Cygwin Issues
Known Issues for Cygwin
Experimental blackboard to get it into the official docs then. Only a few hardcore developers will be able to verify these issues.
I found no document where missing POSIX functionality is listed and where other non-POSIX but expected typical UNIX functionality and/or behaviour are different or missing. The user guide is a bit short on that.
This should help UNIX developers and should go the the official cygwin user guide (or FAQ?) then. Unsolved issues (such as some Ghanshyam items) should go to bugzilla then.
Please fix add or fix as approriate.
Known Issues
First see the user-guide http://cygwin.com/cygwin-ug-net/highlights.html and http://cygwin.com/cygwin-ug-net/using.html ff. for the definitive answers. But some of these answers are known and verified to be outdated.
Strictly NON-POSIX
- fopen(): Ghanshyam listed several NON-POSIX issues starting with
- http://www.cygwin.com/ml/cygwin/2004-03/msg00357.html
- Note on "fopen is successful even if search permission is denied": this isn't a Cygwin issue. Windows disables access checks for directory traversal by default, by granting and enabling the ~SeChangeNotifyPrivilege privilege for all users. Revoke this privilege to your user or group if this is a problem.
- Update: Since 1.3.14 cygwin honors directory traversal security as in POSIX. Turn it off via CYGWIN=notraversal to get back the Windows behaviour not to check all upper dirs for proper perms.
- sysconf():
- strftime():
- sigaction():
- struct lconv *localeconv:
- clock_getres() missing though _POSIX_TIMERS is defined:
- symlink(2) is not atomic:
TODO: find and document more problems.
Unexpected Behaviour
Filesystem Related
case-insensitive search
Windows filesystems are usually case-insensitive in search, but
case-preservant (I know of 5 windows fs in general use, NTFS5 as
best, a new one on the horizon). For POSIX compatibility there's no
optional option or flag to restrict searches
(open,fopen,readdir,unlink,rename,...) to be case-sensitive. %%%
You can only tell your filesystem to be case-sensitive (somewhere
in the registry, which is not recommended and unrealistic), use
special cygwin managed mounts (mount -o managed) or check
case-sensitivity explicitly in your code. Best is to avoid such
issues beforehand, not allowing only case-different filenames.
But with user-created plugins it will get complicated.
_TimRobinson: it's possible to get Windows to use case-sensitive lookups at the kernel level, which is how psxss.exe and Interix work. But doing so can confuse Win32 apps, which clearly don't distinguish case._ _Exercise: Get hold of the Windows Resource Kit. Create a two files file using the cat.exe included, whose names differ only by case. Delete them by name using the del command._
pathname resolution
Cygwin is taking too much liberty when resolving paths with .. in them, as of 1.5.14. POSIX requires that the pathname before the .. exist, and if it is a symlink, that it is resolved, before going to the parent directory. Cygwin is blindly collapsing dir/.. into nothing, without first verifying dir, in violation of POSIX. (_Eric Blake_) %%% It will probably keep this behaviour due to performance reasons. (_cgf_) %%% http://cygwin.com/ml/cygwin/2005-04/msg00380.html
unlink()
* unlink isn't atomic (as on other platforms also)
while (!unlink(..) or end-of-waiting()) { sleep; }
On cygwin =unlink()= is deferred to the end of the process, when unlink fails because of locks or delete on close (=ERROR_SHARING_VIOLATION=).
* unlink a file and subsequent creation of a new file in the same location is not supported.
* unlink(2) doesn't do traverse security checks:
_cygwin unlink() still has the POSIX bug I reported earlier where calling unlink() on a file in a directory with no write permissions still removes the file, thanks to the underlying Windows implementation. Worse, because the directory has no write permissions, you can't directly re-create a new file to replace the one that was erroneously deleted. But Chris already argued that checking for write permissions on the parent directory as part of every unlink() call would slow down the common case._ EricBlake http://sourceware.org/ml/cygwin/2005-04/msg00552.html ...
* "delete on close" (unlinking an open file) is supported.
(_definitive answer: => pierre_)
Passing file descriptors
* You generally cannot pass file descriptors between seperate processes. See http://www.cygwin.com/ml/cygwin-patches/2002-q3/msg00003.html for description and workarounds. Passing the handle via IPC, passing handles to ~MapViewOfFile, ...
Hardlinks
* hardlinking directories is only available to the superuser
ln -d, ...
* *junctions* (NTFS4 directory mount points and NTFS5 volume mount points via Reparse Points)
are almost equivalent to symlinks, but symlinks are implemented in cygwin differently and
slower and do work on filenames and non-local volumes also.
mount
Mounting filesystems and devices are not supported. %%% The =/usr/bin/mount= syntax is similar but different to your favorite mount. See =mount -m=.
/etc/mtab is not linked to /proc/mounts. /proc/mount is not yet implemented, but a few other procfs files are: $ ls /proc <cyg-pids/...> cpuinfo meminfo registry/ uptime loadavg partitions stat version
Device limitations TODO
Explain about our device mapping, special block/char devices, other magic devices like sockets, fifo's, ... %%% /dev/dsp and other audio devices do work. %%% Serial devices have some known limitations. %%% Igor's /usr/sbin/create_devices.sh helper (ml-url?)
Special filenames
See http://cygwin.com/cygwin-ug-net/using-specialnames.html
- using (esp. unlinking)
"NUL"and other special filenames is
problematic (see FAQ and windows-special"\\.\"and"\??\"path-prefixes on Windows)."DEL \\.\c:\somedir\nul"There was a cygwin1.dll patch by Pierre which allows deleting such accidently created files.
getfacl/setfacl
See user guide ( http://www.cygwin.com/cygwin-ug-net/using-utils.html : [ setfacl | http://www.cygwin.com/cygwin-ug-net/using-utils.html#setfacl ] / [ getfacl | http://www.cygwin.com/cygwin-ug-net/using-utils.html#getfacl ] ) %%%
cygwin ACL's are not 1:1 mapped to windows ACL's,
esp. for inherited directory permissions. (?) %%%
TODO
(_definitive answer: => corrinna_)
select()
* http://cygwin.com/cygwin-ug-net/highlights.html#OV-HI-SELECT is outdated. (cgf)
* select with write-to-pipe fails sometimes (rsync). See http://sourceware.org/bugzilla/show_bug.cgi?id=478
* TODO (_definitive answer: => cgf_)
sync()
* sync() is a no-op (although it's possible on Windows, see [1] - the part beginning with "To flush all open files on a volume" - and [2] for a tool that makes use of it), and as specified by the standard it's unclear whether it's even supposed to do anything (all cache managers eventually flush dirty pages...).
* fsync() works as expected.
Security
See http://cygwin.com/cygwin-ug-net/ntsec.html, http://cygwin.com/faq/faq_3.html#SEC78,
sysbash, network shares, samba, windows profiles, ... %%%
Of course cygwin is entirely insecure.
setuid/su/login/cron
* UNIX root uid is 0, Windows SYSTEM uid is 18.
* cygwin cron usually runs as user SYSTEM and therefore ...
(network shares, file permissions, ...) TODO
* TODO: setuid,setgid file permission flag and workarounds.
* setuid()/setgid() process workarounds.
see http://cygwin.com/cygwin-ug-net/ntsec.html#NTSEC-SETUID
* TODO: setsid()/setpgid(): ??
* TODO: su, sudo vs runas ...
* TODO: login limitations
Processes
See the user-guide on fork, IPC, ... %%%
(_definitive answers: => cgf_)
fork
Works as expected. %%%
See http://cygwin.com/faq/faq_3.html#SEC74 and
http://cygwin.com/cygwin-ug-net/highlights.html#OV-HI-PROCESS
PID
- Unfortunately cygwin must use seperate PID's than the underlying windows PID's. %%%
http://cygwin.com/cygwin-ug-net/highlights.html#OV-HI-PROCESS is outdated. (cgf)
IPC
- Requires the *cygserver* daemon (which requires the string "server" in $CYGWIN).
ipc-daemon2 is outdated and should be uninstalled.
Unless you still use postgresql 7.4.3 or other dependent packages.
- IPC is not fully supported on Win95 yet, due to known cygserver problems.
TODO (fixed in which snapshot, when?)
Threads
1.5.11 was rather unstable compared to 1.5.10.
* -lpthread is supported by cygwin1.dll, libpthread.a is just an import library.
* PROCESS_SHARED objects may not work (?, R.Collins, April 2001)
* pshared mutexs (?)
* signals cannot interrupt semaphores: semaphore::_trywait
* as always: The latest snapshot might have fixed it :)
DLL
See http://cygwin.com/cygwin-ug-net/dll.html and
http://cygwin.com/faq/faq_3.html#SEC108 %%%
Use gcc -shared or libtool, or the older helpers dlltool, dllwrap.
- All exported symbols must be relocatable and known at compile-time.
Unless on unix, where they will be resolved at load-time ("lazy").
- Building a .dll might require more steps than building an .so
(unless you use the simple =gcc -shared= of course) %%%
Google for "Microsoft designers must have been on crack",
might need special MSVC-like __declspec IMPORT/EXPORT declarations,
but better use the linker methods.
We use specially named import libraries named like "lib<package>-version>.dll.a",
The shared libs like "cyg<package>-<version>.dll",
and must reside in the PATH!
Most packagers just forget the required -no-undefined libtool switch.
DATA sections (structs, classes) to be exported from DLL's will best work with
this linker switch: %%%
-Wl,--enable-runtime-pseudo-reloc
- Importing symbols from .exe's will not work, unlike on linux or solaris.
you can only import from dll's. %%%
See http://cygwin.com/ml/cygwin/2002-11/msg00615.html
and http://cygwin.com/ml/cygwin/2004-10/msg00339.html
- Lazy linking is not yet supported by gcc.
See wine and MSVC. (Info for windows folks only)
getloadavg() missing
getloadavg() is still a noop on windows. No one came up with a comparable solution yet.
It is used by make, xemacs, top, ...
Console
/bin/sh really is ash (a smaller bourne shell). On linux it's usually /bin/bash. There are subtle syntax differences.
- pty / tty issues: http://www.khngai.com/emacs/tty.php
- pipes TODO (known problems with text/binmode issues, which should have been fixed in a snapshot.)
See also select() above.
- bash PID reuse problem was fixed by bash-2.05b-16. %%%
(TODO: ml-url)
- Foreign characters support:
cygwin does NOT support UTF-8 nor UTF-16 yet, since newlib doesn't support it.
Use better terms, CYGWIN="codepage:oem"
Other magic good to know
- *winsock* requires the 'SYSTEMROOT' environment variable setting. CGI might fail. Fixed since 1.3.14.
http://article.gmane.org/gmane.os.cygwin/55115
Reason: cygwin uses the windows socket library.
Workaround for Apache: %%%
~SetEnv SYSTEMROOT "<path-to-c-windows>"
- setup --help (as of v2.427) does print its options, but to ./setup.log not to STDOUT.
setup --help; tail setup.log