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Date: | Sat, 2 Feb 2019 18:05:08 +0100 |
From: | Corinna Vinschen <corinna-cygwin AT cygwin DOT com> |
To: | cygwin AT cygwin DOT com, Sam Habiel <sam DOT habiel AT gmail DOT com> |
Subject: | Re: 32 bit vs 64 bit Cygwin, followup |
Message-ID: | <20190202170508.GB3532@calimero.vinschen.de> |
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--jI8keyz6grp/JLjh Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Feb 2 09:42, Sam Habiel wrote: > On Sat, Feb 2, 2019 at 9:39 AM Sam Habiel <sam DOT habiel AT gmail DOT com> wrote: > > > > On Thu, Nov 29, 2018 at 11:33 AM Corinna Vinschen > > <corinna-cygwin AT cygwin DOT com> wrote: > > >[...] > > > One way is to create a SYSV wrapper for each C function called from > > > assembler. Assuming this simple scenario: > > > [...] > > I and a colleague started the work to migrate the Linux x64 version to > > Cygwin. The results have been very promising; but I think we found a > > bug in gcc when dealing with va_start in sysv_abi compiled code. I > > have a simple test case. Can somebody confirm? It works fine without > > the attribute on PrintFloats. > > > > /* va_start example */ > > #include <stdio.h> /* printf */ > > #include <stdarg.h> /* va_list, va_start, va_arg, va_end */ > > > > void __attribute__ ((sysv_abi)) PrintFloats (int n, ...) > > { > > int i; > > double val; > > printf ("Printing floats:"); > > va_list vl; > > va_start(vl,n); > > for (i=3D0;i<n;i++) > > { > > val=3Dva_arg(vl,double); > > printf (" [%.2f]",val); > > } > > va_end(vl); > > printf ("\n"); > > } > > > > int main () > > { > > PrintFloats (3,3.14159,2.71828,1.41421); > > return 0; > > } >=20 > Sorry. Should say what the error is: >=20 > Hp AT memphis ~/fis-gtm/build > $ gdb a.exe > [...] > (gdb) r > Starting program: /home/Hp/fis-gtm/build/a.exe > [New Thread 11672.0xdd0] > [New Thread 11672.0x1230] > [New Thread 11672.0x20ac] > [New Thread 11672.0x43f4] >=20 > Thread 1 "a" hit Breakpoint 1, main () at test.c:23 > 23 PrintFloats (3,3.14159,2.71828,1.41421); > (gdb) s > PrintFloats (n=3D1) at test.c:6 > 6 { > (gdb) s > 9 printf ("Printing floats:"); > (gdb) s > 11 va_start(vl,n); > (gdb) s > 12 for (i=3D0;i<n;i++) > (gdb) p vl > $1 =3D (va_list) 0x3000000008 <error: Cannot access memory at address > 0x3000000008> > (gdb) s > [New Thread 11672.0x3f9c] > 14 val=3Dva_arg(vl,double); > (gdb) s >=20 > Thread 1 "a" received signal SIGSEGV, Segmentation fault. > 0x000000010040112f in PrintFloats (n=3D3) at test.c:14 >=20 > PS: Running this outside GDB causes Cygwin x64 to hang--and I don't > know how to make it unhang. That's not GCC fault. You're running varargs, written for the standard stack layout of the MSABI target from a function with a stack laid out in SYSV ABI. That can't work. If you *have to* use varargs from SYSV ABI functions, you have to define your own version of them. But there's another problem with no easy hackaround, the hang you're observing after the SEGV. Keep in mind that *Windows* runs the exception handling in the first place, not Cygwin. Cygwin only installs a structured exception handler, but that requires that windows can *find* the exception handler. To do this, Windows needs to unwind the stack until it finds the exception handler. However, you're running a function using the SYSV stack layout the Windows stack unwinder doesn't know about. As a result, this goes nowhere. The stack unwinder runs wild or, if you're lucky, just stops in the outermost structured exception handler or some vectored exception handler. I'm not sure there is a solution to that. What you could try is to install a structured exception handler inside the SYSV call. You may have to do this inside each and every SYSV function which *may* crash to keep the Windows stack unwinder having to unwind to the calling function at all. Corinna --=20 Corinna Vinschen Cygwin Maintainer --jI8keyz6grp/JLjh Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEoVYPmneWZnwT6kwF9TYGna5ET6AFAlxVzcQACgkQ9TYGna5E T6DhLQ//VHKN0gadCWamJJPQd9sX2q9Q1nMUiW1wjkqkvS9C38L2qFLgHmVdDdRT OZKa7sQqsbw3mvsLbcpC5qjTN4DuqZ/tt2NcSjoDtrqntYOWJj5op0Ozgnbe8j9m hf9AQcHCh1Vvqqgk9LQ1DTjUQ+ltDjK4gRnmQh8kcRagw1RyeeCDpEPfI95c8hHk IKnmRfuP10mJhovkoeszxMBN/S6jGX6F5zQZAJHTJhuq5P9iOc0BXm8dkCvolf5b JtkIEa7K4974Xy8p64suz76L226Hl4fC947cgGcp3qQtsRAqlcDbf5B0c+ImgPwW eWq//Rm8SnMLRJqM+pJfsLX0fxWvY9LZsRY793VK6HKL/ByDpHqS/hyXUkuyRJS+ 9gfFDDOdeVrKnVep+Xed68g2tbESTsqI8/KN5nWM75+q6vrfO4NJPeaQs+PnE2a1 fAZ3tZli1OQa82dDqdyXGAMQ2CWl7qDN1/MWWc/WmLubx2D1Z+eJ3VPN8nqTsLjh vGwVxf1sUHKmmS4Dm9tUKOeosnFHpVN1NTkc6hQDpZRMFxwLeivxWIH4aXt4CeCf B6CLupSyZf0hkkn2NkcHIRFAgMaArwli6jNiCp4krffaBY7RlhuQN4v7HOUhcnRb cSr/H9FeVgINn4sI60dAdP/j8v3q1+t+XG+nuOc0lLJOdXbinJ4= =D98/ -----END PGP SIGNATURE----- --jI8keyz6grp/JLjh--
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