bin\mintty.exe -i /Cygwin-Terminal.ico -. Assuming you are using Bash for your login shell, change it to C:\cygwin\bin\mintty /bin/bash -lx then try running Cygwin Terminal again. The -x option tells Bash to write every command it runs to the terminal before launching it. If the terminal immediately starts filling with lines of text but then pauses, the line where the output paused is your clue as to what's going on. The Cygwin DLL proper probably isn't the cause of the slowdown in this case, since those delays happen before the first line of text appears in the terminal.

4.3. Why is Cygwin suddenly so slow?

If suddenly every command takes a very long time, then something is probably attempting to access a network share. You may have the obsolete //c notation in your PATH or startup files. Using //c means to contact the network server c , which will slow things down tremendously if it does not exist.

4.4. Why can't my services access network shares?

If your service is one of those which switch the user context (sshd, inetd, etc), then it depends on the method used to switch to another user. This problem as well as its solution is described in detail in the Cygwin User's Guide, see https://cygwin.com/cygwin-ug-net/ntsec.html. Workarounds include using public network share that does not require authentication (for non-critical files), providing your password to a net use command, or running the service as your own user with cygrunsrv -u (see /usr/share/doc/Cygwin/cygrunsrv.README for more information).

4.5. How should I set my PATH?

This is done for you in the file /etc/profile, which is sourced by bash when you start it from the Desktop or Start Menu shortcut, created by setup.exe . The line is PATH="/usr/local/bin:/usr/bin:/bin:$PATH" Effectively, this prepends /usr/local/bin and /usr/bin to your Windows system path. If you choose to reset your PATH, say in $HOME/.bashrc, or by editing etc/profile directly, then you should follow this rule. You must have /usr/bin in your PATH before any Windows system directories. (And you must not omit the Windows system directories!) Otherwise you will likely encounter all sorts of problems running Cygwin applications. If you're using another shell than bash (say, tcsh), the mechanism is the same, just the names of the login scripts are different.

4.6. Bash (or another shell) says "command not found", but it's right there!

If you compile a program, you might find that you can't run it: bash$ gcc -o hello hello.c bash$ hello bash: hello: command not found Unlike the Windows default behaviour, Unix shells like bash do not look for programs in . (the current directory) by default. You can add . to your PATH (see above), but this is not recommended (at least on UNIX) for security reasons. Just tell bash where to find it, when you type it on the command line: bash$ gcc -o hello hello.c bash$ ./hello Hello World!

4.7. How do I convert between Windows and UNIX paths?

Use the 'cygpath' utility. Type ' cygpath --help ' for information. For example (on my installation): bash$ cygpath --windows ~/.bashrc D:\starksb\.bashrc bash$ cygpath --unix C:/cygwin/bin/ls.exe /usr/bin/ls.exe bash$ cygpath --unix C:\\cygwin\\bin\\ls.exe /usr/bin/ls.exe Note that bash interprets the backslash '\' as an escape character, so you must type it twice in the bash shell if you want it to be recognized as such.

4.8. Why doesn't bash read my .bashrc file on startup?

Your .bashrc is read from your home directory specified by the HOME environment variable. It uses /.bashrc if HOME is not set. So you need to set HOME (and the home dir in your passwd account information) correctly.

4.9. How can I get bash filename completion to be case insensitive?

Add the following to your ~/.bashrc file: shopt -s nocaseglob and add the following to your ~/.inputrc file: set completion-ignore-case on

4.10. Can I use paths/filenames containing spaces in them?

Cygwin does support spaces in filenames and paths. That said, some utilities that use the library may not, since files don't typically contain spaces in Unix. If you stumble into problems with this, you will need to either fix the utilities or stop using spaces in filenames used by Cygwin tools. In particular, bash interprets space as a word separator. You would have to quote a filename containing spaces, or escape the space character. For example: bash-2.03$ cd '/cygdrive/c/Program Files' or bash-2.03$ cd /cygdrive/c/Program\ Files

4.11. Why can't I cd into a shortcut to a directory?

Cygwin does not follow MS Windows Explorer Shortcuts (*.lnk files). It sees a shortcut as a regular file and this you cannot "cd" into it. Cygwin is also capable to create POSIX symlinks as Windows shortcuts (see the CYGWIN environment variable option "winsymlinks"), but these shortcuts are different from shortcuts created by native Windows applications. Windows applications can usually make use of Cygwin shortcuts but not vice versa. This is by choice. The reason is that Windows shortcuts may contain a bunch of extra information which would get lost, if, for example, Cygwin tar archives and extracts them as symlinks. Changing a Cygwin shortcut in Windows Explorer usually changes a Cygwin shortcut into a Windows native shortcut. Afterwards, Cygwin will not recognize it as symlink anymore.

4.12. I'm having basic problems with find. Why?

Make sure you are using the find that came with Cygwin and that you aren't picking up the Win32 find command instead. You can verify that you are getting the right one by doing a "type find" in bash. If the path argument to find, including current directory (default), is itself a symbolic link, then find will not traverse it unless you specify the -follow option. This behavior is different than most other UNIX implementations, but is not likely to change. If find does not seem to be producing enough results, or seems to be missing out some directories, you may be experiencing a problem with one of find's optimisations. The absence of . and .. directories on some filesystems, such as DVD-R UDF, can confuse find. See the documentation for the option -noleaf in the man page.

4.13. Why doesn't su work?

The su command has been in and out of Cygwin distributions, but it has not been ported to Cygwin and has never worked. It is currently installed as part of the sh-utils, but again, it does not work. You should rather install sshd and use ssh username@localhost as a su replacement. For some technical background into why su doesn't work, read https://www.cygwin.com/ml/cygwin/2003-06/msg00897.html and related messages.

4.14. Why doesn't man -k , apropos or whatis work?

Before you can use man -k , apropos or whatis , you must create the whatis database. Just run the command mandb (it may take a few minutes to complete).

4.15. Why doesn't chmod work?

If you're using FAT32 instead of NTFS, chmod will fail since FAT32 does not provide any permission information. You should really consider converting the drive to NTFS with CONVERT.EXE . FAT and FAT32 are barely good enough for memory cards or USB sticks to exchange pictures... For other cases, understand that Cygwin attempts to show UNIX permissions based on the security features of Windows, so the Windows ACLs are likely the source of your problem. See the Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/ntsec.html for more information on how Cygwin maps Windows permissions.

4.16. Why doesn't my shell script work?

There are two basic problems you might run into. One is the fact that /bin/sh is really bash. It could be missing some features you might expect in /bin/sh, if you are used to /bin/sh actually being zsh (MacOS X "Panther") or ksh (Tru64). Or, it could be a permission problem, and Cygwin doesn't understand that your script is executable. On NTFS or NFS just make the script executable using chmod +x . However, chmod may not work due to restrictions of the filesystem (see FAQ entry above). In this case Cygwin must read the contents of files to determine if they are executable. If your script does not start with #! /bin/sh (or any path to a script interpreter, it does not have to be /bin/sh) then Cygwin will not know it is an executable script. The Bourne shell idiom : # This is the 2nd line, assume processing by /bin/sh also works. Note that you can use the filesystem flag cygexec in /etc/fstab to force Cygwin to treat all files under the mount point as executable. This can be used for individual files as well as directories. Then Cygwin will not bother to read files to determine whether they are executable.

4.17. How do I print under Cygwin?

lpr is available in the cygutils package. Some usage hints are available courtesy of Rodrigo Medina. Jason Tishler has written a couple of messages that explain how to use a2ps (for nicely formatted text in PostScript) and ghostscript (to print PostScript files on non-PostScript Windows printers). Start at https://cygwin.com/ml/cygwin/2001-04/msg00657.html. Note that these are old mails and a2ps as well as file are long available as part of the Cygwin distribution. Alternatively, you can use the Windows print command. Type bash$ print /\? for usage instructions (note the ? must be escaped from the shell). Finally, you can simply cat the file to the printer's share name: bash$ cat myfile > //host/printer You may need to press the formfeed button on your printer or append the formfeed character to your file.

4.18. Why don't international (Unicode) characters work?

Internationalization is a complex issue. The short answer is that Cygwin relies on the setting of the setting of LANG/LC_xxx environment variables. The long answer can be found in the User's Guide in the section Internationalization Cygwin uses UTF-8 by default. To use a different character set, you need to set the LC_ALL, LC_CTYPE or LANG environment variables.

4.19. My application prints international characters but I only see gray boxes

In the case of Cygwin programs, this likely means that the character set as determined by the LC_ALL, LC_CTYPE or LANG environment variables does not match the one set on the Text page of the Cygwin Terminal's options. Setting the locale in the terminal's options will set the LANG variable accordingly. Non-Cygwin programs in the Cygwin Terminal do not usually take heed of the locale environment variables. Instead, they often use the so-called console codepage, which can be determined with the command cmd /c chcp followed by the appropriate Windows codepage number. The codepage number for Cygwin's default UTF-8 character set is 65001.

4.20. Is it OK to have multiple copies of the DLL?

Yes, as long as they are used in strictly separated installations. The Cygwin DLL has to handle various sharing situations between multiple processes. It has to keep a process table. It has to maintain a mount table which is based on the installation path of the Cygwin DLL. For that reason, the Cygwin DLL maintains shared resources based on a hash value created from its own installation path. Each Cygwin DLL on the machine constitutes a Cygwin installation, with the directory the Cygwin DLL resides in treated as "/bin", the parent directory as "/". Therefore, you can install two or more separate Cygwin distros on a single machine. Each of these installations use their own Cygwin DLL, and they don't share the default POSIX paths, nor process tables, nor any other shared resource used to maintain the installation. However, a clean separation requires that you don't try to run executables of one Cygwin installation from processes running in another Cygwin installation. This may or may not work, but the chances that the result is not what you expect are pretty high. If you get the error "shared region is corrupted" or "shared region version mismatch" it means you have multiple versions of cygwin1.dll running at the same time which conflict with each other. Apart from mixing executables of different Cygwin installations, this could also happen if you have one a single Cygwin installation, for example, if you update the Cygwin package without exiting all Cygwin apps (including services like sshd) beforehand. The only DLL that is sanctioned by the Cygwin project is the one that you get by running setup-x86.exe or setup-x86_64.exe, installed in a directory controlled by this program. If you have other versions on your system and desire help from the cygwin project, you should delete or rename all DLLs that are not installed by setup.exe . If you're trying to find multiple versions of the DLL that are causing this problem, reboot first, in case DLLs still loaded in memory are the cause. Then use the Windows System find utility to search your whole machine, not just components in your PATH (as 'type' would do) or cygwin-mounted filesystems (as Cygwin 'find' would do).

4.21. I read the above but I want to bundle Cygwin with a product, and ship it to customer sites. How can I do this without conflicting with any Cygwin installed by the user?

Usually, if you keep your installation separate, nothing bad should happen. However, for the user's convenience, and to avoid potential problems which still can occur, consider to integrate your product with an already existing Cygwin installation on the user's machine, or, if there is none, consider to install the official Cygwin distro on behalf of the user and integrate your tools from there. (If you write a tool to make this easy, consider contributing it for others to use)

4.22. Can I bundle Cygwin with my product for free?

Starting with Cygwin version 2.5.2, which is LGPL licensed, yes, albeit it's not recommended for interoperability reasons. Cygwin versions prior to 2.5.2 were GPL licensed. If you choose to distribute an older cygwin1.dll, you must be willing to distribute the exact source code used to build that copy of cygwin1.dll as per the terms of the GPL. If you ship applications that link with older cygwin1.dll, you must provide those applications' source code under a GPL-compatible license.

4.23. But doesn't that mean that if some application installs an older Cygwin DLL on top of a newer DLL, my application will break?

It depends on what you mean by "break". If the application installs a version of the Cygwin DLL in another location than Cygwin's /bin directory then the rules in Q: 4.21 apply. If the application installs an older version of the DLL in /bin then you should complain loudly to the application provider. Remember that the Cygwin DLL strives to be backwards compatible so a newer version of the DLL should always work with older executables. So, in general, it is always best to keep one version of the DLL on your system and it should always be the latest version which matches your installed distribution.

4.24. Why isn't package XYZ available in Cygwin?

Probably because there is nobody willing or able to maintain it. It takes time, and the priority for the Cygwin Team is the Cygwin package. The rest is a volunteer effort. Want to contribute? See https://cygwin.com/packaging.html.

4.25. Why is the Cygwin package of XYZ so out of date?

(Also: Why is the version of package XYZ older than the version that I can download from the XYZ web site? Why is the version of package XYZ older than the version that I installed on my linux system? Is there something special about Cygwin which requires that only an older version of package XYZ will work on it?) Every package in the Cygwin distribution has a maintainer who is responsible for sending out updates of the package. This person is a volunteer who is rarely the same person as the official developer of the package. If you notice that a version of a package seems to be out of date, the reason is usually pretty simple -- the person who is maintaining the package hasn't gotten around to updating it yet. Rarely, the newer package actually requires complex changes that the maintainer is working out. If you urgently need an update, sending a polite message to the cygwin mailing list pinging the maintainer is perfectly acceptable. There are no guarantees that the maintainer will have time to update the package or that you'll receive a response to your request, however. Remember that the operative term here is "volunteer".

4.26. How can I access other drives?

You have some flexibility here. Cygwin has a builtin "cygdrive prefix" for drives that are not mounted. You can access any drive, say Z:, as '/cygdrive/z/'. In some applications (notably bash), you can use the familiar windows <drive>:/path/, using posix forward-slashes ('/') instead of Windows backward-slashes ('\'). (But see the warning below!) This maps in the obvious way to the Windows path, but will be converted internally to use the Cygwin path, following mounts (default or explicit). For example: bash$ cd C:/Windows bash$ pwd /cygdrive/c/Windows and bash$ cd C:/cygwin bash$ pwd / for a default setup. You could also use backward-slashes in the Windows path, but these would have to be escaped from the shell. Warning: There is some ambiguity in going from a Windows path to the posix path, because different posix paths, through different mount points, could map to the same Windows directory. This matters because different mount points may be binmode or textmode, so the behavior of Cygwin apps will vary depending on the posix path used to get there. You can avoid the ambiguity of Windows paths, and avoid typing "/cygdrive", by explicitly mounting drives to posix paths. For example: bash$ mkdir /c bash$ mount c:/ /c bash$ ls /c Then /cygdrive/c/Windows becomes /c/Windows which is a little less typing. Note that you have to enter the mount point into the /etc/fstab file to keep it indefinitely. The mount command will only add the mount point for the lifetime of your current Cygwin session. You can change the default cygdrive prefix and whether it is binmode or textmode using the /etc/fstab file as well. See the Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/using.html#mount-table for more details.

4.27. How can I copy and paste into Cygwin console windows?

First, consider using mintty instead of the standard console window. In mintty, selecting with the left-mouse also copies, and middle-mouse pastes. It couldn't be easier! In Windows's console window, open the properties dialog. The options contain a toggle button, named "Quick edit mode". It must be ON. Save the properties. You can also bind the insert key to paste from the clipboard by adding the following line to your .inputrc file: "\e[2~": paste-from-clipboard

4.28. What firewall should I use with Cygwin?

We have had good reports about Kerio Personal Firewall, ZoneLabs Integrity Desktop, and the Windows built-in firewall. Other well-known products including ZoneAlarm and Norton Internet Security have caused problems for some users but work fine for others. At last report, Agnitum Outpost did not work with Cygwin. If you are having strange connection-related problems, disabling the firewall is a good troubleshooting step (as is closing or disabling all other running applications, especially resource-intensive processes such as indexed search). On the whole, Cygwin doesn't care which firewall is used. The few rare exceptions have to do with socket code. Cygwin uses sockets to implement many of its functions, such as IPC. Some overzealous firewalls install themselves deeply into the winsock stack (with the 'layered service provider' API) and install hooks throughout. Sadly the mailing list archives are littered with examples of poorly written firewall-type software that causes things to break. Note that with many of these products, simply disabling the firewall does not remove these changes; it must be completely uninstalled. See also https://cygwin.com/faq/faq.html#faq.using.bloda for a list of applications that have been known, at one time or another, to interfere with the normal functioning of Cygwin.

4.29. How can I share files between Unix and Windows?

During development, we have Linux boxes running Samba and NFS as well as Windows machines. We often build with cross-compilers under Linux and copy binaries and source to the Windows system or just toy with them directly off the Samba-mounted partition. Or, we use the Microsoft NFS client and just use NFS shares on Linux from Windows. And then there are tools like scp , ftp , rsync , ...

4.30. Is Cygwin case-sensitive??

Several Unix programs expect to be able to use to filenames spelled the same way, but with different case. A prime example of this is perl's configuration script, which wants Makefile and makefile . Windows can't tell the difference between files with just different case, so the configuration fails. To help with this problem, Cygwin supports case sensitivity. For a detailed description how to use that feature see the Cygwin User's Guide at https://cygwin.com/cygwin-ug-net/using-specialnames.html.

4.31. What about DOS special filenames?

In Windows, files cannot be named com1, lpt1, or aux (to name a few); either as the root filename or as the extension part. If you do, you'll have trouble. Unix programs don't avoid these names which can make things interesting. E.g., the perl distribution has a file called aux.sh . The perl configuration tries to make sure that aux.sh is there, but an operation on a file with the magic letters 'aux' in it will hang. At least that's what happens when using native Windows tools. Cygwin can deal with these filenames just fine. Again, see the User's Guide at https://cygwin.com/cygwin-ug-net/using-specialnames.html for a detailed description of what's possible with filenames and what is not.

4.32. When it hangs, how do I get it back?

If something goes wrong and the tools hang on you for some reason (easy to do if you try and read a file called aux.sh), first try hitting ^C to return to bash or the cmd prompt. If you start up another shell, and applications don't run, it's a good bet that the hung process is still running somewhere. Use the Task Manager, pview, or a similar utility to kill the process. And, if all else fails, there's always the reset button/power switch. In theory this should never be necessary, though.

4.33. Why the weird directory structure?

Why do /lib and /usr/lib (and /bin, /usr/bin) point to the same thing? Why use mounts instead of symbolic links? Can I use a disk root (e.g., C:\) as Cygwin root? Why is this discouraged? After a new installation in the default location, your mount points will look something like this: bash$ mount C:\cygwin\bin on /usr/bin type ntfs (binary,auto) C:\cygwin\lib on /usr/lib type ntfs (binary,auto) C:\cygwin on / type ntfs (binary,auto) C: on /cygdrive/c type ntfs (binary,posix=0,user,noumount,auto) Note that /bin and /usr/bin point to the same location, as do /lib and /usr/lib. This is intentional, and you should not undo these mounts unless you really know what you are doing. Various applications and packages may expect to be installed in /lib or /usr/lib (similarly /bin or /usr/bin). Rather than distinguish between them and try to keep track of them (possibly requiring the occasional duplication or symbolic link), it was decided to maintain only one actual directory, with equivalent ways to access it. Symbolic links had been considered for this purpose, but were dismissed because they do not always work on Samba drives. Also, mounts are faster to process because no disk access is required to resolve them. Note that non-cygwin applications will not observe Cygwin mounts (or most symlinks for that matter). For example, if you use WinZip to unpack the tar distribution of a Cygwin package, it may not get installed to the correct Cygwin path. So don't do this! It is strongly recommended not to make the Cygwin root directory the same as your drive's root directory, unless you know what you are doing and are prepared to deal with the consequences. It is generally easier to maintain the Cygwin hierarchy if it is isolated from, say, C:\. For one thing, you avoid possible collisions with other (non-cygwin) applications that may create (for example) \bin and \lib directories. (Maybe you have nothing like that installed now, but who knows about things you might add in the future?)

4.34. How do anti-virus programs like Cygwin?

Users have reported that NAI (formerly McAfee) VirusScan for NT (and others?) is incompatible with Cygwin. This is because it tries to scan the newly loaded shared memory in cygwin1.dll, which can cause fork() to fail, wreaking havoc on many of the tools. (It is not confirmed that this is still a problem, however.) There have been several reports of NAI VirusScan causing the system to hang when unpacking tar.gz archives. This is surely a bug in VirusScan, and should be reported to NAI. The only workaround is to disable VirusScan when accessing these files. This can be an issue during setup, and is discussed in that FAQ entry. Some users report a significant performance hit using Cygwin when their anti-virus software is enabled. Rather than disable the anti-virus software completely, it may be possible to specify directories whose contents are exempt from scanning. In a default installation, this would be C:\cygwin\bin . Obviously, this could be exploited by a hostile non-Cygwin program, so do this at your own risk. See also https://cygwin.com/faq/faq.html#faq.using.bloda for a list of applications that have been known, at one time or another, to interfere with the normal functioning of Cygwin.

4.35. Is there a Cygwin port of GNU Emacs?

Yes. Install the emacs package. This provides everything you need in order to run GNU emacs in a terminal window. If you also want to be able to use the X11 (https://x.cygwin.com/) GUI, install the emacs-X11 package. In either case, you run emacs by typing 'emacs' or '/usr/bin/emacs'.

4.36. Is there a Cygwin port of XEmacs?

Yes. It can be used in three different modes: X11 (https://x.cygwin.com/) GUI You have to set the DISPLAY environment variable before starting xemacs. bash$ DISPLAY=127.0.0.1:0 xemacs & Windows native GUI You have to unset the DISPLAY environment variable before starting xemacs. bash$ DISPLAY= xemacs & Console mode Start xemacs with -nw in a terminal (native or X11) window bash$ xemacs -nw To use all the standard packages with XEmacs you should download the following two packages: xemacs-sumo - XEmacs standard packages

xemacs-mule-sumo - XEmacs MULE (MUlti Lingual Emacs) packages

4.37. Why don't some of my old symlinks work anymore?

Cygwin supports multiple character sets. Symlinks created with Cygwin are using the UTF-16 character set, which is portable across all character sets. Old symlinks were written using your current Windows codepage, which is not portable across all character sets. If the target of the symlink doesn't resolve anymore, it's very likely that the symlink points to a target filename using native, non-ASCII characters, and you're now using another character set than way back when you created the symlink. Solution: Delete the symlink and create it again under you new Cygwin. The new symlink will be correctly point to the target no matter what character set you're using in future.

4.38. Why don't symlinks work on Samba-mounted filesystems?

Default symlinks on Samba are marked with DOS SYSTEM file attribute. Samba does not enable this attribute by default. To enable it, consult your Samba documentation and then add these lines to your samba configuration file: map system = yes create mask = 0775 Note that the 0775 can be anything as long as the 0010 bit is set. Alternatively, use Windows shortcuts as symlinks. See the CYGWIN environment variable option "winsymlinks:lnk" https://cygwin.com/cygwin-ug-net/using-cygwinenv.html Note that Samba does not support reparse points so some methods to create symlinks are just not available.

4.39. Why does public key authentication with ssh fail after updating to Cygwin 1.7.34 or later?

This is the result of fixing a long-standing security problem in Cygwin's POSIX ACL handling. IEEE 1003.1e draft 17 defines that the permissions of secondary user and group entries in an ACL are reflected in the group permission mask by or'ing the permissions of the file's primary group with all permissions of secondary users and groups in the ACL. The background is that this way the standard POSIX permission bits reflect the fact that somebody else has additional, otherwise potentially invisible permissions on the file. This relatively complex interface has been defined in order to ensure that applications that are compliant with IEEE 1003.1 (“POSIX.1”) will still function as expected on systems with ACLs. So, what does that mean for your situation? Typically this means the private key file, for instance ~/.ssh/id_rsa , has too open permissions. OpenSSH expects the permissions of the private key file to be 0600. Let's use the default SSH2 RSA keyfile as example: $ ls -l .ssh/id_rsa -rw------- 1 user group 1766 Aug 26 2013 .ssh/id_rsa However, if other accounts can read the file, the key is potentially compromised. Consider the file has additional rw- permissions for a group bad_guys . Up to Cygwin 1.7.33 that would have looked like this: $ ls -l .ssh/id_rsa -rw-------+ 1 user group 1766 Aug 26 2013 .ssh/id_rsa Notice the extra + character following the permission string. This shows that additional ACL entries are in the ACL. But an application only checking the POSIX permission bits (and ssh is one of them!), will not notice the fact, because it gets the permissions 0600 for the file. Starting with Cygwin 1.7.34, the extra permissions are reflected in the group permission bits per IEEE 1003.1e draft 17: $ ls -l .ssh/id_rsa -rw-rw----+ 1 user group 1766 Aug 26 2013 .ssh/id_rsa So now ssh will notice that the file has extra permissions and it will complain. The same problem occurs if the file ~/.ssh/authorized_keys has too open permissions. On the client side you won't get any helping text, though, other than that you're suddenly asked for a password. That's a rather good hint to have a closer look at the server's ~/.ssh/authorized_keys file. To fix the permissions of your private key file or your ~/.ssh/authorized_keys file, simply use the setfacl command with the -b option. This removes all additional ACL entries and thus fixes the permissions to be not too open: $ ls -l .ssh/id_rsa -rw-rw----+ 1 user group 1766 Aug 26 2013 .ssh/id_rsa $ setfacl -b .ssh/id_rsa $ ls -l .ssh/id_rsa -rw------- 1 user group 1766 Aug 26 2013 .ssh/id_rsa If the second ls command still gives you -rw-rw---- permissions after running the above commands, it is proably because the file's primary group is your user's personal group: $ ls -l .ssh/id_rsa -rw-rw---- 1 Fred Fred 1766 Aug 26 2013 .ssh/id_rsa Since the Windows security system treats groups and users as much the same thing, a change to the user or group permissions on such a file reflects the change to both user and group. In effect, mode 0600 becomes mode 0660. Because we are saying we want these files to be readable only by our user, the fix for this is easy: $ chgrp `id -g` ~/.ssh/* That resets the group on these files to your default group which should be something like Users , depending on your local configuration. If that doesn't work, you can try something like this instead: $ chgrp None ~/.ssh/* That group always exists, but its name is different on non-English versions of Windows. You might also want to use a domain group instead of a local group if your site uses Windows domains. For example, you might want to use the Domain Users group instead. For more information on setfacl, see https://cygwin.com/cygwin-ug-net/setfacl.html

4.40. Why is my .rhosts file not recognized by rlogin anymore after updating to Cygwin 1.7.34?

The problem is exactly the same as with the key files of SSH. See Q: 4.39. The solution is the same: $ ls -l .rhosts -rw-rw----+ 1 user group 42 Nov 12 2010 .rhosts $ setfacl -b .rhosts $ ls -l .rhosts -rw------- 1 user group 42 Nov 12 2010 .rhosts

4.41. Why do my files have extra permissions after updating to Cygwin 1.7.34?

The problem is exactly the same as with the key files of SSH. See Q: 4.39. The solution is the same: $ ls -l * -rw-rwxr--+ 1 user group 42 Nov 12 2010 file1 -rw-rwxr--+ 1 user group 42 Nov 12 2010 file2 $ setfacl -b * $ ls -l * -rw-r--r-- 1 user group 42 Nov 12 2010 file1 -rw-r--r-- 1 user group 42 Nov 12 2010 file2 You may find that newly-created files also have unexpected permissions: $ touch foo $ ls -l foo -rw-rwxr--+ 1 user group 42 Nov 12 2010 foo This probably means that the directory in which you're creating the files has unwanted default ACL entries that are inherited by newly-created files and subdirectories. The solution is again the same: $ setfacl -b . $ touch bar $ ls -l bar -rw-r--r-- 1 user group 42 Nov 12 2010 bar

4.42. Why do my Tk programs not work anymore?

Previous versions of Tcl/Tk distributed with Cygwin (e.g. tclsh84.exe, wish84.exe) were not actually "Cygwin versions" of those tools. They were built as native libraries, which means they did not understand Cygwin mounts or symbolic links. This lead to all sorts of problems interacting with true Cygwin programs. As of February 2012, this was replaced with a version of Tcl/Tk which uses Cygwin's POSIX APIs and X11 for GUI functionality. If you get a message such as this when trying to start a Tk app: Application initialization failed: couldn't connect to display "" Then you need to start an X server, or if one is already running, set the DISPLAY variable to the proper value. The Cygwin distribution includes an X server; please see the Cygwin/X User Guide for installation and startup instructions.

4.43. What applications have been found to interfere with Cygwin?

From time to time, people have reported strange failures and problems in Cygwin and Cygwin packages that seem to have no rational explanation. Among the most common symptoms they report are fork failures, memory leaks, and file access denied problems. These problems, when they have been traced, often appear to be caused by interference from other software installed on the same PC. Security software, in particular, such as anti-virus, anti-spyware, and firewall applications, often implements its functions by installing hooks into various parts of the system, including both the Explorer shell and the underlying kernel. Sometimes these hooks are not implemented in an entirely transparent fashion, and cause changes in the behaviour which affect the operation of other programs, such as Cygwin. Among the software that has been found to cause difficulties are: AR Soft RAM Disk

ATI Catalyst (some versions)

AVAST (disable FILESYSTEM and BEHAVIOR realtime shields)

Avira AntiVir

BeyondTrust PowerBroker

BitDefender

Bufferzone from Trustware

ByteMobile laptop optimization client

COMODO Firewall Pro

COMODO Internet Security

ConEmu (try disabling "Inject ConEmuHk" or see ConEmuHk documentation)

Citrix Metaframe Presentation Server/XenApp (see Citrix Support page)

Credant Guardian Shield

CylancePROTECT

Earthlink Total-Access

Forefront TMG

Google Desktop

Iolo System Mechanic/AntiVirus/Firewall

Kerio, Agnitum or ZoneAlarm Personal Firewall

LanDesk

Lavasoft Web Companion

Lenovo IPS Core Service (ipssvc)

Lenovo RapidBoot Shield

Logitech webcam software with "Logitech process monitor" service

MacType

NOD32 Antivirus

NVIDIA GeForce (some versions)

Norton/McAfee/Symantec antivirus or antispyware

PC Tools Spyware Doctor

Panda Internet Security

Sonic Solutions burning software containing DLA component (when DLA disabled)

Sophos Anti-Virus 7

Spybot S&D TeaTimer

Various programs by Wave Systems Corp using wxvault.dll, including Embassy Trust Suite and Embassy Security Center

Webroot Spy Sweeper with Antivirus

Windows Defender

Windows LiveOneCare

IBM Security Trusteer Rapport (see its home page) Sometimes these problems can be worked around, by temporarily or partially disabling the offending software. For instance, it may be possible to disable on-access scanning in your antivirus, or configure it to ignore files under the Cygwin installation root. Often, unfortunately, this is not possible; even disabling the software may not work, since many applications that hook the operating system leave their hooks installed when disabled, and simply set them into what is intended to be a completely transparent pass-through mode. Sometimes this pass-through is not as transparent as all that, and the hooks still interfere with Cygwin; in these cases, it may be necessary to uninstall the software altogether to restore normal operation. Some of the symptoms you may experience are: Random fork() failures Caused by hook DLLs that load themselves into every process in the system. POSIX fork() semantics require that the memory map of the child process must be an exact duplicate of the parent process' layout. If one of these DLLs loads itself at a different base address in the child's memory space as compared to the address it was loaded at in the parent, it can end up taking the space that belonged to a different DLL in the parent. When Cygwin can't load the original DLL at that same address in the child, the fork() call has to fail.

File access problems Some programs (e.g., virus scanners with on-access scanning) scan or otherwise operate on every file accessed by all the other software running on your computer. In some cases they may retain an open handle on the file even after the software that is really using the file has closed it. This has been known to cause operations such as deletes, renames and moves to fail with access denied errors. In extreme cases it has been known for scanners to leak file handles, leading to kernel memory starvation.

Networking issues Firewall software sometimes gets a bit funny about Cygwin. It's not currently understood why; Cygwin only uses the standard Winsock2 API, but perhaps in some less-commonly used fashion that doesn't get as well tested by the publishers of firewalls. Symptoms include mysterious failures to connect, or corruption of network data being sent or received.

Memory and/or handle leaks Some applications that hook into the Windows operating system exhibit bugs when interacting with Cygwin that cause them to leak allocated memory or other system resources. Symptoms include complaints about out-of-memory errors and even virtual memory exhaustion dialog boxes from the O/S; it is often possible to see the excess memory allocation using a tool such as Task Manager or Sysinternals' Process Explorer, although interpreting the statistics they present is not always straightforward owing to complications such as virtual memory paging and file caching.

4.44. How do I fix fork() failures?

Unfortunately, Windows does not use the fork/exec model of process creation found in UNIX-like OSes, so it is difficult for Cygwin to implement a reliable and correct fork() , which can lead to error messages such as: unable to remap somedll to same address as parent

to same address as parent couldn't allocate heap

died waiting for dll loading

child -1 - died waiting for longjmp before initialization

STATUS_ACCESS_VIOLATION

resource temporarily unavailable Potential solutions for the above errors: Restart whatever process is trying (and failing) to use fork() . Sometimes Windows sets up a process environment that is even more hostile to fork() than usual.

. Sometimes Windows sets up a process environment that is even more hostile to than usual. Ensure that you have eliminated (not just disabled) all software on the BLODA.

Switch from 32-bit Cygwin to 64-bit Cygwin, if your OS and CPU support that. With the bigger address space fork() is less likely to fail.

is less likely to fail. Try setting the environment variable CYGWIN to "detect_bloda", which enables some extra debugging, which may indicate what other software is causing the problem. See this mail for more information.

Force a full rebase: Run rebase-trigger fullrebase, exit all Cygwin programs and run Cygwin setup. By default, Cygwin's setup program automatically performs an incremental rebase of newly installed files. Forcing a full rebase causes the rebase map to be cleared before doing the rebase. See /usr/share/doc/rebase/README and /usr/share/doc/Cygwin/_autorebase.README for more details. Please note that installing new packages or updating existing ones undoes the effects of rebase and often causes fork() failures to reappear. See the process creation section of the User's Guide for the technical reasons it is so difficult to make fork() work reliably.

4.45. How do I fix find_fast_cwd warnings?

Older Cygwin releases asked users to report problems to the mailing list with the message: find_fast_cwd: WARNING: Couldn't compute FAST_CWD pointer. Please report this problem to the public mailing list cygwin@cygwin.com Recent Cygwin releases changed this to the message: This typically occurs if you're using an older Cygwin version on a newer Windows. Please update to the latest available Cygwin version from https://cygwin.com/. If the problem persists, please see https://cygwin.com/problems.html. This is not serious, just a warning that Cygwin may not always be able to exactly emulate all aspects of Unix current directory handling under your Windows release. Unfortunately some projects and products still distribute older Cygwin releases which may not fully support newer Windows releases, instead of installing the current release from the Cygwin project. They also may not provide any obvious way to keep the Cygwin packages their application uses up to date with fixes for security issues and upgrades. The solution is simply downloading and running Cygwin Setup, following the instructions in the Internet Setup section of Setting Up Cygwin in the Cygwin User's Guide. Please exit from all applications before running Cygwin Setup. When running Setup, you should not change most of the values presented, just select the Next button in most cases, as you already have a Cygwin release installed, and only want to upgrade your current installation. You should make your own selection if the internet connection to your system requires a proxy; and you must always pick an up to date Cygwin download (mirror) site, preferably the site nearest to your system for faster downloads, as shown, with more details to help you choose, on the Mirror Sites web page. Cygwin Setup will download and apply upgrades to all packages required for Cygwin itself and installed applications. Any problems with applying updates, or the application after updates, should be reported to the project or product supplier for remedial action. As Cygwin is a volunteer project, unable to provide support for older releases installed by projects or products, it would be helpful to let other users know what project or product you installed, in a quick email.

5. Cygwin API Questions

5.1. How does everything work?

There's a C library which provides a POSIX-style API. The applications are linked with it and voila - they run on Windows. The aim is to add all the goop necessary to make your apps run on Windows into the C library. Then your apps should (ideally) run on POSIX systems (Unix/Linux) and Windows with no changes at the source level. The C library is in a DLL, which makes basic applications quite small. And it allows relatively easy upgrades to the Win32/POSIX translation layer, providing that DLL changes stay backward-compatible. For a good overview of Cygwin, you may want to read the Cygwin User's Guide.

5.2. Are development snapshots for the Cygwin library available?

Yes. They're made whenever anything interesting happens inside the Cygwin library (usually roughly on a nightly basis, depending on how much is going on). They are only intended for those people who wish to contribute code to the project. If you aren't going to be happy debugging problems in a buggy snapshot, avoid these and wait for a real release. The snapshots are available from https://cygwin.com/snapshots/.

5.3. How is the DOS/Unix CR/LF thing handled?

Let's start with some background. On POSIX systems, a file is a file and what the file contains is whatever the program/programmer/user told it to put into it. In Windows, a file is also a file and what the file contains depends not only on the program/programmer/user but also the file processing mode. When processing in text mode, certain values of data are treated specially. A

(new line, NL) written to the file will prepend a \r (carriage return, CR) so that if you `printf("Hello

") you in fact get "Hello\r

". Upon reading this combination, the \r is removed and the number of bytes returned by the read is 1 less than was actually read. This tends to confuse programs dependent on ftell() and fseek(). A Ctrl-Z encountered while reading a file sets the End Of File flags even though it truly isn't the end of file. One of Cygwin's goals is to make it possible to mix Cygwin-ported POSIX programs with generic Windows programs. As a result, Cygwin allows to open files in text mode. In the accompanying tools, tools that deal with binaries (e.g. objdump) operate in POSIX binary mode and many (but not all) tools that deal with text files (e.g. bash) operate in text mode. There are also some text tools which operate in a mixed mode. They read files always in text mode, but write files in binary mode, or they write in the mode (text or binary) which is specified by the underlying mount point. For a description of mount points, see the Cygwin User's Guide. Actually there's no really good reason to do text mode processing since it only slows down reading and writing files. Additionally many Windows applications can deal with POSIX

line endings just fine (unfortunate exception: Notepad). So we suggest to use binary mode as much as possible and only convert files from or to DOS text mode using tools specifically created to do that job, for instance, dos2unix and unix2dos from the dos2unix package. It is rather easy for the porter of a Unix package to fix the source code by supplying the appropriate file processing mode switches to the open/fopen functions. Treat all text files as text and treat all binary files as binary. To be specific, you can select binary mode by adding O_BINARY to the second argument of an open call, or "b" to second argument of an fopen call. You can also call setmode (fd, O_BINARY) . To select text mode add O_TEXT to the second argument of an open call, or "t" to second argument of an fopen call, or just call setmode (fd, O_TEXT) . You can also avoid to change the source code at all by linking an additional object file to your executable. Cygwin provides various object files in the /usr/lib directory which, when linked to an executable, changes the default open modes of any file opened within the executed process itself. The files are binmode.o - Open all files in binary mode. textmode.o - Open all files in text mode. textreadmode.o - Open all files opened for reading in text mode. automode.o - Open all files opened for reading in text mode, all files opened for writing in binary mode. Note Linking against these object files does not change the open mode of files propagated to a process by its parent process, for instance, if the process is part of a shell pipe expression. Note that of the above flags only the "b" fopen flags are defined by ANSI. They exist under most flavors of Unix. However, using O_BINARY, O_TEXT, or the "t" flag is non-portable.

5.4. Is the Cygwin library multi-thread-safe?

Yes. There is also extensive support for 'POSIX threads', see the file cygwin.din for the list of POSIX thread functions provided.

5.5. How is fork() implemented?

Cygwin fork() essentially works like a non-copy on write version of fork() (like old Unix versions used to do). Because of this it can be a little slow. In most cases, you are better off using the spawn family of calls if possible. Here's how it works: Parent initializes a space in the Cygwin process table for child. Parent creates child suspended using Win32 CreateProcess call, giving the same path it was invoked with itself. Parent calls setjmp to save its own context and then sets a pointer to this in the Cygwin shared memory area (shared among all Cygwin tasks). Parent fills in the child's .data and .bss subsections by copying from its own address space into the suspended child's address space. Parent then starts the child. Parent waits on mutex for child to get to safe point. Child starts and discovers if has been forked and then longjumps using the saved jump buffer. Child sets mutex parent is waiting on and then blocks on another mutex waiting for parent to fill in its stack and heap. Parent notices child is in safe area, copies stack and heap from itself into child, releases the mutex the child is waiting on and returns from the fork call. Child wakes from blocking on mutex, recreates any mmapped areas passed to it via shared area and then returns from fork itself. When the executable or any dll in use by the parent was renamed or moved into the hidden recycle bin, fork retries with creating hardlinks for the old executable and any dll into per-user subdirectories in the /var/run/cygfork/ directory, when that one exists and resides on NTFS.

5.6. How does wildcarding (globbing) work?

If the DLL thinks it was invoked from a DOS style prompt, it runs a `globber' over the arguments provided on the command line. This means that if you type LS *.EXE from DOS, it will do what you might expect. Beware: globbing uses malloc . If your application defines malloc , that will get used. This may do horrible things to you.

5.7. How do symbolic links work?

Cygwin knows of five ways to create symlinks. This is really complicated stuff since we started out way back when Windows didn't know symlinks at all. The rest is history... Starting with Cygwin 3.1.5 in 2020, symlinks are created by default as a special reparse point type known as "WSL symlinks". These have been introduced on Windows 10 with the advent of WSL, "Windows Subsystem for Linux". WSL symlinks created by Cygwin are understood by WSL and vice versa. They contain a normal POSIX path as used in the Cygwin and WSL environments. Windows itself recognizes them as arbitrary reparse points (CMD's "dir" command shows them as "[JUNCTION]") but it doesn't know how to follow them to the target. Older Windows versions handle these symlinks exactly the same way, so there's no point using different symlink types on older Windows. These symlinks only work on filesystems supporting reparse points, but fortunately there's another symlink type Cygwin creates, right the next bullet point...

The original default method creating symlinks in Cygwin since pre-2000 generates symlinks as simple files with a magic header and the DOS SYSTEM attribute set. When you open a file or directory through such a symlink, Cygwin opens the file, checks the magic header, and if it's correct, reads the target of the symlink from the remainder of the file. Because we don't want having to open every referenced file to check symlink status, Cygwin only opens files with DOS SYSTEM attribute set to inspect them for being a Cygwin symlink. These symlinks also work on filesystems not supporting reparse points, i. e., FAT/FAT32/ExFAT.

A very special case are NFS filesystems, supported by Cygwin since 2008 via the Microsoft NFS driver, unfortunately only available in Enterprise versions of Windows. Filesystems shared via NFS usually support symlinks all by themselves, and the Microsoft driver has special functionality to support them. Cygwin utilizes this interface to create "real" symlinks on filesystems mounted via NFS.

Starting 2013, Cygwin also supports NTFS symlinks, introduced with Windows Vista. These symlinks are reparse points containing a Windows path. Creating them is enabled by setting 'winsymlinks:native' or 'winsymlinks:nativestrict' in the environment variable CYGWIN. The upside of this symlink type is that the path is stored as Windows path so they are understood by non-Cygwin Windows tools as well. The downsides are: The path is stored as Windows path, so the path has perhaps to be rearranged to result in a valid path. This may result in a divergence from the original POSIX path the user intended. Creating NTFS symlinks require administrative privileges by default. You have to make certain settings in the OS (depending on the Windows version) to allow creating them as a non-privileged user. NTFS symlinks have a type. They are either a "file" or a "directory", depending on the target file type. This information is utilized especially by Windows Explorer to show the correct file or directory icon in file listings without having to check on the target file and to know what actions are provided by clicking on the symlink. However, if a NTFS symlink points to a file "foo", and "foo" is deleted and replaced by a directory "foo", the symlink type of an NTFS symlink pointing to "foo" is unchanged and subsequently Windows Explorer will misbehave. Consequentially, creating dangling NTFS symlinks is a nuisance, since the library does not know what type the still-to-be-created symlink target will be. Cygwin will not create dangling NTFS symlinks, but fallback to creating the default symlink type (winsymlinks:native) or just fail (winsymlinks:nativestrict).

Another method, available since 2001, is enabled if `winsymlinks' or 'winsymlinks:lnk' is set in the environment variable CYGWIN. Using this method, Cygwin generates symlinks by creating Windows shortcuts . Cygwin created shortcuts have a special header (which is never created by Explorer that way) and the DOS READONLY attribute set. A Windows path is stored in the shortcut as usual and the POSIX path is stored in the remainder of the file. While the POSIX path is stored as is, the Windows path has perhaps to be rearranged to result in a valid path. This may result in a divergence between the Windows and the POSIX path when symlinks are moved crossing mount points. When a user changes the shortcut, this will be detected by Cygwin and it will only use the Windows path then. While Cygwin shortcuts are shown without the ".lnk" suffix in `ls' output, non-Cygwin shortcuts are shown with the suffix. For enabling this or the preceeding symlink type, see https://cygwin.com/cygwin-ug-net/using-cygwinenv.html

5.8. Why do some files, which are not executables have the 'x' type.

When working out the POSIX-style attribute bits on a file stored on certain filesystems (FAT, FAT32), the library has to fill out some information not provided by these filesystems. It guesses that files ending in .exe and .bat are executable, as are ones which have a "#!" as their first characters. This guessing doesn't take place on filesystems providing real permission information (NTFS, NFS), unless you switch the permission handling off using the mount flag "noacl" on these filesystems.

5.9. How secure is Cygwin in a multi-user environment?

As of version 1.5.13, the Cygwin developers are not aware of any feature in the cygwin dll that would allow users to gain privileges or to access objects to which they have no rights under Windows. However there is no guarantee that Cygwin is as secure as the Windows it runs on. Cygwin processes share some variables and are thus easier targets of denial of service type of attacks.

5.10. How do the net-related functions work?

The network support in Cygwin is supposed to provide the POSIX API, not the Winsock API. There are differences between the semantics of functions with the same name under the API. E.g., the POSIX select system call can wait on a standard file handles and handles to sockets. The select call in Winsock can only wait on sockets. Because of this, the Cygwin dll does a lot of nasty stuff behind the scenes, trying to persuade various Winsock/Win32 functions to do what a Unix select would do. If you are porting an application which already uses Winsock, then porting the application to Cygwin means to port the application to using the POSIX net functions. You should never mix Cygwin net functions with direct calls to Winsock functions. If you use Cygwin, use the POSIX API.

5.11. I don't want Unix sockets, how do I use normal Win32 winsock?

You don't. Look for the Mingw-w64 project to port applications using native Win32/Winsock functions. Cross compilers packages to build Mingw-w64 targets are available in the Cygwin distro.

5.12. What version numbers are associated with Cygwin?

Cygwin versioning is relatively complicated because of its status as a shared library. First of all, since October 1998 every Cygwin DLL has been named cygwin1.dll and has a 1 in the release name. Additionally, there are DLL major and minor numbers that correspond to the name of the release, and a release number. In other words, cygwin-2.4.1-1 is cygwin1.dll , major version 2, minor version 4, release 1. -1 is a subrelease number required by the distro versioning scheme. It's not actually part of the Cygwin DLL version number. The cygwin1.dll major version number gets incremented only when a change is made that makes existing software incompatible. For example, the first major version 5 release, cygwin-1.5.0-1, added 64-bit file I/O operations, which required many libraries to be recompiled and relinked. The minor version changes every time we make a new backward compatible Cygwin release available. There is also a cygwin1.dll release version number. The release number is only incremented if we update an existing release in a way that does not effect the DLL (like a missing header file). There are also Cygwin API major and minor numbers. The major number tracks important non-backward-compatible interface changes to the API. An executable linked with an earlier major number will not be compatible with the latest DLL. The minor number tracks significant API additions or changes that will not break older executables but may be required by newly compiled ones. Then there is a shared memory region compatibility version number. It is incremented when incompatible changes are made to the shared memory region or to any named shared mutexes, semaphores, etc. For more exciting Cygwin version number details, check out the /usr/include/cygwin/version.h file.

5.13. Why isn't timezone set correctly?

(Please note: This section has not yet been updated for the latest net release.) Did you explicitly call tzset() before checking the value of timezone? If not, you must do so.

5.14. Is there a mouse interface?

If you're using X then use the X API to handle mouse events. In a Windows console window you can enable and capture mouse events using the xterm escape sequences for mouse events.

6. Programming Questions

6.1. How do I contribute a package?

If you are willing to be a package maintainer, great! We urgently need volunteers to prepare and maintain packages, because the priority of the Cygwin Team is Cygwin itself. The Cygwin Package Contributor's Guide at https://cygwin.com/packages.html details everything you need to know about Cygwin packaging. For questions about package maintenance, use the cygwin-apps mailing list (start at https://cygwin.com/lists.html) after searching and browsing the cygwin-apps list archives, of course. Be sure to look at the Submitting a package checklist at https://cygwin.com/packaging-contributors-guide.html#submitting before sending an ITP (Intent To Package) email to cygwin-apps. You should also announce your intentions to the general cygwin list, in case others were thinking the same thing.

6.2. How do I contribute to Cygwin?

If you want to contribute to Cygwin itself, see https://cygwin.com/contrib.html.

6.3. Why are compiled executables so huge?!?

By default, gcc compiles in all symbols. You'll also find that gcc creates large executables on UNIX. If that bothers you, just use the 'strip' program, part of the binutils package. Or compile with the -s option to gcc.

6.4. What do I have to look out for when porting applications to 64 bit Cygwin?

The Cygwin x86_64 toolchain is using the LP64 data model. That means, in contrast to Windows, which uses an LLP64 data model, sizeof(long) != sizeof(int), just as on Linux. For comparison: Cygwin Windows Cygwin Linux x86_64 Linux Windows x86_64 i686 sizeof(int) 4 4 4 sizeof(long) 4 4 8 sizeof(size_t) 4 8 8 sizeof(void*) 4 8 8 This difference can result in interesting problems, especially when using Win32 functions, especially when using pointers to Windows datatypes like LONG, ULONG, DWORD. Given that Windows is LLP64, all of the aforementioned types are 4 byte in size, on 32 as well as on 64 bit Windows, while `long' on 64 bit Cygwin is 8 bytes. Take the example ReadFile: ReadFile (HANDLE, LPVOID, DWORD, LPDWORD, LPOVERLAPPED); In the 32 bit Cygwin and Mingw-w64 environments, as well as in the 64 bit Mingw-w64 environment, it is no problem to substitute DWORD with unsigned long: unsigned long number_of_bytes_read; [...] ReadFile (fhdl, buf, buflen, &number_of_bytes_read, NULL); However, in 64 bit Cygwin, using LP64, number_of_bytes_read is 8 bytes in size. But since ReadFile expects a pointer to a 4 byte type, the function will only change the lower 4 bytes of number_of_bytes_read on return, while the content of the upper 4 bytes stays undefined. Here are a few donts which should help porting applications from the known ILP32 data model of 32 bit Cygwin, to the LP64 data model of 64 bit Cygwin. Note that these are not Cygwin-only problems. Many Linux applications suffered the same somewhat liberal handling of datatypes when the AMD64 CPU was new. Don't mix up int and long in printf/scanf. This: int i; long l; printf ("%d %ld

", l, i); may not print what you think it should. Enable the gcc options -Wformat or -Wall, which warn about type mismatches in printf/scanf functions. Note Using -Wall (optionally with -Werror to drive the point home) makes a lot of sense in general, not only when porting code to a new platform.

Don't mix int and long pointers. long *long_ptr = (long *) &my_int; /* Uh oh! */ *long_ptr = 42; The assignment will write 8 bytes to the address of my_int. Since my_int is only 4 bytes, something else gets randomly overwritten. Finding this kind of bug is very hard, because you will often see a problem which has no immediate connection to the actual bug.

Don't mix int and pointers at all! This will not work as expected anymore