Shared memory handling for mixed C/FORTRAN program

Sun Apr 12 12:18:00 GMT 2015

On Apr 12 13:23, Corinna Vinschen wrote:
> On Apr 11 20:25, Christoph Weise wrote:
> > Please see below, I provide minimal C code for three separate
> > executables, one creates the shm section, another finds it, the third
> > removes it. I include also a bash test script that executes the
> > routines in order.
>> Thanks,
>> > Please beware as I removed some checks to reduce
> > the length of the code, but it should run ok. The PAGESIZE parameter
> > is hardcoded (here 512 bytes). The desired shared mem section size is
> > set interactively as input to makeshm, or automatically with the bash
> > script. 
>> Ok, but there are bugs in the code which result in GCC warnings. I
> don't know which of them are part of your original code, but they are
> really a problem.
>> if ((int) -1 == p)
>> Don't check a pointer against an int value. It won't work on a 64 bit
> platform. Make that
>> if ((void *) -1 == p)
>> For the same reason, don't use %x to printf a pointer. Use %tx.
>> > I can write to the shm section with the second routine when the requested section <= 4096 bytes. Otherwise it's not happy. 
>> The problem is the call to shmget:
>> #undef PAGESIZE
> #define PAGESIZE 512
> shmid = shmget(key, PAGESIZE, IPC_ALLOC);
>> Since you're requesting only 512 bytes, the shared memory segment the
> following shmat call returns is only 4K. The shmget call should request
> as much memory as it needs so that the OS call is called with the right
> view size.
>> I re-read the POSIX man page for shmget, and it doesn't mention anything
> which would point out that Cygwin's behaviour here is wrong. If anybody
> has more information on this, please share them.

On second thought, adjusting Cygwin's behaviour to Linux here is rather
trivial. I applied a patch to the git repo and uploaded new developer
snapshots (2015年04月12日) to https://cygwin.com/snapshots/
You only have to replace the DLL itself, cygserver is not affected by
this patch. Please give it a try.
Thanks,
Corinna
-- 
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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