Hi Ryan,
Thanks for the FAQ entry. I had a look now, finally. Two nits:
On Aug 25 22:08, Ryan Johnson wrote:
Index: winsup/doc/faq-using.xml
===================================================================
RCS file: /cvs/src/src/winsup/doc/faq-using.xml,v
retrieving revision 1.35
diff -u -r1.35 faq-using.xml
--- winsup/doc/faq-using.xml 4 Aug 2011 18:25:41 -0000 1.35
+++ winsup/doc/faq-using.xml 26 Aug 2011 01:58:44 -0000
@@ -1199,3 +1199,92 @@
</listitem>
</itemizedlist></para>
</answer></qandaentry>
+<qandaentry id='faq.using.fixing-fork-failures'>
+<question><para>Calls to<literal>fork</literal> fail a lot. How can
+ I fix the problem?</para></question>
+<answer>
+
+<para>Unix-like applications make extensive use of
+<literal>fork</literal>, a function which spawns an exact copy of
+ the running process. Notable fork-using applications include bash
+ (and bash scripts), emacs, gcc, make, perl, python, and
+ ruby. Unfortunately, the Windows ecosystem is quite hostile to a
+ reliable fork implementation, leading to error messages such as:</para>
+<para><itemizedlist>
+<listitem>unable to remap<emphasis>$dll</emphasis> to same address as
parent</listitem>
+<listitem>couldn't allocate heap</listitem>
+<listitem>died waiting for dll loading</listitem>
+<listitem>child -1 - died waiting for longjmp before initialization</listitem>
+<listitem>STATUS_ACCESS_VIOLATION</listitem>
+<listitem>resource temporarily unavailable</listitem>
+</itemizedlist></para>
+<para>If you find that frequent fork failures interfere with normal
+ use of cygwin, please try the following:</para>
+<para><itemizedlist>
+<listitem>Restart whatever process is trying (and failing) to use
+<literal>fork</literal>. Sometimes Windows sets up a process
+ environment that is even more hostile to fork than usual.</listitem>
+<listitem>Ensure that you have eliminated (not just disabled) all
+ software on the BLODA (see<ulink
+ url="http://cygwin.com/faq/faq.using.html#faq.using.bloda";
+ />)</listitem>
+<listitem>Install the 'rebase' package, read its README in
+<literal>/usr/share/doc/Cygwin</literal>, and follow the
+ instructions there to run 'rebaseall'.</listitem>
The rebase package is always installed since it's part of the Base
category. This entry should be rephrased accordingly.
+</itemizedlist></para>
+<para>Please note that installing new packages or updating existing
+ ones often undoes the effects of rebaseall and cause fork failures
+ to reappear. If so, just run rebaseall again.
+</para></answer>
+</qandaentry>
+<qandaentry id='faq.using.why-fork-fails'>
+<question><para>Why does<literal>fork</literal> fail so much,
+ anyway? (or: Why does<literal>fork</literal> still fail even though
+ I ran rebaseall?)</para></question>
+<answer>
+<para>The semantics of<literal>fork</literal> require that a forked
+ child process have<emphasis>exactly</emphasis> the same address
+ space layout as its parent. However, Windows provides no native
+ support for cloning address space between processes and several
+ features actively undermine a reliable<literal>fork</literal>
+ implementation.
Everything else which follows from here is a good description of the
inner workings, but that shouldn't go into the FAQ. What about creating
a link to the user's guide "Process Creation" section here. The technical
details could then go into the "Process Creation" section.
+ Three issues are especially prevalent:</para>
+<para><itemizedlist>
+<listitem>DLL base address collisions. Unlike *nix shared
+ libraries, which use "position-independent code", Windows shared
+ libraries assume a fixed base address. Whenever the hard-wired
+ address ranges of two DLLs collide (which occurs quite often), the
+ Windows loader must "rebase" one of them to a different
+ address. However, it does not resolve collisions consistently, and
+ may rebase a different dll and/or move it to a different address
+ every time. Cygwin can usually compensate for this effect when it
+ involves libraries opened dynamically, but collisions among
+ statically-linked dlls (dependencies known at compile time) are
+ resolved before<literal>cygwin1.dll</literal> initializes and
+ cannot be fixed afterward. This problem can only be solved by
+ removing the base address conflicts which cause the problem,
+ usually using the<literal>rebaseall</literal> package.</listitem>
+
+<listitem>Address space layout randomization (ASLR). Starting with
+ Vista, Windows implements ASLR, which means that thread stacks,
+ heap, memory-mapped files, and statically-linked dlls are placed
+ at different (random) locations in each process. This behavior
+ interferes with a proper<literal>fork</literal>, and if an
+ unmovable object (process heap or system dll) ends up at the wrong
+ location, Cygwin can do nothing to compensate (though it will
+ retry a few times automatically). In a 64-bit system, marking
+ executables as large address-ware and rebasing dlls to high
+ addresses has been reported to help, as ASLR affects only the
+ lower 2GB of address space.</listitem>
+
+<listitem>DLL injection by BLODA. Badly-behaved applications which
+ inject dlls into other processes often manage to clobber important
+ sections of the child's address space, leading to base address
+ collisions which rebasing cannot fix. The only way to resolve this
+ problem is to remove (usually uninstall) the offending
+ app.</listitem></itemizedlist></para>
+<para>In summary, current Windows implementations make it
+ impossible to implement a perfectly reliable fork, and occasional
+ fork failures are inevitable. PTC.
+</para>
+</answer>
+</qandaentry>
Corinna
