* new-features.sgml (ov-new1.7.7): Change to describe partial
reversion to pre-1.7.6 behaviour. (ov-new1.7.6): Deprecate description of CWD handling. * pathnames.sgml (pathnames-win32-api): Rewrite to reflect new CWD behaviour.
This commit is contained in:
Corinna Vinschen
parent
1121c57f54
commit
6c43f249ac
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@ -1,3 +1,11 @@
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2010-08-27 Corinna Vinschen <corinna@vinschen.de>
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* new-features.sgml (ov-new1.7.7): Change to describe partial
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reversion to pre-1.7.6 behaviour.
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(ov-new1.7.6): Deprecate description of CWD handling.
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* pathnames.sgml (pathnames-win32-api): Rewrite to reflect new CWD
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behaviour.
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2010-08-19 Corinna Vinschen <corinna@vinschen.de>
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* new-features.sgml (ov-new1.7.7): New section.
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@ -5,8 +5,11 @@
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<itemizedlist mark="bullet">
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<listitem><para>
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Add a new CW_SYNC_WINCWD command to the <function>cygwin_internal</function>
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call, to allow to synchronize the Win32 CWD with the Cygwin CWD.
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Partially revert the 1.7.6 change to set the Win32 current working directory
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(CWD) always to an invalid directory, since it breaks backward compatibility
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too much. The Cygwin CWD and the Win32 CWD are now kept in sync again, unless
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the Cygwin CWD is not usable as Win32 CWD. See the reworked
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<xref linkend="pathnames-win32-api"></xref> for details.
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</para></listitem>
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</itemizedlist>
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@ -61,10 +64,9 @@ clock_gettime(3) and clock_getres(3) accept CLOCK_MONOTONIC.
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</para></listitem>
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<listitem><para>
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Cygwin handles the current working directory entirely on its own. The Win32
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current working directory is set to an invalid path to be out of the way.
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This affects calls to the Win32 file API (CreateFile, etc.). See
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<xref linkend="pathnames-win32-api"></xref> for details.
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DEPRECATED with 1.7.7: Cygwin handles the current working directory entirely
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on its own. The Win32 current working directory is set to an invalid path to
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be out of the way. [...]
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</para></listitem>
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</itemizedlist>
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@ -373,88 +373,64 @@ relative pathnames, or if your application uses functions like
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<function>CreateProcess</function> or <function>ShellExecute</function>
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to start other applications.</para>
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<para>When a Cygwin application is started, the Win32 idea of the current
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working directory (CWD) is set to an <emphasis role='bold'>invalid</emphasis>
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directory. This works around the problem that the Win32 CWD is locked
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in a way which restricts certain POSIX functionality. However, the side
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effect is that a call to, for instance, <function>CreateFile ("foo",
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...);</function> will fail, since the Win32 notion of the CWD is
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not the same as the Cygwin notion of the CWD, and worse, it's a
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directory entirely unsuitable for normal file operations.</para>
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<para>When a Cygwin application is started, the Windows idea of the current
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working directory (CWD) is not necessarily the same as the Cygwin CWD.
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There are a couple of restrictions in the Win32 API, which disallow certain
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directories as Win32 CWD:</para>
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<itemizedlist spacing="compact">
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<listitem>
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<para>The Windows subsystem only supports CWD paths of up to 258 chars.
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This restriction doesn't apply for Cygwin processes, at least not as
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long as they use the POSIX API (chdir, getcwd). This means, if a Cygwin
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process has a CWD using an absolute path longer than 258 characters, the
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Cygwin CWD and the Windows CWD differ.</para>
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</listitem>
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<listitem>
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<para>The Win32 API call to set the current directory,
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<function>SetCurrentDirectory</function>, fails for directories for which
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the user has no permissions, even if the user is an administrator. This
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restriction doesn't apply for Cygwin processes, if they are running under
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an administrator account.</para>
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</listitem>
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<listitem>
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<para><function>SetCurrentDirectory</function> does not support
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case-sensitive filenames.
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</para>
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</listitem>
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<listitem>
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<para>Last, but not least, <function>SetCurrentDirectory</function> can't
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work on virtual Cygwin paths like /proc or /cygdrive. These paths only
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exists in the Cygwin realm so they have no meaning to a native Win32
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process.</para>
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</listitem>
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</itemizedlist>
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<para>As long as the Cygwin CWD is usable as Windows CWD, the Cygwin and
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Windows CWDs are in sync within a process. However, if the Cygwin process
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changes its working directory into one of the directories which are
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unusable as Windows CWD, we're in trouble. If the process uses the
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Win32 API to access a file using a relative pathname, the resulting
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absolute path would not match the expectations of the process. In the
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worst case, the wrong files are deleted.</para>
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<para>To workaround this problem, Cygwin sets the Windows CWD to a special
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directory in this case. This special directory points to a virtual
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filesystem within the native NT namespace (<filename>\??\PIPE\</filename>).
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Since it's not a real filesystem, the deliberate effect is that a call to,
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for instance, <function>CreateFile ("foo", ...);</function> will fail,
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as long as the processes CWD doesn't work as Windows CWD.</para>
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<para>So, in general, don't use the Win32 file API in Cygwin applications.
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If you <emphasis role='bold'>really</emphasis> need to access files using
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the Win32 API, or if you <emphasis role='bold'>really</emphasis> have to use
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<function>CreateProcess</function> to start applications, rather than
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the POSIX <function>exec(3)</function> familiy of functions, you have to
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make sure that the Win32 CWD is set to some valid directory. To
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accomplish that, you can choose from several methods.</para>
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<itemizedlist spacing="compact">
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<listitem>
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<para>The easiest method is to call</para>
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<screen>
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#include <sys/cygwin.h>
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cygwin_internal (CW_SYNC_WINCWD);
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</screen>
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<para>prior to calling the Win32 functions which require a valid Win32 CWD.
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This function synchronizes the Win32 CWD with the Cygwin CWD.</para>
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<para>Note that the <function>cygwin_internal (CW_SYNC_WINCWD)</function>
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call may fail. In that case, it returns with a non-zero value and
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errno is set appropriately:</para>
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<segmentedlist>
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<?dbhtml list-presentation="table"?>
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<seglistitem>
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<seg><emphasis role='bold'>ENOTDIR</emphasis></seg>
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<seg>The Cygwin CWD is a virtual path, like /proc, or //, which does not
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exist as valid directory in the Win32 namespace.</seg>
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</seglistitem>
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<seglistitem>
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<seg><emphasis role='bold'>EACCES</emphasis></seg>
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<seg>The Cygwin CWD is a directory with restrictive permissions,
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which make it unusable as Win32 directory.</seg>
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</seglistitem>
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<seglistitem>
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<seg><emphasis role='bold'>ENAMETOOLONG</emphasis></seg>
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<seg>The Cygwin CWD is too long to be used as Win32 CWD.
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The Win32 CWD is restricted to 258 characters.</seg>
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</seglistitem>
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</segmentedlist>
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<para>You should make sure that you test the return value of
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<function>cygwin_internal (CW_SYNC_WINCWD)</function>, otherwise
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your application will potentially not work correctly. If the call
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failed, you can use the Win32 call <function>SetCurrentDirectory</function>
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to move to some well-known directory.</para>
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<note><para>After you've synchronized the Win32 CWD with the Cygwin CWD,
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be aware that the directory is locked, until the process exited, or
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until the process set the Win32 CWD to some other directory. During that
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period it will not be possible to rename or remove the directory from
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other Cygwin applications.</para></note>
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</listitem>
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<listitem>
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<para>If you know where to go to, you can also just call the Win32 function
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<function>SetCurrentDirectory</function> immediately.</para>
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</listitem>
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<listitem>
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<para>If you need a valid Win32 CWD only for a child application started
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via <function>CreateProcess</function> and friends, you don't have to
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set your own Win32 CWD. In that case, just utilize the lpCurrentDirectory
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parameter. See the description of the <function>CreateProcess</function>
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function in the MSDN manual pages.</para>
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</listitem>
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<listitem>
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<para>Last, but not least, if you don't need any POSIX function from
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Cygwin in your specific applciation, consider to compile your application
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as native Win32 (mingw) executable, rather than as Cygwin executable.</para>
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</listitem>
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</itemizedlist>
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the POSIX <function>exec(3)</function> family of functions, you have to
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make sure that the Cygwin CWD is set to some directory which is valid as
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Win32 CWD.</para>
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</sect2>
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