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Date: Thu, 9 Apr 2015 09:58:18 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Shared memory handling for mixed C/FORTRAN program
Message-ID: <20150409075818.GP2819@calimero.vinschen.de>
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On Apr  8 11:49, Christoph Weise wrote:
> I am porting to cygwin a program in FORTRAN/C that relies on C
> routines to create a shared memory region allowing various independent
> FORTRAN routines to share data. Program compiles and runs ok on Linux
> with g77/gcc compilers. I am compiling on cygwin 1.7.33-2(0.280/5/3)
> with gfortran/gcc (4.8.3).
>=20
> One C routine creates the shared memory section of a user-defined
> size. This seems to work just fine, although I had to add two lines to
> the sys/shm.h file:
>=20
>=20
> #define SHM_R        0400        /* or S_IRUGO from <linux/stat.h> */
> #define SHM_W        0200        /* or S_IWUGO from <linux/stat.h> */

These are non-standard Linux extensions.  You should use the normal
permission bits from sys/stat.h, like S_IRUSR, S_IWUSR, etc.

> The shm library functions seem to return reasonable info (page size
> and address, for removal of the shm section).
>=20
>=20
> Each FORTRAN routine then calls a C routine to find the shared memory,
> with a C routine returning pointers to two positions in the section
> intended for different kinds of data:
>=20
>=20
>=20
> #define PAGESIZE 1024

PAGESIZE on Cygwin is not 1024, and the right value to use for
XSI SHM is SHMLBA (=3D=3D 64K on Cygwin)

> int findshm(char**pptr,  /* Address of the parameter pointer */
> float**cptr)  /* Address of the data pointer */
>=20
> ....calls to shm library functions ....
>=20
> shmaddr =3D0; p =3Dshmat(shmid,shmaddr,(SHM_R |SHM_W)); *pptr =3Dp; *cptr
> =3D(float*)(p +PAGESIZE); return npages;
>=20
>=20
> The calling FORTRAN code looks like this:
>=20
> integer pptr,cptr integer npages npages =3Dfindshm(pptr,cptr)
>=20
> Although the total size of the created memory section npages is ok,
> the amount of memory following cptr is too small on cygwin (but not in
> Linux) and the program crashes for larger datasets with
>=20
> Program received signal SIGSEGV:Segmentationfault -invalid memory
> reference.

Does shmat actually return a non-NULL value?  Are you running
cygserver?  Did you check if it works from plain C?  If not,
do you have a simple testcase in plain C?


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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