newlib/winsup/doc/faq-api.xml

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<qandadiv id="faq.api">
<title>Cygwin API Questions</title>

<!-- faq-api.xml --> 
<qandaentry id="faq.api.everything">
<question><para>How does everything work?</para></question>
<answer>

<para>There's a C library which provides a POSIX-style API.  The
applications are linked with it and voila - they run on Windows.
</para>
<para>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.
</para>
<para>The C library is in a DLL, which makes basic applications quite small.
And it allows relatively easy upgrades to the Windows/POSIX translation
layer, providing that DLL changes stay backward-compatible.
</para>
<para>For a good overview of Cygwin, you may want to read the Cygwin
User's Guide.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.snapshots">
<question><para>Are development snapshots for the Cygwin library available?</para></question>
<answer>

<para>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
<ulink url="https://cygwin.com/snapshots/"/>.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.cr-lf">
<question><para>How is the DOS/Unix CR/LF thing handled?</para></question>
<answer>

<para>Let's start with some background.
</para>
<para>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.
</para>
<para>When processing in text mode, certain values of data are treated
specially.  A \n (new line, NL) written to the file will prepend a \r
(carriage return, CR) 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.
</para>
<para>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.
</para>
<para>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 \n 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.
</para>
<para>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
<literal>O_BINARY</literal> to the second argument of an
<literal>open</literal> call, or <literal>"b"</literal> to second argument
of an <literal>fopen</literal> call.  You can also call
<literal>setmode (fd, O_BINARY)</literal>.  To select text mode add
<literal>O_TEXT</literal> to the second argument of an <literal>open</literal>
call, or <literal>"t"</literal> to second argument of an
<literal>fopen</literal> call, or just call
<literal>setmode (fd, O_TEXT)</literal>.
</para>
<para>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 <filename>/usr/lib</filename> directory which,
when linked to an executable, changes the default open modes of any
file opened within the executed process itself.  The files are
<screen>
  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.
</screen>
</para>
<para>
<note>
  Linking against these object files does <emphasis>not</emphasis> 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>
</para>
<para>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.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.threads">
<question><para>Is the Cygwin library multi-thread-safe?</para></question>
<answer>

<para>Yes.
</para>
<para>There is also extensive support for 'POSIX threads', see the file
<literal>cygwin.din</literal> for the list of POSIX thread functions provided.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.fork">
<question><para>How is fork() implemented?</para></question>
<answer>

<para>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.
</para>
<para>Here's how it works:
</para>
<para>Parent initializes a space in the Cygwin process table for child.
Parent creates child suspended using Windows 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.
</para>
<para>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.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.globbing">
<question><para>How does wildcarding (globbing) work?</para></question>
<answer>

<para>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 <literal>LS *.EXE</literal> from DOS, it will do what you might
expect.
</para>
<para>Beware: globbing uses <literal>malloc</literal>.  If your application defines
<literal>malloc</literal>, that will get used.  This may do horrible things to you.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.symlinks">
<question><para>How do symbolic links work?</para></question>
<answer>

<para>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...
</para>

<itemizedlist spacing="compact">

<listitem><para>
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...
</para></listitem>

<listitem><para>
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.
</para></listitem>

<listitem><para>
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.
</para></listitem>

<listitem><para>
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:
<itemizedlist spacing="compact">
<listitem><para>
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.
</para></listitem>
<listitem><para>
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.
</para></listitem>
<listitem><para>
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).
</para></listitem>
</itemizedlist>
</para></listitem>

<listitem><para>
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.
</para>

<para>For enabling this or the preceeding symlink type, see
<ulink url="https://cygwin.com/cygwin-ug-net/using-cygwinenv.html"/>
</para></listitem>

</itemizedlist>

</answer></qandaentry>

<qandaentry id="faq.api.executables">
<question><para>Why do some files, which are not executables have the 'x' type.</para></question>
<answer>

<para>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.
</para>
<para>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.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.secure">
<question><para>How secure is Cygwin in a multi-user environment?</para></question>
<answer>

<para>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.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.net-functions">
<question><para>How do the net-related functions work?</para></question>
<answer>

<para>The network support in Cygwin is supposed to provide the POSIX API, not
the Winsock API.
</para>
<para>There are differences between the semantics of functions with the same
name under the API.
</para>
<para>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/Windows functions to do what
a Unix select would do.
</para>
<para>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.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.winsock">
<question><para>I don't want Unix sockets, how do I use normal Windows winsock?</para></question>
<answer>

<para>You don't.  Look for the Mingw-w64 project to port applications using
native Windows API/Winsock functions.  Cross compilers packages to build Mingw-w64
targets are available in the Cygwin distro.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.versions">
<question><para>What version numbers are associated with Cygwin?</para></question>
<answer>

<para>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 <literal>cygwin1.dll</literal> 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 <literal>cygwin1.dll</literal>, 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.
</para>
<para>The <literal>cygwin1.dll</literal> 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
<literal>cygwin1.dll</literal> 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).
</para>
<para>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.
</para>
<para>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
<literal>/usr/include/cygwin/version.h</literal> file.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.timezone">
<question><para>Why isn't timezone set correctly?</para></question>
<answer>

<para><emphasis role='bold'>(Please note: This section has not yet been updated for the latest net release.)</emphasis>
</para>
<para>Did you explicitly call tzset() before checking the value of timezone?
If not, you must do so.
</para>
</answer></qandaentry>

<qandaentry id="faq.api.mouse">
<question><para>Is there a mouse interface?</para></question>
<answer>

<para>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.
</para>
</answer></qandaentry>
</qandadiv>