How does everything work?

There's a C library which provides a Unix-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 run on Unix 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/Unix translation layer, providing that DLL changes stay backward-compatible.

For a good overview of Cygwin, you may want to read the paper on Cygwin published by the Usenix Association in conjunction with the 2d Usenix NT Symposium in August 1998. It is available in HTML format on the project WWW site.

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 http://cygwin.com/snapshots/.

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

Let's start with some background.

In UNIX, 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 \n (new line) written to the file will prepend a \r (carriage return) so that if you `printf("Hello\n") you in fact get "Hello\r\n". 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 easily mix Cygwin-ported Unix programs with generic Windows programs. As a result, Cygwin opens files in text mode as is normal under Windows. In the accompanying tools, tools that deal with binaries (e.g. objdump) operate in Unix binary mode and tools that deal with text files (e.g. bash) operate in text mode.

Some people push the notion of globally setting the default processing mode to binary via mount point options or by setting the CYGWIN environment variable. But that creates a different problem. In binary mode, the program receives all of the data in the file, including a \r. Since the programs will no longer deal with these properly for you, you would have to remove the \r from the relevant text files, especially scripts and startup resource files. This is a porter "cop out", forcing the user to deal with the \r for the porter.

It is rather easy for the porter 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)`.

Note that because the open/fopen switches are defined by ANSI, they exist under most flavors of Unix; open/fopen will just ignore the switch since they have no meaning to UNIX.

Explanation adapted from mailing list email by Earnie Boyd <earnie_boyd (at) yahoo.com>.

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.

Why is some functionality only supported in Windows NT?

Windows 9x: n. 32 bit extensions and a graphical shell for a 16 bit patch to an 8 bit operating system originally coded for a 4 bit microprocessor, written by a 2 bit company that can't stand 1 bit of competition.

But seriously, Windows 9x lacks most of the security-related calls and has several other deficiencies with respect to its version of the Win32 API. See the calls.texinfo document for more information as to what is not supported in Win 9x.

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.

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.

How do symbolic links work?

Cygwin knows of two ways to create symlinks.

The old method is the only valid one up to but not including version 1.3.0. If it's enabled (from 1.3.0 on by setting `nowinsymlinks' in the environment variable CYGWIN) Cygwin generates link files with a magic header. When you open a file or directory that is a link to somewhere else, it opens the file or directory listed in the magic header. Because we don't want to have to open every referenced file to check symlink status, Cygwin marks symlinks with the system attribute. Files without the system attribute are not checked. Because remote samba filesystems do not enable the system attribute by default, symlinks do not work on network drives unless you explicitly enable this attribute.

The new method which is introduced with Cygwin version 1.3.0 is enabled by default or if `winsymlinks' 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 in that way never created by Explorer) and the R/O attribute set. A DOS path is stored in the shortcut as usual and the description entry is used to store the POSIX path. While the POSIX path is stored as is, the DOS path has perhaps to be rearranged to result in a valid path. This may result in a divergence between the DOS 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 DOS path then. While Cygwin shortcuts are shown without the ".lnk" suffix in `ls' output, non-Cygwin shortcuts are shown with the suffix. However, both are treated as symlinks.

Both, the old and the new symlinks can live peacefully together since Cygwin treats both as symlinks regardless of the setting of `(no)winsymlinks' in the environment variable CYGWIN.

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

When working out the Unix-style attribute bits on a file, the library has to fill out some information not provided by the WIN32 API.

It guesses that files ending in .exe and .bat are executable, as are ones which have a "#!" as their first characters.

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.

How do the net-related functions work?

(Please note: This section has not yet been updated for the latest net release.)

The network support in Cygwin is supposed to provide the Unix API, not the Winsock API.

There are differences between the semantics of functions with the same name under the API.

E.g., the select system call on Unix can wait on a standard file handles and handles to sockets. The select call in Winsock can only wait on sockets. Because of this, 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 using the net support in Cygwin is wrong.

But you can still use native Winsock, and use Cygwin. The functions which cygwin.dll exports are called 'cygwin_<name>'. There are a load of defines which map the standard Unix names to the names exported by the DLL-- check out include/netdb.h:

    ..etc..
    void		cygwin_setprotoent (int);
    void		cygwin_setservent (int);
    void		cygwin_setrpcent (int);
    ..etc..
    #ifndef __INSIDE_CYGWIN_NET__
    #define endprotoent cygwin_endprotoent
    #define endservent cygwin_endservent
    #define endrpcent  cygwin_endrpcent
    ..etc..

The idea is that you'll get the Unix->Cygwin mapping if you include the standard Unix header files. If you use this, you won't need to link with libwinsock.a - all the net stuff is inside the DLL.

The mywinsock.h file is a standard winsock.h which has been hacked to remove the bits which conflict with the standard Unix API, or are defined in other headers. E.g., in mywinsock.h, the definition of struct hostent is removed. This is because on a Unix box, it lives in netdb. It isn't a good idea to use it in your applications.

As of the b19 release, this information may be slightly out of date.

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

To use the vanilla Win32 winsock, you just need to #define __USE_W32_WINSOCK and #include "windows.h" (or "winsock2.h" at the top of your source file(s). You may find it easier to add "-D__USE_W32_WINSOCK" to the CFLAGS settings in your makefile, if you are using one, as this will then apply to all your source files. It is also worth using "#define WIN32_LEAN_AND_MEAN" before you include the windows header file, as this will prevent it from pulling in lots of header files for all sorts of unrelated windows APIs when all you want is the Winsock definitions; again, this could be set for the entire project in your CFLAGS.

You'll also need to add -lwsock32 to the compiler's command line (or the makefile's list of link libs) so that you link against libwsock32.a.

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-1.5.10-2 is `cygwin1.dll`, major version 5, minor version 10, release 2.

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. Finally there is a mount point registry version number which keeps track of non-backwards-compatible changes to the registry mount table layout. This has been `mounts v2` for a long time. For more exciting Cygwin version number details, check out the `/usr/include/cygwin/version.h` file.

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.

Is there a mouse interface?

There is no way to capture mouse events from Cygwin. There are currently no plans to add support for this.