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diff --git a/winsup/doc/overview2.sgml b/winsup/doc/overview2.sgml deleted file mode 100644 index 203e32578..000000000 --- a/winsup/doc/overview2.sgml +++ /dev/null @@ -1,422 +0,0 @@ -<sect1 id="ov-ex-win"> -<title>Quick Start Guide for those more experienced with Windows</title> -<para> -If you are new to the world of UNIX, you may find it difficult to -understand at first. This guide is not meant to be comprehensive, -so we recommend that you use the many available Internet resources -to become acquainted with UNIX basics (search for "UNIX basics" or -"UNIX tutorial"). -</para> -<para> -To install a basic Cygwin environment, run the -<command>setup.exe</command> program and click <literal>Next</literal> -at each page. The default settings are correct for most users. If you -want to know more about what each option means, see -<xref linkend="internet-setup"></xref>. Use <command>setup.exe</command> -any time you want to update or install a Cygwin package. If you are -installing Cygwin for a specific purpose, use it to install the tools -that you need. For example, if you want to compile C++ programs, you -need the <systemitem>gcc-g++</systemitem> package and probably a text -editor like <systemitem>nano</systemitem>. When running -<command>setup.exe</command>, clicking on categories and packages in the -package installation screen will provide you with the ability to control -what is installed or updated. -</para> -<para> -Another option is to install everything by clicking on the -<literal>Default</literal> field next to the <literal>All</literal> -category. However, be advised that this will download and install -several hundreds of megabytes of software to your computer. The best -plan is probably to click on individual categories and install either -entire categories or packages from the categories themselves. -After installation, you can find Cygwin-specific documentation in -the <literal>/usr/share/doc/Cygwin/</literal> directory. -</para> -<para> -Developers coming from a Windows background will be able to write -console or GUI executables that rely on the Microsoft Win32 API instead -of Cygwin using the -mno-cygwin option to GCC. The <command>-shared</command> -option allows to write Windows Dynamically Linked Libraries (DLLs). The -resource compiler <command>windres</command> is also provided. -</para> -</sect1> - -<sect1 id="ov-ex-unix"> -<title>Quick Start Guide for those more experienced with UNIX</title> -<para> -If you are an experienced UNIX user who misses a powerful command-line -environment, you will enjoy Cygwin. -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 will find that the Cygwin library provides -an easy way to port many UNIX packages, with only minimal source code -changes. -</para> -<para> -Note that there are some workarounds -that cause Cygwin to behave differently than most UNIX-like operating -systems; these are described in more detail in -<xref linkend="using-effectively"></xref>. -</para> -<para> -Use the graphical command <command>setup.exe</command> any time you want -to update or install a Cygwin package. This program must be run -manually every time you want to check for updated packages since Cygwin -does not currently include a mechanism for automatically detecting -package updates. -</para> -<para> -By default, <command>setup.exe</command> only installs a minimal subset of -packages. Add any other packages by clicking on the <literal>+</literal> -next to the Category name and selecting the package from the displayed -list. You may search for specfic tools by using the -<ulink url="http://cygwin.com/packages/">Setup Package Search</ulink> -at the Cygwin web site. -</para> -<para> -Another option is to install everything by clicking on the -<literal>Default</literal> field next to the <literal>All</literal> -category. However, be advised that this will download and install -several hundreds of megabytes of software to your computer. The best -plan is probably to click on individual categories and install either -entire categories or packages from the categories themselves. -After installation, you can find Cygwin-specific documentation in -the <literal>/usr/share/doc/Cygwin/</literal> directory. -</para> -<para> -For more information about what each option in -<command>setup.exe</command> means, see <xref -linkend="internet-setup"></xref>. -</para> - -</sect1> - -<sect1 id="highlights"><title>Highlights of Cygwin Functionality</title> - -<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 and global synchronization objects that other -processes using separate instances of the DLL can access. This is used to keep track of open file descriptors and to assist fork and exec, among other -purposes. 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 as a standard DLL in the Win32 subsystem. Under -the hood it's using the Win32 API, as well as the native NT API, where -appropriate.</para> - -<para>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>The native NT API is used mainly for speed, as well as to access -NT capabilities which are useful to implement certain POSIX features, but -are hidden to the Win32 API. -</para> - -<para>Due to some restrictions in Windows, it's not always possible -to strictly adhere to existing UNIX standards like POSIX.1. Fortunately -these are mostely border cases.</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). Cygwin maps Win32 file ownership and permissions to -ACLs by default, on file systems supporting them (usually NTFS). Solaris -style ACLs and accompanying function calls are also supported. -The chmod call maps UNIX-style permissions back to the Win32 equivalents. -Because many programs expect to be able to find the -<filename>/etc/passwd</filename> and -<filename>/etc/group</filename> files, we provide <ulink -url="http://cygwin.com/cygwin-ug-net/using-utils.html">utilities</ulink> -that can be used to construct them from the user and group information -provided by the operating system.</para> - -<para>Users with Administrator rights are permitted to chown files. -With version 1.1.3 Cygwin introduced a mechanism for setting real and -effective UIDs. This is described in <xref linkend="ntsec"></xref>. 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> - -</sect2> - -<sect2 id="ov-hi-files"><title>File Access</title> <para>Cygwin supports -both POSIX- and Win32-style paths, using either forward or back slashes as the -directory delimiter. Paths coming into the DLL are translated from POSIX to -native NT as needed. From the application perspective, the file system is -a POSIX-compliant one. The implementation details are safely hidden in the -Cygwin DLL. UNC pathnames (starting with two slashes) are supported for -network paths.</para> - -<para>Since version 1.7.0, the layout of this POSIX view of the Windows file -system space is stored in the <filename>/etc/fstab</filename> file. Actually, -there is a system-wide <filename>/etc/fstab</filename> file as well as a -user-specific fstab file <filename>/etc/fstab.d/${USER}</filename>.</para> - -<para>At startup the DLL has to find out where it can find the -<filename>/etc/fstab</filename> file. The mechanism used for this is simple. -First it retrieves it's own path, for instance -<filename>C:\Cygwin\bin\cygwin1.dll</filename>. From there it deduces -that the root path is <filename>C:\Cygwin</filename>. So it looks for the -<filename>fstab</filename> file in <filename>C:\Cygwin\etc\fstab</filename>. -The layout of this file is very similar to the layout of the -<filename>fstab</filename> file on Linux. Just instead of block devices, -the mount points point to Win32 paths. An installation with -<command>setup.exe</command> installs a <filename>fstab</filename> file by -default, which can easily be changed using the editor of your choice.</para> - -<para>In addition to selecting the root partition, the -<filename>fstab</filename> file allows mounting arbitrary Win32 paths into -the POSIX file system space. A special case is the so-called cygdrive prefix. -It's the path under which every available drive in the system is mounted -under its drive letter. The default value is <filename>/cygdrive</filename>, -so you can access the drives as <filename>/cygdrive/c</filename>, -<filename>/cygdrive/d</filename>, etc... The cygdrive prefix can be set to -some other value (<filename>/mnt</filename> for instance) in the -<filename>fstab</filename> file(s).</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 -<command>cygpath</command> utility program that we provide with Cygwin.</para> - -<para>Win32 applications handle filenames case preserving but case -insensitive. Cygwin supports case sensitivity on file systems supporting -that. Since Windows XP, the OS only supports case sensitivity when a -specific registry value is changed. Therefore case sensitivity is not -the default usually.</para> - -<para>Symbolic links are not present and supported on Windows up to and -including Windows Server 2003 R2. Only starting with Windows Vista, -native symlinks are available. Unfortunately they are strangly implemented -and so not very useful for a POSIX emulation layer. Consequentially -Cygwin recognizes them as symlinks but does not create them.</para> - -<para>Symbolic links are potentially created in two different ways. -The file style symlinks are files containing a magic cookie followed by -the path to which the link points. They are marked with the System DOS -attribute so that only files with that attribute have to be read to -determine whether or not the file is a symbolic link. The shortcut style -symlinks are Windows shortcut files with a special header and the -Readonly DOS attribute set. The advantage of file symlinks is speed, -the advantage of shortcut symlinks is the fact that they can be utilized -by non-Cygwin Win32 tools as well.</para> - -<para>Hard links are fully supported on NTFS and NFS file systems. On FAT -and some other file systems, the call falls back to simply copying the file, -a strategy that works in many cases.</para> - -<para>On file systems which don't support unique persistent file IDs (FAT, -older Samba shares) 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 <literal>d_ino</literal> of the <literal>dirent</literal> -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> - -<para><function>chroot(2)</function> is supported since Cygwin 1.1.3. -However, chroot is not a concept known by Windows. This implies some -restrictions. First of all, the <function>chroot</function> call isn't a -privileged call. Each user may call it. Second, the chroot environment -isn't safe against native windows processes. If you want to support a -chroot environment as, for example, by allowing an anonymous ftp with -restricted access, you'll have to care that only native Cygwin applications -are accessible inside of the chroot environment. Since those applications -are only using the Cygwin POSIX API to access the file system their access -can be restricted as it is intended. This includes not only POSIX paths but -Win32 paths containing drive letter and/or backslashes as well as UNC paths -(<filename>//server/share</filename> or <filename>\\server\share</filename>). -</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 Red Hat -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.</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 Red Hat's 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. Other functions, which are not supported by newlib have -been added to the Cygwin sources using BSD implementations as much as -possible.</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 <function>fork</function> 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 basically call the functions by the -same name in Winsock, Microsoft's implementation of Berkeley sockets, but -with lots of tweaks. All sockets are non-blocking under the hood to allow -to interrupt blocking calls by POSIX signals. Additional bookkeeping is -necessary to implement correct socket sharing POSIX semantics and especially -for the select call. Some socket-related functions are not implemented at -all in Winsock, as, for example, socketpair. Starting with Windows Vista, -Microsoft removed the legacy calls <function>rcmd(3)</function>, -<function>rexec(3)</function> and <function>rresvport(3)</function>. -Recent versions of Cygwin now implement all these calls internally.</para> - -<para>An especially troublesome feature of Winsock is that it must be -initialized before the first socket function is called. As a result, Cygwin -has to perform this initialization on the fly, as soon as the first -socket-related function is called by the application. In order to support -sockets across fork calls, child processes initialize Winsock if any -inherited file descriptor is a socket.</para> - -<para>AF_UNIX (AF_LOCAL) sockets are not available in Winsock. They are -implemented in Cygwin by using local AF_INET sockets instead. This is -completely transparent to the application. Cygwin's implementation also -supports the getpeereid BSD extension. A yet missing feature is -descriptor passing, though.</para> - -<para>Starting with release 1.7.0, Cygwin gets IPv6 support. However, this -depends on the availability of the Windows IPv6 stack. Up to Windows 2003, -the IPv6 stack is treated as "experimental" and it's not feature complete. -Full support is only available starting with Windows Vista and Windows Server -2008. The newly implemented <function>getaddrinfo</function> and -<function>getnameinfo</function> functions are not dependent on the OS, -though. Cygwin 1.7.0 adds replacement functions which implement the full -functionality for IPv4.</para> - -</sect2> - -<sect2 id="ov-hi-select"><title>Select</title> -<para>The UNIX <function>select</function> 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> -</sect1> - |