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Subject: fork error when trying to call VirtualAlloc with size==0
Date: Mon, 19 May 2025 23:49:23 +0800
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From: Yuyi Wang via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Yuyi Wang <Strawberry_Str AT hotmail DOT com>
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Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

Recently I'm trying to build rustc on Cygwin (actually MSYS2). Here is the error:

      0 [main] rustc 3299 child_info_fork::abort: couldn't allocate memory 0x1FD100000(0) for '\??\D:\Straw\Documents\Git\rust\build\x86_64-pc-cygwin\stage1-rustc\release\deps\zerovec_derive-4b870d338ef8cbdd.dll' alignment, Win32 error 87
      0 [main] rustc 3299 child_info_fork::abort: couldn't allocate memory 0x1FD100000(0) for '\??\D:\Straw\Documents\Git\rust\build\x86_64-pc-cygwin\stage1-rustc\release\deps\zerovec_derive-4b870d338ef8cbdd.dll' alignment, Win32 error 87
error: could not exec the linker `x86_64-pc-msys-gcc`

Seems that reserve_at() in file cygwin/winsup/cygwin/dll_init.cc is trying to call
VirtualAlloc() with size == 0. That's why it returns 87 (ERROR_INVALID_PARAMETER).
This error is rare and I'm not able to create an MWE now.

I looked into the source code in dll_init.cc and confirmed that size is possible
to be zero. I guess that end == dll_end in this case. (here != dll_base because of
the conditions in callee.) So the dll straddles our left edge. Is this a bug of
the code?

Sincerely,
Yuyi Wang

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