newlib/winsup/doc/overview2.sgml

<sect1 id="ov-ex-unix"><title>Expectations for UNIX Programmers</title>

<para>Developers coming from a UNIX background will find a set of utilities
they are already comfortable using, including a working UNIX shell.  The
compiler tools are the standard GNU compilers most people will have previously
used under UNIX, only ported to the Windows host.  Programmers wishing to port
UNIX software to Windows NT or 9x will find that the Cygwin library provides
an easy way to port many UNIX packages, with only minimal source code
changes.</para>
</sect1>

<sect1 id="ov-ex-win"><title>Expectations for Windows Programmers</title>
<para>Developers coming from a Windows background will find a set of tools capable
of writing console or GUI executables that rely on the Microsoft Win32 API.
The linker and dlltool utility may be used to write Windows Dynamically Linked
Libraries (DLLs).  The resource compiler "windres" is also provided with the
native Windows GNUPro tools.  All tools may be used from the Microsoft command
line prompt, with full support for normal Windows pathnames.</para>
</sect1>

<sect2 id="ov-hi-intro"><title>Introduction</title> <para>When a binary linked
against the library is executed, the Cygwin DLL is loaded into the
application's text segment.  Because we are trying to emulate a UNIX kernel
which needs access to all processes running under it, the first Cygwin DLL to
run creates shared memory areas that other processes using separate instances
of the DLL can access.  This is used to keep track of open file descriptors and
assist fork and exec, among other purposes.  In addition to the shared memory
regions, every process also has a per_process structure that contains
information such as process id, user id, signal masks, and other similar
process-specific information.</para>

<para>The DLL is implemented using the Win32 API, which allows it to run on all
Win32 hosts.  Because processes run under the standard Win32 subsystem, they
can access both the UNIX compatibility calls provided by Cygwin as well as
any of the Win32 API calls.  This gives the programmer complete flexibility in
designing the structure of their program in terms of the APIs used.  For
example, they could write a Win32-specific GUI using Win32 API calls on top of
a UNIX back-end that uses Cygwin.</para>

<para>Early on in the development process, we made the important design
decision that it would not be necessary to strictly adhere to existing UNIX
standards like POSIX.1 if it was not possible or if it would significantly
diminish the usability of the tools on the Win32 platform.  In many cases, an
environment variable can be set to override the default behavior and force
standards compliance.</para>
</sect2>

<sect2 id="ov-hi-win9xnt"><title>Supporting both Windows NT and 9x</title>
<para>While Windows 95 and Windows 98 are similar enough to each other that we
can safely ignore the distinction when implementing Cygwin, Windows NT is an
extremely different operating system.  For this reason, whenever the DLL is
loaded, the library checks which operating system is active so that it can act
accordingly.</para>

<para>In some cases, the Win32 API is only different for
historical reasons.  In this situation, the same basic functionality is
available under Windows 9x and NT but the method used to gain this
functionality differs.  A trivial example: in our implementation of
uname, the library examines the sysinfo.dwProcessorType structure
member to figure out the processor type under Windows 9x.  This field
is not supported in NT, which has its own operating system-specific
structure member called sysinfo.wProcessorLevel.</para>

<para>Other differences between NT and 9x are much more fundamental in
nature.  The best example is that only NT provides a security model.</para>
</sect2>

<sect2 id="ov-hi-perm"><title>Permissions and Security</title>
<para>Windows NT includes a sophisticated security model based on Access
Control Lists (ACLs).  Although some modern UNIX operating systems include
support for ACLs, Cygwin maps Win32 file ownership and permissions to the
more standard, older UNIX model.  The chmod call maps UNIX-style permissions
back to the Win32 equivalents.  Because many programs expect to be able to find
the /etc/passwd and /etc/group files, we provide utilities that can be used to
construct them from the user and group information provided by the operating
system.</para>

<para>Under Windows NT, the administrator is permitted to chown files.  There
is currently no mechanism to support the setuid concept or API call.  Although
we hope to support this functionality at some point in the future, in practice,
the programs we have ported have not needed it.</para>

<para>Under Windows 9x, the situation is considerably different.  Since a
security model is not provided, Cygwin fakes file ownership by making all
files look like they are owned by a default user and group id.  As under NT,
file permissions can still be determined by examining their read/write/execute
status.  Rather than return an unimplemented error, under Windows 9x, the
chown call succeeds immediately without actually performing any action
whatsoever.  This is appropriate since essentially all users jointly own the
files when no concept of file ownership exists.</para>

<para>It is important that we discuss the implications of our "kernel" using
shared memory areas to store information about Cygwin processes.  Because
these areas are not yet protected in any way, in principle a malicious user
could modify them to cause unexpected behavior in Cygwin processes.  While
this is not a new problem under Windows 9x (because of the lack of operating
system security), it does constitute a security hole under Windows NT.
This is because one user could affect the Cygwin programs run by
another user by changing the shared memory information in ways that
they could not in a more typical WinNT program.  For this reason, it
is not appropriate to use Cygwin in high-security applications.  In
practice, this will not be a major problem for most uses of the
library.</para>
</sect2>

<sect2 id="ov-hi-files"><title>File Access</title> <para>Cygwin supports
both Win32- and POSIX-style paths, using either forward or back slashes as the
directory delimiter.  Paths coming into the DLL are translated from Win32 to
POSIX as needed.  As a result, the library believes that the file system is a
POSIX-compliant one, translating paths back to Win32 paths whenever it calls a
Win32 API function.  UNC pathnames (starting with two slashes) are
supported.</para>

<para>The layout of this POSIX view of the Windows file system space is stored
in the Windows registry.  While the slash ('/') directory points to the system
partition by default, this is easy to change with the Cygwin mount utility.
In addition to selecting the slash partition, it allows mounting arbitrary
Win32 paths into the POSIX file system space.  Many people use the utility to
mount each drive letter under the slash partition (e.g. C:\ to /c, D:\ to /d,
etc...).</para>

<para>The library exports several Cygwin-specific functions that can be used
by external programs to convert a path or path list from Win32 to POSIX or vice
versa.  Shell scripts and Makefiles cannot call these functions directly.
Instead, they can do the same path translations by executing the cygpath
utility program that we provide with Cygwin.</para>

<para>Win32 file systems are case preserving but case insensitive.  Cygwin
does not currently support case distinction because, in practice, few UNIX
programs actually rely on it.  While we could mangle file names to support case
distinction, this would add unnecessary overhead to the library and make it
more difficult for non-Cygwin applications to access those files.</para>

<para>Symbolic links are emulated by files containing a magic cookie followed
by the path to which the link points.  They are marked with the System
attribute so that only files with that attribute have to be read to determine
whether or not the file is a symbolic link.  Hard links are fully supported
under Windows NT on NTFS file systems.  On a FAT file system, the call falls
back to simply copying the file, a strategy that works in many cases.</para>

<para>The inode number for a file is calculated by hashing its full Win32 path.
The inode number generated by the stat call always matches the one returned in
d_ino of the dirent structure.  It is worth noting that the number produced by
this method is not guaranteed to be unique.  However, we have not found this to
be a significant problem because of the low probability of generating a
duplicate inode number.</para>
</sect2>

<sect2 id="ov-hi-textvsbinary"><title>Text Mode vs. Binary Mode</title>
<para>Interoperability with other Win32 programs such as text editors was
critical to the success of the port of the development tools.  Most Cygnus
customers upgrading from the older DOS-hosted toolchains expected the new
Win32-hosted ones to continue to work with their old development
sources.</para>

<para>Unfortunately, UNIX and Win32 use different end-of-line terminators in
text files.  Consequently, carriage-return newlines have to be translated on
the fly by Cygwin into a single newline when reading in text mode.  The
control-z character is interpreted as a valid end-of-file character for a
similar reason.</para>

<para>This solution addresses the compatibility requirement at the expense of
violating the POSIX standard that states that text and binary mode will be
identical. Consequently, processes that attempt to lseek through text files can
no longer rely on the number of bytes read as an accurate indicator of position
in the file.  For this reason, the CYGWIN environment variable can be
set to override this behavior.</para>
</sect2>

<sect2 id="ov-hi-ansiclib"><title>ANSI C Library</title>
<para>We chose to include
Cygnus' own existing ANSI C library
"newlib" as part of the library, rather than write all of the lib C
and math calls from scratch.  Newlib is a BSD-derived ANSI C library,
previously only used by cross-compilers for embedded systems
development.</para>

<para>The reuse of existing free implementations of such things
as the glob, regexp, and getopt libraries saved us considerable
effort.  In addition, Cygwin uses Doug Lea's free malloc
implementation that successfully balances speed and compactness.  The
library accesses the malloc calls via an exported function pointer.
This makes it possible for a Cygwin process to provide its own
malloc if it so desires.</para>
</sect2>

<sect2 id="ov-hi-process"><title>Process Creation</title>
<para>The fork call in Cygwin is particularly interesting because it
does not map well on top of the Win32 API.  This makes it very
difficult to implement correctly.  Currently, the Cygwin fork is a
non-copy-on-write implementation similar to what was present in early
flavors of UNIX.</para>

<para>The first thing that happens when a parent process
forks a child process is that the parent initializes a space in the
Cygwin process table for the child.  It then creates a suspended
child process using the Win32 CreateProcess call.  Next, the parent
process calls setjmp to save its own context and sets a pointer to
this in a Cygwin shared memory area (shared among all Cygwin
tasks).  It then fills in the child's .data and .bss sections by
copying from its own address space into the suspended child's address
space.  After the child's address space is initialized, the child is
run while the parent waits on a mutex.  The child discovers it has
been forked and longjumps using the saved jump buffer.  The child then
sets the mutex the parent is waiting on and blocks on another mutex.
This is the signal for the parent to copy its stack and heap into the
child, after which it releases the mutex the child is waiting on and
returns from the fork call.  Finally, the child wakes from blocking on
the last mutex, recreates any memory-mapped areas passed to it via the
shared area, and returns from fork itself.</para>

<para>While we have some
ideas as to how to speed up our fork implementation by reducing the
number of context switches between the parent and child process, fork
will almost certainly always be inefficient under Win32.  Fortunately,
in most circumstances the spawn family of calls provided by Cygwin
can be substituted for a fork/exec pair with only a little effort.
These calls map cleanly on top of the Win32 API.  As a result, they
are much more efficient.  Changing the compiler's driver program to
call spawn instead of fork was a trivial change and increased
compilation speeds by twenty to thirty percent in our
tests.</para>

<para>However, spawn and exec present their own set of
difficulties.  Because there is no way to do an actual exec under
Win32, Cygwin has to invent its own Process IDs (PIDs).  As a
result, when a process performs multiple exec calls, there will be
multiple Windows PIDs associated with a single Cygwin PID.  In some
cases, stubs of each of these Win32 processes may linger, waiting for
their exec'd Cygwin process to exit.</para>
</sect2>

<sect2 id="ov-hi-signals"><title>Signals</title>
<para>When
a Cygwin process starts, the library starts a secondary thread for
use in signal handling.  This thread waits for Windows events used to
pass signals to the process.  When a process notices it has a signal,
it scans its signal bitmask and handles the signal in the appropriate
fashion.</para>

<para>Several complications in the implementation arise from the
fact that the signal handler operates in the same address space as the
executing program.  The immediate consequence is that Cygwin system
functions are interruptible unless special care is taken to avoid
this.   We go to some lengths to prevent the sig_send function that
sends signals from being interrupted.  In the case of a process
sending a signal to another process, we place a mutex around sig_send
such that sig_send will not be interrupted until it has completely
finished sending the signal.</para>

<para>In the case of a process sending
itself a signal, we use a separate semaphore/event pair instead of the
mutex.  sig_send starts by resetting the event and incrementing the
semaphore that flags the signal handler to process the signal.  After
the signal is processed, the signal handler signals the event that it
is done.  This process keeps intraprocess signals synchronous, as
required by POSIX.</para>

<para>Most standard UNIX signals are provided.  Job
control works as expected in shells that support
it.</para>
</sect2>

<sect2 id="ov-hi-sockets"><title>Sockets</title>
<para>Socket-related calls in Cygwin simply
call the functions by the same name in Winsock, Microsoft's
implementation of Berkeley sockets.  Only a few changes were needed to
match the expected UNIX semantics - one of the most troublesome
differences was that Winsock must be initialized before the first
socket function is called.  As a result, Cygwin has to perform this
initialization when appropriate.  In order to support sockets across
fork calls, child processes initialize Winsock if any inherited file
descriptor is a socket.</para>

<para>Unfortunately, implicitly loading DLLs
at process startup is usually a slow affair.  Because many processes
do not use sockets, Cygwin explicitly loads the Winsock DLL the
first time it calls the Winsock initialization routine.  This single
change sped up GNU configure times by thirty
percent.</para>
</sect2>

<sect2 id="ov-hi-select"><title>Select</title>
<para>The UNIX select function is another
call that does not map cleanly on top of the Win32 API.  Much to our
dismay, we discovered that the Win32 select in Winsock only worked on
socket handles.  Our implementation allows select to function normally
when given different types of file descriptors (sockets, pipes,
handles, and a custom /dev/windows Windows messages
pseudo-device).</para>

<para>Upon entry into the select function, the first
operation is to sort the file descriptors into the different types.
There are then two cases to consider.  The simple case is when at
least one file descriptor is a type that is always known to be ready
(such as a disk file).  In that case, select returns immediately as
soon as it has polled each of the other types to see if they are
ready.  The more complex case involves waiting for socket or pipe file
descriptors to be ready.  This is accomplished by the main thread
suspending itself, after starting one thread for each type of file
descriptor present.  Each thread polls the file descriptors of its
respective type with the appropriate Win32 API call.  As soon as a
thread identifies a ready descriptor, that thread signals the main
thread to wake up.  This case is now the same as the first one since
we know at least one descriptor is ready.  So select returns, after
polling all of the file descriptors one last time.</para>
</sect2>